ULTRAVIOLET RADIATION
In general scientific usage, ultraviolet radiation refers to radiant
emissions in that portion of the electromagnetic spectrum
between X-rays and visible radiation, i.e. in the wavelength
range 0.01–0.4 ␮m approximately. However, outside the
Earth’s atmosphere, 99% of the solar irradiance occurs in the
spectral range 0.15–4␮m; hence, climatologically significant
ultraviolet radiation is that lying between 0.15␮m and 0.4␮m
in the spectrum, a band containing about 9% of the radiant
energy of solar origin prior to atmospheric attenuation. Solar
ultraviolet radiation is conventionally subdivided into three
wavelength bands, based primarily on potential biological
activity: UV-A (0.32–0.4␮m), UV-B (0.28–0.32␮m), and UVC
(Ͻ0.28␮m).
Radiation with wavelengths shorter than 0.21 ␮m (most of
the UV-C) is absorbed by oxygen and nitrogen in processes
of ionization, dissociation, and heating at elevations in excess
of 80 km in the thermosphere and ionosphere. However, most
absorption takes place in the stratosphere, where ozone
absorbs radiation in the wavelength range 0.21–0.31 ␮m.
Collectively, these attenuation processes reduce the solar irradiance
at the surface by only about 3% on a global, annual
basis. Nevertheless, ozone’s absorption of ultraviolet radiation
to a large extent maintains the temperature structure of
the stratosphere. As a result, solar radiation at the Earth’s surface
lacks UV-C and is deficient in UV-B, possessing little
radiant energy at wavelengths shorter than 0.31 ␮m.
Large percentage fluctuations have been detected in the
monochromatic irradiance received outside the atmosphere at
ultraviolet wavelengths, primarily associated with solar activity.
Because the magnitude of these fluctuations is greatest at
the extreme short end of the solar ultraviolet, and decreases
with increasing wavelength, the effect on the spectrally integrated
solar irradiance is small (probably less than 0.1%).
However, changes in stratospheric warming resulting from
irradiance fluctuations in the far ultraviolet have been linked to
dynamic processes occurring in the troposphere by some
authors, and speculation has ensued that longer-term variations
in ultraviolet emissions by the sun may be responsible for climate
changes.
Concern has been expressed about the role of stratospheric
ozone depletion, brought about by the introduction of chlorofluorocarbons
and other byproducts of human activity into the
atmosphere, in increasing UV-B irradiances at ground level.
Ultraviolet radiation, particularly at wavelengths shorter than
0.3␮m, has strong actinic effects, and excessive exposure to
such radiation has been linked to sunburn, skin cancer, and eye
disease in humans. Deleterious effects on terrestrial and marine
ecosystems also have been suggested, as well as the potential
for adverse effects on air quality. Nevertheless, this radiation
possesses beneficial attributes such as forming vitamin D in the
skin and devitalizing bacteria and similar microorganisms of
disease.
A. John Arnfield
Bibliography
Kemp, D.D., 1994. Global Environmental Issues: A Climatological
Approach. New York: Routledge.
Liou, K.N., 2002. An Introduction to Atmospheric Radiation. New York:
Academic Press.
Paltridge, G.W., and Platt, C.M.R., 1976. Radiative Processes in
Meteorology and Climatology. New York: Elsevier.
United Nations Environment Programme, 2003. Environmental Effects of
Ozone Depletion and its Interactions with Climate Change: 2002
Assessment. Nairobi: UNEP.
Cross-references
Energy Budget Climatology
Electromagnetic Radiation
Ozone
Seasonal Affective Disorder
Sunspots
U
UNITS AND CONVERSIONS 763
UNITS AND CONVERSIONS
The strength and utility of a measurement system depends on
the existence of a consistent set of units and standards around
which a complete system can develop. These fundamental
standards of weights and measures are maintained on a federal
level in the United States by the National Bureau of Standards.
On a worldwide level the International Bureau of Weights and
Measures in Paris, which was established by the Treaty of
Meter in 1875, sets the standard.
The Treaty of Meter also provided for an International
Conference of Weights and Measures to meet at regular intervals.
In 1957 this conference changed the world standard
of length from the platinum–iridium meter bar to a multiple
(1650763.73) of the wavelength of orange-red light of krypton
86. This new wavelength standard has several advantages over
the metal one. It is indestructible, immutable, a constant of
nature, and can be reproduced anywhere in the world with an
accuracy of one part in one hundred million. In 1960 this same
conference adopted an International System of Units (abbreviated
SI for Système International). The SI is a metric system
based on seven fundamental physical quantities in terms of
which all others are to be defined so as to be consistent with the
generally accepted equations of physics. These quantities and
their units are mass (kilogram), length (meter), time (second),
temperature (Kelvin), current (ampere), and luminous intensity
(candela). The seventh unit, the mole, was adopted in 1971.
The SI supplants the older cgs and mks systems, although they
are still the two most commonly used. In the United States the
customary system still employed by established tradition uses the
foot, pound, and second; these are defined in terms of the SI units
by simple numerical ratios. This customary system, inherited
from Britain, is widely used in industry and commerce in both
the United States and much of the English-speaking world, but it
is not inherently consistent and is little used for strictly scientific
measurements. Certain units of measurement, e.g. the nautical
mile, which is equivalent to one degree of latitude, have such
obvious value that it will be difficult to replace them.
Metric units of measurement
In the metric system of measurement, designations of multiples
and subdivisions of any unit may be arrived at by combining
with the name of the unit the prefixes deka, hecto, and kilo
meaning, respectively, 10, 100, and 1000, and deci, centi, and
milli, meaning, respectively, one-tenth, one-hundredth, and
one-thousandth. In certain cases, particularly in scientific
usage, it becomes convenient to provide for multiples larger
than 1000 and for subdivisions smaller than one-thousandth.
Accordingly, the following prefixes have been introduced and
these are now generally recognized:
yotta (Y), meaning 1024 deci (d), meaning 10Ϫ1
zetta (Z), meaning 1021 centi (c), meaning 10Ϫ2
exa (E), meaning 1018 milli (m), meaning 10Ϫ3
peta (P), meaning 1015 micro (␮), meaning 10Ϫ6
tera (T), meaning 1012 nano (n), meaning 10Ϫ9
giga (G), meaning 109 pico (p), meaning 10Ϫ12
mega (M), meaning 106 femto (f), meaning 10Ϫ15
kilo (k), meaning 103 atto (a), meaning 10Ϫ18
hecto (h), meaning 102 zepto (z), meaning 10Ϫ21
deka (da), meaning 101 yocto (y), meaning 10Ϫ24
These prefixes increasingly are used with data exchanges. The
bit (b) and byte (B) are the standard unit of data quantity, and
Gb, MB and GB are commonly used.
Time
Second: the standard unit of time. It was originally defined as
1/86400 of a mean solar day, but the standard was changed in
1960 by the Eleventh General Conference on Weights and
Measures to be 1/31556925.9747 of the tropical year 1900. The
second is the duration of 9 192 631 770 periods of the radiation
corresponding to the transition between the two hyperfine levels
of the ground state of the cesium 133 atom. The present standard
avoids the variability caused by changes in the Earth’s period of
rotation about its axis.
1 minute ϭ 60 seconds
1 hour ϭ 3600 seconds
1 sidereal month ϭ 27.32167 days
1 synodical month ϭ 29.53059 days
1 tropical (ordinary) year ϭ 31556925.9747 seconds
1 calendar year (common) ϭ 31536000 seconds
Temperature
Temperature: a term used to express the relative intensity of
heat. It is identified with the kinetic energy of translation of
molecules and, in accordance with the kinetic theory of gases,
has an absolute zero where all motion has ceased. It is also an
aspect of matter that governs the ability to transfer heat from or
to other matters. Heat will not transfer between two bodies if
they are at the same temperature. There are three temperature
scales currently employed: the Fahrenheit scale, the Centigrade
(Celsius) scale, and the Kelvin scale. By international agreement,
Celsius is favored over Centigrade, but the latter is still
widely preferred.
Temperature scales – Centigrade (Celsius) to Fahrenheit
Equivalents
1 CЊ ϭ 1.8 FЊ
1 FЊ ϭ 0.5556 CЊ
1 K (Kelvin) ϭ 1 CЊ Ϫ273ЊC ϭ Ϫ459.4ЊF ϭ 0ЊK
Conversion formulas
ЊF ϭ 9/5ЊC + 32
ЊC ϭ 5/9 (ЊF Ϫ32)
Note: ЊF is read “degrees Fahrenheit”
FЊ is read “Fahrenheit degrees”
ЊC is read “degrees Centigrade or Celsius”
CЊ is read “Centigrade or Celsius degrees”
1ЊF per foot ϭ 0.0182269ЊC per centimeter
1ЊC per centimeter ϭ 54.864ЊF per foot
1ft per ЊF ϭ 0.54864m per ЊC
1m per ЊC ϭ 1.82269ft per ЊF
Mass and weight
Gram: the unit of mass in the metric system that was first
defined to be equal to the mass of a cubic centimeter of pure
UNITS AND CONVERSIONS764
water at the temperature of its maximum density (4ЊC). A platinum
cylinder was made, known as the kilogram, and declared
to be the standard for 1000 grams.
Gram-atomic weight: the atomic weight of an element
expressed in grams, customarily based on a scale on which the
gram-atomic weight of oxygen is 16000 grams, but more precisely,
based on a scale on which the gram-atomic weight of
carbon 12 is 12000. The gram-atomic weight of all elements
contains the same number of atoms. This number, called
Avogadro’s number, is 6.032 ϫ 1023 and is termed a gram atom.
Gram-equivalent weight: the equivalent weight of an element
of compound expressed in grams, customarily based on a scale
on which the equivalent weight of oxygen is 8000 grams. It is
the gram-atomic weight of an element (or formula weight of a
radical) divided by the absolute value of its valence (oxidation
state).
Gram-molecular weight: the molecular weight of an element
or compound expressed in grams. The gram-molecular weight
of all elements or compounds contains the same number of
molecules, 6.032 ϫ 1023 (Avogadro’s number), and is termed
one mole.
Ton: in the United States a ton is a unit of weight equal to
2000 pounds, commonly called a short ton. The long ton, more
widely used in England, is equal to 2240 pounds. The metric
ton is equal to 1000 kilograms.
1 short ton ϭ 0.90718 metric ton ϭ 0.892857 long ton
1 long ton ϭ 1.12 short tons ϭ 1.016047 metric tons
1 metric ton ϭ 0.984207 long ton ϭ 1.10232 short tons
1 pound (avoirdupois) ϭ 7000 grains ϭ 16 ounces
1 ounce (av.) ϭ 437.5 grains
1 ounce (troy) ϭ 480 grains
1 pound (troy) ϭ 5760 grains
1 ounce (apothecaries) ϭ 480 grains
1kg ϭ 103 g ϭ 2.20462lb
1lb ϭ 0.453592kg ϭ 453.592g
Linear measures
Meter: the standard unit of length in the metric system. It was
first defined by a platinum end standard, the meter of the
archives, in Paris. This length, based on a measure of an arc
from Dunkirk, France, to Montzuich, Spain, was to be one tenmillionth
part of the meridional quadrant of the Earth. In 1889
at the first general conference of the International Bureau of
Weights and Measures, the meter was redefined in terms of the
platinum–iridium bar known as the international prototype
meter at the international bureau. This standard meter is now
defined as 1650763.73 times the wavelength in a vacuum of
krypton 86.
The micron is a unit of length equal to 10Ϫ6 meters. It is
commonly employed in the measure of short distances such
as the wavelength of light and is represented by the Greek
letter ␮Å. The Angstrom unit (Å) is also used for wavelengths;
one Å ϭ 10Ϫ10 m.
Mile: the mile most commonly used in English-speaking
countries is the English or statute mile of 5280 feet. This distance
was supposedly designated by 1000 paces of the Roman
Legions stationed in Britain. Navigators use the nautical mile,
which was originally defined as the length of one minute of arc
on a great circle drawn on the surface of a sphere with the same
area as the Earth. This length is 6080.27 feet. However, since
the Earth is an oblate spheroid flattened at the poles, the length
of one minute of arc measured along a meridian varies in
different latitudes. It is shortest near the poles and longest near
the equator with an average length of 6077.015 feet. To resolve
this confusion, an international agreement was made defining
the nautical mile as 1852 meters, the present value of the international
nautical mile. The United States nautical mile, equal to
1.00067387 international nautical miles, and the British nautical
mile, equal to 1.00063931 international nautical miles, are
occasionally used.
Foot: a unit of length used in the British system of units and
employed in English-speaking countries. It was originally
standardized in England in 1870 as one-third the length of a
yard. In 1959 the foot was defined as 0.3048 meter in accordance
with an agreement among the directors of the standards
laboratories of English-speaking countries. As a linear measure
the foot is of great antiquity, originally identified as the length
of a man’s foot.
Yard: the fundamental distance in the English measuring
system is taken as the distance at 62ЊF between two fine lines
on gold plugs in a bronze bar at Westminster, England, known
as the Troughton scale. In 1893 the yard was redefined as
3600/3937 meter.
1cm ϭ 0.39370 in ϭ 0.032808ft
1km ϭ 105 cm ϭ 0.62137 mile
1 fathom ϭ 6ft ϭ 1.8288m
1 nautical mile ϭ 1.85325km
1in ϭ 2.54001cm
1ft ϭ 30480cm
1 statute mile ϭ 1.60935km ϭ 5280ft
1 astronomical unit ϭ 1.496 ϫ 108 km ϭ 92957000 miles
1 light year ϭ 9460 ϫ 1012 km ϭ 5.878 ϫ 1012 miles
1 parsec ϭ 3.085 ϫ 1013 km ϭ 1.917 ϫ 1013 miles
Metric to English units – equivalents of length
1 micron (␮) ϭ 0.001 millimeter (mm) ϭ 0.00004 inch (in)
1mm ϭ 0.1 centimeter (cm) ϭ 0.03937in
1000mm ϭ 100cm ϭ 1 meter (m) ϭ 39.27in ϭ
3.2808 feet (ft)
1m ϭ 0.001 kilometer (km) ϭ 1.0936 yards (yd)
1000m ϭ 1km ϭ 0.62137 mile
1in ϭ 2.54cm
12in ϭ 1ft ϭ 0.3048m
Square measures
1 squareft ϭ 0.00002295684 acre
1 acre ϭ 43560ft2 ϭ 0.0015625 mile2
1 square yard ϭ 0.836127m2
1 hectare ϭ 2.471054 acres
1 square mile (statute) ϭ 640 acres
1 square cm ϭ 0.1550 square in ϭ 0.0010764 square ft
1 square km ϭ 1010 square cm ϭ 0.3861 square mile
1 square in ϭ 6452 square cm
1 square ft ϭ 929.0 square cm
1 square mile ϭ 2.5900 square km
1mm2 ϭ 0.00155 in2 1in2 ϭ 6.452cm2
1m2 ϭ 10.764ft2 1ft2 ϭ 0.09290m2
1km2 ϭ 0.3861 mile2 1 mile2 ϭ 2.5900km2
Cubic measures
1gal (UK) ϭ 4.5461 liters ϭ 1.200956gal (US)
1 liter ϭ 0.219969gal (UK) ϭ 0.264173gal (US)
UNITS AND CONVERSIONS 765
1gal (US) ϭ 3.7854 liters ϭ 0.832670gal (UK)
1cc ϭ 0.0610cuin ϭ 0.000035314cuft
1cuin ϭ 16.387cc
1cuft ϭ 28317cc
1mm3 ϭ 0.000061in3 1in3 ϭ 16.387cm3 (cc)
1cm3 (cc) ϭ 0.0610in3 1ft3 ϭ 0.028317m3
1m3 ϭ 35.315ft3 1mile3 ϭ 4.1681km3
1km3 ϭ 0.239911 mile3
Velocity
1 knot ϭ 51.4cm/s
1ft/s ϭ 30.480cm/s ϭ 1.0973km/h
1mi/h ϭ 1.6093km/h ϭ 44.704cm/s
1cm/s ϭ 3.728 ϫ 10Ϫ4 mile/min ϭ 0.02237mile/h
1km/h ϭ 27.7778cm/s
1Њ c per day ϭ 1.2863m/s ϭ 2.8774mile/h ϭ 2.4987 knots
Force
As units of force, frequency gram weight, kilogram weight,
pound weight, etc., are used. We denote them here by g*, kg*,
and lb*.
1 dyne ϭ 0.0010197g* ϭ 2.2481 ϫ 10Ϫ6 lb* ϭ 1g/s2
1 megadyne ϭ 106 dynes
1g* ϭ 980.665 dynes
1kg* ϭ 980.665 ϫ 103 dynes
1lb* ϭ 4.4482 ϫ 105 dynes
Pressure
The SI unit of pressure is the pascal, symbol Pa, the special
name given to a pressure of one newton per square metre
(N/m2). The relationships between the pascal and some other
pressure units are shown in Table U1, but note that not all are,
or can be, expressed exactly.
Following the Eighth Congress of the World Meteorological
Organization in 1986, the term hectopascal (hPa) is preferred to
the numerically identical millibar (mb) for meteorological purposes.
This choice was made, despite the fact that hecto
(ϫ 100) is not a preferred multiple in the SI system.
The so-called “manometic” pressure unit definitions such as
millimeters of mercury and inches of mercury depend on an
assumed liquid density and acceleration due to gravity, assumptions
which inherently limit knowledge of their relationship
with the pascal. In order to encourage the demise of non-SI
units, whose definitions are becoming inadequate for the most
precise measurement of pressure, there is international effort to
exclude them from conversion tables or, in the meantime,
restrict the precision of newly published conversion factors.
The torr is defined as exactly 101325/760 Pa – the “760”
coming from the original and arbitrary definition of standard
atmosphere. Its value differs from the conventional millimeter
of mercury by about 1 part in 7 million.
Other pressure units and conversions include the following.
1 barye ϭ 1 dyne/cm2 ϭ 9.8692 ϫ 10Ϫ7 atmosphere ϭ 10Ϫ6
bar ϭ 1.4504 ϫ 10Ϫ5 lb/sqin
1 bar ϭ 106 baryes ϭ 106 dynes/cm2 ϭ 0.98692 atmosphere ϭ
14.504lb/sq in ϭ pressure of 750.06mm of Hg
1 millibar ϭ 10Ϫ3 ϭ 103 dynes/cm2 ϭ pressure of 0.75006
mmHg
1 megabar ϭ 106 bars
1kg/cm2 ϭ 0.980665 ϫ 106 dynes/cm2 ϭ 14.233lb/sq in
1lb/sqft ϭ 4.7254 ϫ 10Ϫ4 atmosphere ϭ 478.80 dynes/cm2 ϭ
47.880 newtons/m2
1lb/sqin ϭ 0.068046 atmosphere ϭ 0.068947 bar ϭ 6.8947 ϫ
104 dynes/cm2 ϭ 6894.8 newtons/m2
1 atmosphere ϭ 1.0332kg/cm2 ϭ 1.01325 bars ϭ 14.7lb/sqin
ϭ pressure of 760mmHg at 0ЊC and g ϭ 980.665cm/s2
Energy–work
Foot candle: the unit of illumination in general use in the
United States. This is defined as the illumination on an area,
one foot square, on which there is a luminous flux of one lumen
uniformly distributed. It may also be defined as the illumination
on a surface of which all points are at a distance of one foot
from a uniform point source of one candle.
Foot-lambert: a unit of luminance defined as the uniform
luminance of a perfectly diffusing surface either emitting or
reflecting light at the rate of one lumen per square foot. It may
also be defined as 1/n candle per square foot and is sometimes
referred to as the apparent foot-candle.
Foot-pound: a unit in the English gravitational system; it may
be defined in two ways: (1) a unit of energy equal to the work
done when one pound of force displaces a point to which the
force is applied one foot in the direction of the applied force;
(2) a unit of torque equal to the time-rate of change of angular
momentum produced by a pound of force acting at a perpendicular
distance of one foot from the axis of rotation.
Foot-poundal: a unit in the English absolute system; it may
be defined either as a unit of energy, equal to the work done by
a force of magnitude one poundal when the point at which the
force is applied is displaced one foot in the direction of the
force, or as a unit of torque equal to the time rate of change of
angular momentum produced by a force of magnitude one
poundal acting at a perpendicular distance of one foot from the
axis of rotation.
Calorie: used in the cgs system, this unit represents the
quantity of heat required to raise 1 gram of water through 1ЊC
(at 15ЊC).
Kilocalorie: equal to 103 calories, this is often cited as a
calorie in metabolic studies. This use has led to considerable
confusion.
Langley: proposed in 1942 as a solar radiation unit. Initially
given as calories per sq cm per minute, the time dimension was
dropped in 1947. Thus 1 Langley ϭ 1cal/cm2 ϭ 697.8 WmϪ2.
The unit was named for S.P. Langley (1834–1906) of the
Smithsonian Institution.
British thermal unit (BTU): the energy required to raise the
temperature of one pound of water through 1ЊF (39Њ to 40ЊF).
1erg ϭ 1 dyne centimeter ϭ 1gcm2/s2
1 joule ϭ 107 ergs ϭ 0.102mkg* ϭ 0.737ft-lb
Table U1 Units of pressure
Unit Symbol No. of pascals
bar bar 1 ϫ 105
millibar mb 100
hectopascal hPa 100
conventional millimeter of mercury mmHg 133.322...
conventional inch of mercury inHg 3 386.39...
torr torr 101325/760
pound-force per square inch lbf/in2 6 894.76...
URBAN CLIMATOLOGY766
1 gram calorie corresponds to 4.19 ϫ 107 ergs
1 watt ϭ 107 ergs per second ϭ 1 joule per second
1 horsepower ϭ 746 watts
British thermal unit (39ЊF) (Btu) ϭ 1060.4 joules (absolute)
British thermal unit (60ЊF) (Btu) ϭ 1054.6 joules (absolute)
Gram-calorie (mean) ϭ 1.5593 ϫ 10Ϫ6 horsepower
Energy flow representation
In attempting to standardize the representation of energy flow,
the World Meteorological Organization (1971) suggests the
following symbols:
Q (Q ϭ K + L ) Downward radiation
K (K ϭ S + D) Global solar radiation
S Vertical component of direct solar
radiation
D Diffuse solar radiation
L (A )(L ϭ A ) Downward atmospheric radiation
Q↑ (Q↑ ϭ K↑ + L↑) Upward radiation
K↑ Reflected solar radiation
R Upward solar radiation reflected by
Earth’s surface alone
L↑ Upward terrestrial radiation
Lq Upward terrestrial radiation – surface
r Reflected atmospheric radiation
A↑ Upward atmospheric radiation
Q Net radiation
K* Net solar radiation
L* Net terrestrial radiation
Given these symbols, components of the Earth’s budget can be
represented by equations. Of particular importance is net
radiation.
Net radiation ϭ Incoming Ϫ Outgoing
alternatively
John E. Oliver
Bibliography
Brown, J.M., 1973. Tables and Conversions for Microclimatology. USDA
Forest Service Technical Report NC-8. Washington, DC: US
Government Printing Office.
Cardarelli, F., 1997. Scientific Unit Conversion. London: Springer-
Verlag.
Carmichael, R.D., and Smith, E.R., 1962. Mathematical Tables and
Formulas. New York: Dover.
Clark, S.E., Jr, (ed.), 1966. Handbook of Physical Constants, rev. edn.
Geological Society of America Memorandum 97. Boulder, Co:
Geological Society of America.
Clason, W.E. (comp.), 1964. Elsevier’s Lexicon of International and
National Units. New York: Elsevier.
Forsythe, W.E. (comp.), 1954. Smithsonian Physical Tables. Washington,
DC: Smithsonian Institution.
Gilbert, J.A., 1967. Units, numbers, constants and symbols. In Fairbridge,
R.W., ed., The Encyclopedia of Atmospheric Sciences and
Astrogeology. New York: Reinhold.
Gray, D.E. (ed.), 1957. American Institute of Physics Handbook. NewYork:
McGraw-Hill.
Hodgman, C.D. (ed.), 1960. Handbook of Chemistry and Physics.
Cleveland, OH: Chemical Rubber Publishing Co.
Jeffreys, H., 1948. The figures of the Earth and of the Moon (3rd paper),
Royal Astonomical Society Monthly Notices, 5: 219–247.
Jerrard, H.G., and McNeill, D.B., 1980. A Dictionary of Scientific Units.
London: Chapman & Hall.
Kayan, C.E. (ed.), 1959. Systems of Units, Publication No. 57. Washington,
DC: American Association for the Advancement of Science.
National Physical Laboratory, United Kingdom. www.npl.co.uk
NIST References on Constants, Units and Uncertainties. http:.physics.
nist.gov
McNish, A.G., 1957. Dimensions, units and standards. Physics Today, 10:
19–25.
United Nations Studies in Methods, 1987. Energy Statistics, Units of
Measure and Conversion Factors. Series F, No. 44. New York: United
Nations.
World Meteorological Organization, 1971. Guide to Meteorological
Instruments and Observing Practices, 4th edn. W.M.O. No. 8TP3.
Geneva: World Meteorological Organization.
URBAN CLIMATOLOGY
Urbanization and climate
The process of urbanization significantly alters natural surface
and atmospheric conditions. Oke, in Thompson and Perry
(1997), suggests that urban atmospheres demonstrate the
strongest evidence we have of the potential for human activities
to change climate. In the twentieth century rapid urbanization
has occurred on a worldwide scale and the majority of the
world’s population lives in cities. Rapid expansion of cities has
produced concurrent alterations in the urban climatic environment
(Landsberg, 1981).
In general there are many apparent anthropogenic impacts on
our atmospheric environment (Changnon, 1983). These range
from microscale (e.g. replacing trees with a parking lot) to
macroscale (e.g. carbon dioxide effects on global climate by
fossil fuel combustion and emissions). This discussion focuses
on the causes of alterations in climate in urban areas. Processes
are reviewed that are keys to the formation of an urban climate
and the underlying energy, moisture, and air movement patterns
that distinguish urban climates from their surroundings.
An extensive literature addresses the specific problem of air
pollution on regional and global scales as it relates to general
processes of urbanization. The field of urban climatology has
developed to the level of theory and detailed investigations that
we see today in part due to early interests in air quality in cities.
This discussion, however, does not focus on air pollution
effects per se. Urban climate can be understood, to a large
degree, by the study of modifications which develop primarily
through the effects of land-use changes and feedbacks into the
energy, moisture, and local air motion systems. Air quality
certainly plays a role. The following discussion is limited
to micro-, local, and meso-scales. It is also restricted to
processes taking place in what are called the urban canopy
then Q* ϭ 1KT ϩ LT2 Ϫ 1Kc ϩ Lc2
Since QT ϭ KT ϩ LT and QcKc ϩ Lc
Q* ϭ 1KT Ϫ Kc2 ϩ 1LT Ϫ Lc2
Since K* ϭ KT Ϫ Kc and L* ϭ LT Ϫ Lc
or Q* ϭ K* ϩ L
Q ϭ QT Ϫ Qc
URBAN CLIMATOLOGY 767
layer (UCL – beneath roof level) and the urban boundary layer
(UBL – extending from roof level to the height at which urban
influences are absent; Oke, 1998, see Figure U1).
The first real growth of urban climatology dates from the
1920s, followed by rapid increases in interest in urban climates
between the 1930s and 1960s (especially in Germany, Austria,
France, and North America). After World War II, and into the
environmental era of the 1960s and 1970s and beyond, there was
an exponential increase in urban climatic investigations.
Investigations have simultaneously become less descriptive,
more oriented to quantitative and theoretical modeling, and more
integrative and interdisciplinary (e.g. World Meteorological
Organization, 1970; de Dear et al., 2000). There are many
scholarly reviews of the subject and accompanying bibliographies
that illustrate the overall problem of how cities alter their
climatic environment (e.g. Brazel, 1987; Chandler, 1976;
Beryland and Kondratyev, 1972; Landsberg, 1981; Lee, 1984;
Oke, 1974, 1979, 1979, 1980; and in Bonan, 2002). Today,
urban climatology has achieved its own recognition as a
subdiscipline in climatology and among allied disciplines such
Figure U1 The idealized vertical structure of urban-modified air. (a) The whole city (mesoscale) in near-calm conditions with its “dome”, and
(b) in a steady regional air flow with its urban “plume”. (c) A single urban terrain zone (local scale) showing the internal layering of the urban
canopy (UCL) and lower portion of the urban boundary (UBL) layers. (d) A single street canyon (microscale) and building elements
(after Oke, 1998).
URBAN CLIMATOLOGY768
as planning, ecology, environmental science, and meteorology
(e.g. as evidenced in de Dear et al., 2000).
Factors controlling urban climates
Many aspects of urbanization change the physical environment
and lead to alterations in energy exchanges, thermal conditions,
moisture fluxes (evaporation, precipitation, and runoff) and
wind circulation systems. These include: (1) air pollution,
(2) anthropogenic heat, (3) surface waterproofing, (4) thermal
properties of the surface materials, and (5) surface geometry
(Oke, 1981). Other factors that must be considered relate to the
setting of the city, such as relief, proximity to water bodies, size
of the city, population density, and land-use distributions. Oke
(1997) provides a summary of the typical resulting alterations
of climatic elements in cities compared to rural environs (see
Table U2).
The magnitude–frequency concept is important but less studied
in urban climatology. How large are climatic alterations in a
city and how often are they that large? The latter part of the question
demands analysis of the linkages of microscale–mesoscale
alteration magnitudes with more macroscale conditions, such as
the role of synoptic climatology on urban climate variations (e.g.
Unwin, 1980). Typically, cloudy and/or windy days reduce heat
island magnitudes for a city.
Urbanization causes changes in the energy, moisture, and
momentum systems, but few studies demonstrate pre–post
urban climate conditions (Lowry, 1977). One excellent example
is the experiment conducted to specifically illustrate how a city,
as it developed from a rural environment, affected the local and
regional climate (Landsberg, 1981). Most urban climate studies
rely on geographic comparisons between the city and its
surroundings in order to produce an estimate of the urban effect
on local climate. Considerable attention has been given to the
study of historical weather records in cities to evaluate temperature
trends that are more urban in origin versus trends more
attributable to global signals of change (e.g. global warming).
A large number of stations with long records are needed for
trend analyses, and many of the world’s weather stations are in
or near urban-affected locales. Thus, it is often difficult to decipher
global change from such sites (e.g. Hansen, et al., 1999,
2001). Furthermore, placing typical weather stations in urban
areas to study how cities alter climate is a most challenging
endeavor in and of itself (Oke, 1999). This arises because of the
complex structure of the urban atmosphere, processes operating
at differing scales, and the resultant climate patterns in the city
(see Figure U1, after Oke, 1998).
Methods of evaluating urban climate
Many methods are used to determine how a city affects climate.
Early methodologies included sampling the differences between
urban–rural environments, upwind–downwind portions of the
urban area, urban–regional ratios of various climatic variables,
time trends of differences and ratios, time segment differences
such as weekday versus weekend, and point sampling in mobile
surveys throughout the urban environment (Lowry, 1977). Much
of this sampling led to the discovery of the famous heat-island
phenomenon.
Methodological inadequacies of many of the early studies
have been pointed out in recent decades, and accurate time-series
data remain a problem of analysis (Lowry, 1977; myriad studies
on filtering out urban effects to reveal global trends). These
inadequacies led to increased studies of processes involved,
particularly fluxes of energy, moisture, and momentum in urban
environments. Process studies help to provide a better characterization
of how urbanization alters the surface–atmospheric
system (Oke, 1979). Much attention has been given to internal
variability of climate conditions within the urban environment
and to the importance of the UCL (Arnfield, 1982; Grimmond,
1992; Grimmond and Oke, 1995, 1999; Johnson and Watson,
1984; Oke, 1981).
The urban heat island
Cities, no matter what their size, tend to be warmer than their
surroundings. One can observe this biking through the urban
landscape (Melhuish and Pedder, 1998). This fact was discovered
by scientists well over a century ago (e.g. Howard, 1833),
and is well known as evidenced by its mention in virtually
every book on modern climatology and ecology (e.g.
Thompson and Perry, 1997; Bonan, 2002). Since cities are
regional agglomerations of people, buildings, and urban activities,
they are spots on the broader, more rural surrounding land.
These spots produce a heat-island effect on the spatial temperature
distribution in an area (see Figures U7a and b in the later
section on remote sensing).
The reasons for heat-island formation are many-fold (Table U3
after Oke, 1979). City size, morphology, land-use configuration,
and geographic setting (e.g. relief, elevation, regional
climate) dictate the intensity of the heat island, its geographic
extent, orientation, and its persistence through time. Oke (1981)
illustrates the effect of city size (as indicated by population size)
on the maximum urban heat-island intensity (see Figure U2).
Although population is only a surrogate measure for many of
the causes of a heat island, there is a significant correlation
Table U2 Urban climate effects for a mid-latitude city with about
1 million inhabitants (values for summer unless otherwise noted)
Variable Change Magnitude/comments
Turbulence intensity Greater 10–50%
Wind speed Decreased 5–30% at 10m in strong flow
Increased In weak flow with heat island
Wind direction Altered 1–10 degrees
UV radiation Much less 25–90%
Solar radiation Less 1–25%
Infrared input Greater 5–40%
Visibility Reduced
Evaporation Less About 50%
Convective heat flux Greater About 50%
Heat storage Greater About 200%
Air temperature Warmer 1–3ЊC per 100 years; 1–3ЊC
annual mean up to 12ЊC
hourly mean
Humidity Drier Summer daytime
More moist Summer night, all day winter
Cloud More haze In and downwind of city
More cloud Especially in lee of city
Fog More or less Depends on aerosol and
surroundings
Precipitation
Snow Less Some turns to rain
Total More? To the lee of rather than in city
Thunderstorms More
Tornadoes Less
Source: Adapted after Oke (1997), p. 275.
URBAN CLIMATOLOGY 769
indicated, and population serves as a useful reference for
expected heat-island formation.
In evaluating the heat-island intensity of cities, several
cautionary notes must be emphasized. The method of sampling
and/or the accuracy of historical weather records from available
rural and urban locales must be carefully considered (Oke,
1999). Much of the earlier work in heat-island detection is of
somewhat limited value due to non-specification of probable
pre-urban conditions in an area, and biases in spatial sampling
of the extent of the heat island (Lowry, 1977; Landsberg, 1981).
Numerical adjustments of historical temperature records must
be considered by the urban researcher, since available data may
be biased (e.g. observation time of various stations may be different)
and influences of local changes in station locations on
temperature and precipitation statistics may go undetected (e.g.
Winkler and Skaggs, 1981). The more process-oriented urban
climatic studies in the last two decades are not only providing
measures of explanation of the heat-island phenomenon, but
also are allowing for a better understanding of the efficacy of
the earlier, more descriptive studies of the urban heat island.
Radiation and energy balance of the city
Shortwave radiation
An urban area affects the exchanges of shortwave and longwave
radiation by air pollution and complex changes of surface radiative
characteristics. Figure U3 illustrates radiative fluxes and
processes in a city atmospheric system and the profiles of shortwave
radiative heating and longwave radiative cooling due to the
existence of an urban aerosol layer. The attenuation of incoming
shortwave radiation (fluxes 2 and 4 divided by 1; see Figure U3)
has been analyzed in numerous urban climatic environments. It
is thought that the attenuation in the city is 2–10% more than in
the country. Generally, the ultraviolet portion of the electromagnetic
spectrum (< 0.4␮m), the so-called UV, is depleted by as
much as 50% (e.g. Petersen et al., 1978). However, total depletion
across all solar wavelengths (0.15–4.0␮m) is much smaller,
less than 10% (Petersen and Flowers, 1977; Petersen and Stoffel,
1980). The processes of scattering and absorption, which account
for the attenuation, require further study to predict their relative
contribution to total shortwave radiation depletion.
Table U3 Mechanisms hypothesized to cause the urban heat island
effect
• Urban Boundary Layer
᭺ Anthropogenic heat from roofs and stacks
᭺ Entrainment of air scoured from warmer canopy layer
᭺ Entrainment of heat from overlying stable air by the process of
penetrative convection
᭺ Shortwave radiative flux convergence within polluted air
• Urban Canopy Layer
᭺ Anthropogenic heat from building sides
᭺ Greater shortwave absorption due to canyon geometry
᭺ Decreased net longwave loss due to reduction of sky view factor
by canyon geometry
᭺ Greater daytime heat storage (and nocturnal release) due to
thermal properties of building materials
᭺ Greater sensible heat flux due to decreased evaporation resulting
from removal of vegetation and surface waterproofing
᭺ Convergence of sensible heat due to reduction of wind speed
in the canopy
Source: After Oke (1979).
Figure U2 The maximum urban heat island intensity (⌬TuϪr [max]) versus population for 32 cities in Europe, North America, and Australia
(data from Oke, 1981).
URBAN CLIMATOLOGY770
Albedo
The second major effect by the city is the change in the ratio of
outgoing shortwave radiation (flux 3) to that of incident shortwave
radiation at the complex, three-dimensional city interface
(fluxes 2 and 4). This ratio, the albedo, is typically less in urban
areas than in the surrounding landscape. Lower albedoes are due
to darker surface materials making up the urban landscape mosaic
and also due to effects of trapping of shortwave radiation by the
vertical walls and the urban canyon-like morphology. There is
considerable variation of albedo within the city depending on the
percent vegetative cover, building material variation and roof
composition, and land-use characteristics (see Table U4).
The difference in albedo between a city and its environs
depends also, of course, on the surrounding terrain. A city
surrounded by dark forest may experience very little albedo
difference from the forest cover (both may range from 10% to
20%). In winter a midlatitude to high-latitude city with surrounding
snow cover may display a much lower albedo than its
surroundings. Thus, since cities receive 2–10% less shortwave
radiation than their surroundings, yet have slightly lower albedoes
(by less than 10%), most cities experience very small
overall differences in absorbed shortwave radiation from rural
locations nearby.
Longwave radiation
Longwave radiation is affected by city pollution and the fact
that most urban surface areas are warmer. Warmer surfaces
promote greater thermal emission of energy vertically upward
from the city surface compared to rural areas, particularly at
night (flux 5). Some longwave radiation is reradiated by urban
aerosols back to the surface and also from the warmer urban air
layer (fluxes 7 and 8). Thus, increases in incoming longwave
radiation and outgoing longwave radiation are usually experienced
in urban areas (that is, fluxes 7 and 8 are greater than flux
6 would be at the urban surface).
It is thought that outgoing longwave radiation increases are
slightly greater than the incoming increases in the city, again
especially on clear, calm nights. During daytime there is little
difference between the city and its surroundings. However, surface
emissivity (amount emitted relative to black-body amounts
for a given temperature) can be quite different between country
and city areas, and can account for considerable longwave radiation
variations (Yap, 1975). A major consideration is that in
the city a three-dimensional surface temperature must be characterized
to accurately estimate the flux values of radiation and
the energy budget (e.g. Voogt and Oke, 1997, 1998).
Figure U3 Radiative exchanges in a polluted urban boundary layer, including generalized profiles of shortwave radiative heating (left) and
longwave cooling (right) due to the aerosol layer (shaded). Numbered fluxes are referred to in the text (after Atwater, 1971; Oke, 1982).
Table U4 Albedo values of urban surface materials
Material Albedo (%)
Concrete 27.1
Blacktop/asphalt 10.3
Brick, red 32.0
Brick, yellow/buff 40.0
Brick, white/cream 60.0
Glass 9.0
Paint, dark 27.5
Paint, white 68.7
Roofing shingles 25.0
Snow, weathered 55.0
Stone 31.7
Tar-gravel roof 13.5
Yard (90% lawn, 10% soil) 24.0
Source: Data from Arnfield (1982), p. 104.
URBAN CLIMATOLOGY 771
Net all-wave radiation
Generally, variations in the shortwave and longwave radiation
fluxes between a city and its environs are relatively small, and
the net all-wave radiation (the balance between incoming and
outgoing shortwave and longwave radiation) may actually be
less in urban areas (e.g. in St Louis there is 4% less net radiation
in the city; White et al., 1978). The exact differences
depend on land use, city building density and arrangement,
aerosol composition of the urban atmosphere and climate of the
city surroundings, as well as other factors such as artificial heat
emissions and the topography of the region. In the UCL, what
is called the urban sky view factor (USVF) is decreased
considerably below a totally unobstructed horizon. This latter
condition is more typical of the countryside. An obstructed
horizon consisting of structures that make up the UCL reduces
radiative loss and can account for excess heat in the city (Oke,
1981). The emission of heat from the UCL structures has been
hypothesized as explaining a city’s heat-island intensity.
Figure U4 shows a correlation of the UCL sky view factor with
the maximum heat island intensity for 31 cities plotted in
Figure U4. The USVF is specifically defined as:
where: H ϭ height of UCL
W ϭ width of the UCL.
Note the tightening of the correlation, indicating that for a
given population, a European city has a more open geometry,
whereas most US cities have deeper canyons in their core areas.
The individual correlations of European–Australian and North
American population versus maximum heat-island intensities
now merge into one generalized correlation shown in Figure U4.
Urban climatologists are increasingly using photographic and
GIS methods to derive USVF variability within cities.
Figures U5a and b illustrate results for downtown Salt Lake City
for USVF and building heights (Brown and Grimmond, 2001).
Total energy balance
Not many long-term studies of a city’s entire energy balance
have been made (Oke, 1979). One specific reason relates to
important paradigms in climatology, shifting only in the last
five decades to considering physical principles of the energy,
moisture and mass exchanges at the Earth–atmosphere interface
(Miller, 1965). However, over the last decade, many cities
worldwide have been studied through the use of tall towers with
energy flux sensors, GIS, remote sensing and modeling procedures
(e.g. Grimmond, 1992; Grimmond and Oke, 1995, 1999).
The city energy budget or balance can be simplified in
Figure U6a to:
where: Q* ϭ net all-wave radiation
ϭ K* ϩ L* (net shortwave and longwave radiation)
Qf ϭ anthropogenic heat emission (Qfv ϩ Qfh ϩ Qfm)
Qe ϭ latent heat flux
Qh ϭ sensible heat flux
⌬Qs ϭ net heat storage in the city
⌬Qa ϭ net advection into or out of the city.
Since Q* is not that different between a city and its surroundings,
the observed heating in cities is likely accounted for by
Qf, (Qe, Qh), and ⌬Qs. Usually, ⌬Qa is negligible. The contribution
of Qf to the total energy balance is highly city-dependent
and seasonally variable. Qf ranges from nil to 300% of Q*,
depending on the degree of industrialization (Qf is high in more
industrialized cities), latitude (Qf is high in higher-latitude
cities), and season (Qf is higher in winter). Qf is composed of
heat produced by combustion of vehicle fuels (Qfv), stationary
source releases such as within buildings (Qfh), and heat
released by metabolism (Qfm). Qfv is a function of type and
amount of gasoline used, number of vehicles, distance traveled,
and fuel efficiency. Qfh requires an analysis of consumer usage
of fuel such as gas and electricity. Qfm can be evaluated by
active and sleep rates. For people metabolic rates have been
given (Oke, 1979). Bach (1970) suggests rates for animals.
Sensible, latent, and storage heat
The repartitioning of energy in urban areas among sensible,
latent, and storage heat processes primarily depends on the
mosaic of land uses in the city as compared to rural areas.
Generally, the drier urban building and road materials induce
higher Qh, less Qe, and higher ⌬Qs in urban areas. A term
labeled Qg is specified in the energy budget equation often as
part of ⌬Qs and is the soil heat flux. Significant Qe does occur
in cities. This is theorized to be due to urban irrigation effects
and vegetation in the city (Kalanda et al., 1980). Marotz and
Coiner (1973) indicate that vegetation in urban areas is not as
limited as supposed, and Oke (1978) shows that ⌬Qs/Q* ratios
for rural areas vary by only about 0.10 from those for suburban
and urban areas.
Q* ϩ Qf ϭ Qe ϩ Qh ϩ ¢Qs ϩ ¢Qa
u ϭ tanϪ1
1H>0.5W2
UWVF ϭ 0.51sin2
u ϩ cos u Ϫ 12 1cos u2Ϫ1
USVF ϭ 11 Ϫ 2UWVF2
Figure U4 The maximum urban heat-island intensity (⌬TuϪr [max])
versus urban sky view factor (USVF) for 31 cities. The equation is
⌬TuϪr (max) ϭ Ϫ13.3 USVF ϩ 14.86 (r2 ϭ 0.87, standard error of
the estimate ϭ 0.96ЊC).
URBAN CLIMATOLOGY772
Table U5 shows some recent results for selected US cities
after Grimmond and Oke (1995). Note the substantive fluxes of
Qh and ⌬Qs, also significant Qe, especially for the more moist
city of Chicago. In a detailed evapotranspiration study of nine
places, Grimmond and Oke (1999) show that the ratio of Qe/Q*
ranges from 0.09 (Mexico City with very little external water
use) to Chicago’s ratio of 0.46 (Table U5).
Goward (1981) lists thermal properties of typical interface
materials, noting that urban area materials have similar thermal
properties and that urban thermal inertias are higher than dry
soils but lower than wet soils (see Table U6). Differences in
UCL structure and composition are the keys to explaining heat
excesses in cities, rather than just the thermal properties of city
materials per se. Much research remains on this point, and has
become a focal point in the field of urban climatology
(Arnfield, 1982; Barring et al., 1985; Goldreich, 1985; Goward,
1981; Grimmond and Oke, 1995, 1999; Johnson and Watson,
1984; Oke, 1981, 1982; Terjung and O’Rourke, 1980).
0
20
40
60
80
100
120
0 - 3.5 3.5 - 7 7 - 10.5 10.5 - 17.5 17.5 - 35 35 - 52.5 52.5 - 87.5 87.5 - 140
building height range, m
no.ofbuildings
0
2
4
6
8
10
12
14
16
18
20
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1
sky view factor
count
Figure U5 Upper: histogram of the computed sky view factor for street-level positions in downtown Salt Lake City. Lower: histogram of the
building heights for downtown Salt Lake City (after Brown and Grimmond, 2001).
URBAN CLIMATOLOGY 773
Remote sensing of urban surface temperature
The extreme land cover and spatial heterogeneity of the city
landscape is problematic to the synoptic collection of in-situ
measurements of energy balance flux parameters. Longwave
radiation emitted upward from the urban surface (L*), however,
can be measured as a separate flux across the city landscape
using thermal infrared (TIR) remote sensing. A number of
investigators have provided evidence using TIR remote sensing
data that urban areas are strong daytime longwave emitters
(Rao, 1972; Pease and Nichols, 1976; Auer, 1978, Carlson and
Boland, 1978; White et al., 1978; Roth et al., 1989). Other
researchers have used remote sensing data to observe nighttime
longwave radiation characteristics of cities (Dabberdt and
Davis, 1974; Roth et al., 1989). As indicated by Table U6, however,
the thermal properties of surfaces typical of the urban
landscape are highly variable, and this variability is compounded
even further by other factors, such as inconsistencies
and deterioration caused by weathering or residue formation
from automobiles. Flynn (1980), therefore, suggests that the
problem related to the effects of surface material types and
patterns on urban climatological phenomena, including the urban
heat island effect, is a spatial one. A solution is to measure the
attributes of space at a scale commensurate with that where
the process becomes evident. The object of study then becomes
the urban space that describes the energy budget or climatic
processes within an areal unit. Flynn (1980) notes that approaching
the problem in this way focuses on the pattern analysis and
measurement of a basic urban surface (i.e. surface material type)
and may explain some of the variability in urban temperatures
across the city landscape. Thus, although TIR remote sensing
data obtained from satellites are useful for discerning the general
temperature characteristics of urban surfaces, these data must be
obtained at spatial scales at which energy budgets for discrete
surfaces material types can be identified (e.g. concrete, trees,
rooftops, asphalt) to adequately quantify how thermal energy
emanating from the composite urban landscape forces development
of the urban heat island effect.
In this respect, high spatial resolution (i.e. Ͻ20m) TIR
remote sensing data have been shown to be extremely useful for
measuring urban thermal energy balance characteristics
(Quattrochi and Ridd, 1994, 1998; Lo et al., 1997; Quattrochi
et al., 2000). For example, Figure U7a is a mosaic of individual
Figure U6 Schematic depiction of the fluxes involved in (a) the
energy and (b) the water balance of an urban building air volume
(after Oke, 1978, p. 242).
Table U5 Summary of daytime mean summer energy balance
fluxes (MJ/m2/day) for selected cities derived from tall tower
observations (Grimmond and Oke, 1995).
Location Q* Qh Qe ⌬Qs Qg
Tucson, AZ 16.27 7.54 4.11 4.62 Na
Sacramento, CA 12.65 5.19 3.79 3.67 12.73
Chicago, ILl 17.20 5.58 7.11 4.51 2.65
Los Angeles, CA 16.40 5.74 4.12 6.54 1.37
Table U6 Thermal properties of typical interface materials
Thermal
Thermal Specific admittance
conductivity heat Density JmϪ2 ЊCϪ1
Material WmϪ1 ЊCϪ1 JkgϪ1 ЊCϪ1 ϫ 103 kgmϪ3 ϫ 103 sec Ϫ1/2 ϫ 103
Asphalt 0.7454 0.92 2.114 1.204
Brick 0.6910 0.84 1.970 1.067
Concrete 0.9338 0.67 2.307 1.185
Glass 0.8794 0.67 2.600 1.213
Granite 2.7219 0.67 2.600 2.176
Limestone 0.9338 0.92 1.650 1.182
Sand (dry) 0.3308 0.80 1.515 0.633
Wood 0.2094 1.38 0.500 0.377
Soil (wet) 2.4288 1.48 2.000 2.681
Soil (dry) 0.2513 0.80 1.600 0.567
Water (20ЊC) 0.5988 4.15 0.998 1.579
Air (20ЊC) 0.0251 1.01 1.001 0.056
Source: After Goward (1981), p. 24. Note: Thermal admittance is the square root of the product of the other three quantities.
URBAN CLIMATOLOGY774
flight lines of airborne TIR data acquired at a 10m spatial
resolution during the day in summertime over the Atlanta,
Georgia, USA, central business district (CBD). Given these are
TIR data, surfaces that emit high surface temperatures (i.e.
exhibit a high thermal energy response) are depicted in white to
very light gray tones on Figure U7a. On the other hand, surfaces
that are comparatively low emitters of surface temperatures
(and subsequently, exhibit lower thermal energy responses in
relation to other surfaces) are represented as dark gray to black
in tone in the figure. The A–AЈ line in Figure U7a represents a
transect centered on the Atlanta CBD and extending 5km to the
northwest and southeast of the CBD. There is a mixture of land
covers along the transect that, in turn, is reflected in the fluctuations
of temperatures along the graph given in Figure U7b. It
may be seen in reference to Figures U7a and b that, in general,
the residential land covers that inherently have more vegetation
(e.g. trees, grass) are relatively cool (~20ЊC) as opposed to the
surfaces in the CBD that have considerably less vegetation and
are predominantly composed of pavements, rooftops and other
impervious surfaces, exhibit considerably higher surface temperatures
of approximately 35ЊC. Hence, these high spatial resolution
TIR aircraft remote sensing data can be used to identify
thermal energy responses from discrete urban surfaces to assist
in more accurately measuring longwave energy upwelling from
the city surface as a factor in the overall city landscape regime
and, ultimately, in assessing their effects on urban climatology.
Moisture environment of cities
The processes of urbanization affect the surficial and atmospheric
water budget. The water balance of an urban area is
(Figure U6b):
where: p ϭ precipitation
I ϭ water supply from rivers and reservoirs
F ϭ water released to air by combustion
E ϭ evapotranspiration
⌬r ϭ net runoff
⌬S ϭ moisture storage
⌬A ϭ net moisture advection.
Precipitation
Many studies have shown that precipitation has appeared to
have increased up to 30% in, and/or downwind of, large urban
areas (e.g. Changnon, 1969; Dettwiller and Changnon, 1976;
Atkinson, 1971). Stemming from Changnon’s famous La
Porte, Indiana, anomaly (1968) of increased precipitation
downwind of the Chicago-Gary industrial area, much attention
has been given recently to this subject. Detailed climatic investigations
as part of the Metropolitan Meteorological
Experiment (METROMEX) in St Louis, Missouri, have
yielded estimates of urban effects on precipitation (Changnon
et al., 1977; Ackerman et al., 1978; Changnon, 1981). Results
indicate that summer is the time of maximum urban effect on
precipitation (Figure U8) due to the dominance of convection.
Analysis of rain cells, radar echo counts, detailed raingauge
measurements, and temperature and wind field studies in and
around St Louis have led to causal explanations of rainfall
excesses that include: (1) pollution; (2) modification of the
thermal structure of the UBL; and (3) deepening of the mixing
layer and stronger vertical entrainment of heat and moisture in
portions of the metropolitan area. A convergence zone over the
city is set up by increased sensible heating. The city is aerodynamically
rougher, has more pollution (condensation
nuclei sources), causes more thermal and mechanical turbulence,
and particularly affects convective cloud formation and
precipitation. However, topographic settings of cities must be
clearly understood relative to induced differences in precipitation
receipt in the urban region, before causal explanations
such as the above can be applied to other cities in different
environments.
Sheppard et al. (2002) use the Tropical Rainfall Measuring
Mission (TRMM) satellite’s precipitation radar (PR) data to
identify warm-season rainfall patterns in and around six
selected cities in the southeast United States (Atlanta, Georgia;
Montgomery, Alabama; Nashville, Tennessee; San Antonio,
Waco, and Dallas, Texas). The results substantiated earlier
p ϩ I ϩ F ϭ E ϩ ¢r ϩ ¢S ϩ ¢A
Figure U7 (a) Mosaic of individual flight lines of daytime airborne
thermal infrared remote sensing data obtained over the Atlanta,
Georgia, USA, central business district in summer. The A–AЈ line
represents a transect across the area to illustrate the variability in
surface temperature energy responses across the Atlanta urban
landscape (refer to b). (b) Graph of the mean surface temperature
(solid line) across the Atlanta, Georgia, USA, central business district
area as identified from daytime high-resolution TIR aircraft remote
sensing data. Reproduced with permission, the American Society for
Photogrammetry and Remote Sensing. Quattrochi, D.A., et al.
“A Decision Support Information System for Urban Landscape
Management Using Thermal Infrared Data.” Photogrammetric
Engineering and Remote Sensing (PE&RS). October 2000: 1195–1207.
URBAN CLIMATOLOGY 775
METROMEX findings of downwind increases in the rainfall
of about 28% on average some 30–60 km downwind of the
cities studied, and highlight the usefulness of satellite data to
determine urban effects. Precipitation increases due to human
influences may show cyclic behavior due to pollution effects,
even at the weekly time scale, as suggested in a study of the
regional pollution and precipitation enhancement toward end
of workweeks on the Atlantic seaboard of the US (Cerveny and
Balling, 1999). Table U7 illustrates results from several cities
focusing on the fluxes of Qe relative to Q* and the expression
of water loss in millimeters per day, after Grimmond and Oke
(1999).
Runoff
Since I and F are extra sources in cities, there is a net gain of
moisture by urbanization. Water loss through E would be less
in a city due to a waterproofing effect (Oke, 1980). The ⌬S term
would correspondingly be less. Assuming ⌬A to be negligible,
⌬r would thus increase in cities. Indeed, many studies have
indicated increased r in city environments, as well as changed
hydrographic characteristics (e.g. Mather, 1978). There are four
effects of land-use changes on the hydrology of the urban
area: (1) peak flow is increased; (2) total runoff is increased;
(3) water quality is lowered; and (4) hydrologic amenities –
appearance of river channel and aesthetic impressions – are
lowered (Mather, 1978).
Figure U9 illustrates two factors that control discharge characteristics
before and after urbanization, shown here as a ratio
related to percent impervious area and percent area served by
storm sewers. These two factors are controls on the discharge
of an area. Note that urban–preurban ratios of runoff increase
as percent impervious surface area and percent storm sewers
increase. Geographic information system processing of data is
now used in hydrological studies, merged with simulation programs
for stream flow analysis in urban areas (e.g. Brun and
Band, 2000). In their work, relationships among runoff, base
flow, impervious cover, and percent soil saturation for upper
Gwynns Falls watershed, Baltimore, Maryland, were studied. It
was possible to determine through modeling procedures the
threshold percent impervious cover that would significantly
alter runoff in the urbanizing watershed. In this case, current
percent impervious surface cover is approximately 18%,
whereas modeling suggests 20% is a threshold beyond which
runoff alterations by land cover would significantly occur.
Humidity
Since E and ⌬S are less, dewpoint is usually less in daytime in
cities (e.g. Sisterson and Dirks, 1978; Brazel and Balling,
1986).At night a humidity island may result (e.g. Chandler, 1967;
Kopec, 1973), due to extra E at night and contributions of F.
Since recent studies have not shown Qe and E to be that
Table U7 Daily mean fluxes of Qe and water loss (E) for selected cities and conditions (after Grimmond
and Oke, 1999)
E P P (mean
Locationa Q* Qe Qe/Q* (mm.day) (mm/month) monthly) Practicesb
C95 14.89 6.80 0.46 2.76 80.5 92.0 1
A93 13.74 4.93 0.36 2.00 0.0 0.0 2
T90 12.50 4.90 0.39 1.99 16.3 5.4 3
A94 15.58 4.70 0.30 1.94 0.0 0.0 4
Mi95 13.74 4.58 0.33 1.87 79.8 180.9 5
S91 9.72 4.38 0.45 1.77 0.0 1.8 6
Vs92 8.88 2.68 0.30 1.10 23.2 41.1 7
Sg94 12.45 3.46 0.28 1.40 0.0 0.25 8
Vl92 11.41 1.48 0.13 0.60 23.2 41.1 9
Me93 3.38 0.31 0.09 0.14 0.0 0.0 10
a In order, Chicago, ILl June/Aug 1995; Arcadia, CA July/Aug 1993; Tucson, AZ , June 1990; Arcadia, CA July 1994; Miami,
FL, May/June 1995; Sacramento, CA, Aug 1991; Vancouver, BC, July/Sept, 1992; San Gabriel, CA, July, 1994; Vancouver, BC,
Aug, 1992; Mexico City, D.F., Dec 1993.
b 1 ϭ extensive irrigation; 2 ϭ extensive irrigation (see A94); 3 ϭ xeric landscaping; low water use vegetation, greenspace irrigated
automatically at night/early a.m.; 4 ϭ As A93. irrigation ~1.37mm/day; 5 ϭ frequent rainfall, also irrigation; 6 ϭ irrigation
on alternate days; 7 ϭ irrigation ban Ϫ external use ~0.25mm/day; 8 ϭ As A93 but more hand watering, irrigation ~1.32
mm/day; 9 ϭ irrigation ban in Vs92; 10 ϭ some street washing, very little external use.
Figure U8 Average rural–urban ratios of summer rainfall in the
St Louis area in the period 1949–1968. The rural–urban ratio is the
ratio of the precipitation recorded at any station to that at two
stations near the urban center (after Oke, 1978, p. 266).
URBAN CLIMATOLOGY776
different between city and surroundings, evaluations of F are
important in the understanding of the humidity contrasts. Its
absence may mean a negative humidity island in the city
(drier); its presence a positive humidity island (wetter). Fog differences
are not great between cities and rural areas although,
due to pollution, cities have more haze, smog, and perhaps
more slight fog. More cloud condensation nuclei and ice nuclei
have been observed in cities, but knowledge is very unsatisfactory
at this point about the effects on urban precipitation
(Landsberg, 1981). Holmer and Eliasson (1999) report vapor
pressure results between urban and rural areas for nine locations
from previous studies plus their own study in Goteborg,
Sweden. They identified an urban moisture excess, particularly
at night, that also was related to the urban heat island and longwave
radiative fluxes.
Air movement in cities
Urban areas affect air flow, both speed and direction, on microand
meso-scales. The aerodynamic roughness of urban areas
(level above the ground where wind speed becomes greater
than zero on average) is larger by some 0.5–4.0m than in rural
areas, and the city therefore produces more frictional drag. A
heat-island effect in cities influences the pressure field and also
vertical stability of the air. This will affect local airflow. These
effects also depend on overall gradient wind strength across the
urban region. Many studies have shown that, as wind increases,
the magnitude of the urban heat island diminishes.
Wind speeds
When regional air flow is strong, the aerodynamic roughness of
the city is dominant and is important in inducing increases in
mechanical turbulence on the order of 30–50% (Oke, 1980).
Under light winds, air pressure differences and atmospheric
stability effects, in addition to roughness, are critical in
determining city wind speeds from place to place. The built
environment affects the flow regimes of air through and over
the city (Hunter et al., 1990). Generally, the length/height ratio
and height/width ratio of buildings control the flow regime
among isolated roughness flow, wake interference flow, and
skimming flow (viewed in Figures U10a and b). Generally,
under light regional flow, winds tend to accelerate in cities compared
to rural areas because of strong rural atmospheric stability
and the increased turbulence over the warmer, rougher city
surface. Under strong flow, winds generally decelerate in the
urban area due to greater frictional effects of the city on airflow.
Studies have shown that there appears to be a critical regional
flow speed separating the accelerating from decelerating wind
response in the city (Chandler, 1965; Bornstein and Johnson,
1977; Lee, 1979). This speed, however, is quite variable and
ranges from 0.8 to 5.6m/s.
Wind direction
Wind direction differences in cities and in rural areas relate to
frictional retardation effects. Decelerating winds will promote
cyclonic turning toward lower pressure; accelerating winds produce
anticyclonic turning (Ackerman, 1974; Angell and
Bernstein, 1975; Lee, 1973). Wind direction changes induced
by a city also vary diurnally in relation to the depth of the
mixing layer. During the day the turning effect lessens, whereas
at night it reaches a maximum.
Figure U9 Two factors that cause increased discharge in the urban
area as related to the ratio of discharge after urbanization to that
before urbanization (after Mather, 1978).
Figure U10 Threshold H/W ratios for the transition flow between
flow regimes (a) skimming → wake interference; (b) wake interference
→ isolated roughness (from Hunter et al., 1992, used by permission
of Pergamon Press Ltd).
URBAN CLIMATOLOGY 777
Urban circulations, much like the classic sea breeze, have
been documented in addition to being theorized from laws of
thermodynamics (Bach, 1970; Hjelmfelt, 1982; Shreffler, 1978;
Vukovich and King, 1980). Landsberg (1981) illustrates the
summer air circulation for Washington, DC (Figure U11). Note
the convergence locally in the city boundary. Other air flow
systems, such as synoptic fronts and sea breezes, also can be
affected by cities. Manhattan, New York, for example, slows
cold frontal passages upwind of the city and speeds them up
downwind, especially when a strong heat island prevails (Loose
and Bornstein, 1977). Wind conditions at street level and in the
UCL are very complex and relate to physical barriers, building
sizes, orientations, building densities, and general land-use
patterns. As a generalization, winds are slowed in the UCL.
However, effects of wind channeling and funneling can occur,
depending on prevailing winds and the overall orientation and
morphology of the city landscape.
Conclusions
The climate of cities will continue to be of importance to the
ever-expanding urban population of the world. The application
of research findings of urban climatology in building designs
and urban environmental planning is beginning to emerge but is
not yet widespread (Bonan, 2002). Due to the complexity of the
urban landscape and the variability of dimensions, land use,
morphology, and other characteristics, much research still
remains on just how a city affects the surface and atmospheric
climatic environment, and the city’s overall urban ecology.
Equally, if not more important, are the interactions of the city
climate system with other elements of the entire urban ecosystem
(Douglas, 1981; Bonan, 2002). The discovery of these
interrelationships will eventually aid in planning solutions
related to pollution, health, comfort, water supplies, and general
quality of life among urban dwellers.
Anthony J. Brazel and Dale Quatrocchi
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Cross-references
Air Pollution Climatology
Bioclimatology
Energy Budget Climatology
Local Climatology
Microclimatology
Water Budget Analysis