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VII. Spatio-temporal variability of other
meteorological elements in urban areas
URBAN CLIMATOLOGY
7.1 Urban
climate effects
7.2 Humidity in urban areas
• Spatial and temporal variability of air humidity in urban areas
is the result of evapotranspiration, condensation and
advection processes.
There are several positive and negative feedbacks.
• Higher temperature -> higher intensity of evapotranspiration
(that is however low due to lack of vegetation
• No consumption of latent heat -> rising temperature
• Fast runoff -> les intensity of evaporation
• Urban dry island
Humidity in urban areas
• Atmospheric humidity is generally lower in cities during daytime
(due to lower evapotranspiration compared to rural areas – there
is smaller fraction of vegetation cover)
• At night and in winter there is an urban moisture excess
(UME) in mid- and high latitude cities. The reason is: i) additional
water vapor from anthropogenic activities; ii) weak
evapotranspiration in unstable atmosphere
• In some situations (dry spells, arid climate) humidity can be
higher in cities due to extensive irrigation compared to
neighborhoods
• Bad air quality mostly cause increase of fog frequency and
intensity
Relative humidity in Brno
Annual variations in daily means of relative humidity at three stations in Brno area
in the period 1987–2010
Variability during a year
Mean, min a max relative humidity in Brno area in individual seasons during days
with radiation driven weather for urban stations (red) and rural stations (blue)
Relative humidity in Brno
Seasonal differences
dryer city
spring summer
autumn winter
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Variability during a day
Mean daily variations of
relative humidity at urban
(M) and rural (P) stations in
Brno area (a) and their
differences P – M (b) in
individual seasons
Relative humidity in Brno
spring summer
autumn
winter
dryer city
7.3 Wind field in urban environment
Wind velocity vertical profile in different environment
(http://www.wind-power-program.com/windestimates.htm)
According to Landsberg (1981) in urban areas there is:
• mean annual wind velocity 20–30% lower
• maximum wind spead 10–20% lower
• frequency of calms 5–20% higher compared to rural areas
Wind field in urban environment
Wind filed in urban environment is modified due to mechanical
(left figure) and thermal (right figure) effects
Wind field modification in
urban environment in vertical
(above) and horizontal
direction (below)
Local circulation resulting from
temperature differences between
rural and urban areas (modified
after Munn 1968)
warmercooler
Wind velocity in Brno
Daily course of mean wind speed at urban (blue) and rural (red) stations in the
2005–2011 period
Variability of mean annual wind speed at rural (blue, 1961–2011) and urban (red,
1987–2011) stations
Intensity of turbulence
The ratio between maximum daily wind speed and mean daily wind
speed may be used as a simple measure of intensity of turbulence.
The ratio is clearly higher at the urban station.
Intensity of turbulence
Daily variation of the intensity of turbulence defined as a ratio of maximum wind speed
(Fmax) and mean wind speed (Fprum) at urban and rural stations, period 2005–2011
Daily variation of mean maximum wind speed at Tuřany and Žabovřesky stations in the period
2005–2011
urban
rural
rural
urban
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Modelled mean annual wind velocity
in Brno area and wind roses for
selected points at 10 m above
terrain (model WAsP, data from
Tuřany station, period 1961–1998)
Localities:
1 – Brněnská přehrada
2 – oblast Staré dálnice
3 – Troubsko
4 – ul. Hroznová
5 – ul. Šumavská
6 – údolí Svitavy
7 – Tuřany, letiště.
Wind field modification Wind field modification
Modification of wind speed near obstacles calculated using WAsP model –
an example for for high-rise buildings (60 m) at Šumavská str. a) study
area (black – buildings, green – roughness, red – altitude); b) mean wind
speed near buildings for NW wind direction; c) relative reduction of mean
wind speed near buildings for NW wind direction
7.4 Final remarks and questions
1. How does the vegetation in urban areas influence humidity?
2. What is a typical variability of humidity in urban area
during a day and during a year?
3. What are main factors modifying wind field in urban areas
4. What can be the most important negative effects of wind
field modification in urban areas?