Geog. 310 Urban Climatology
Plants and the Urban Climates
A. Solar radiation
1. Vegetation (trees, shrubs, grass) ameliorate temperatures in the city by controlling
solar radiation.
a. Leaves intercept, reflect, absorb and transmit solar radiation.
b. Plants reduce diurnal, seasonal temperature range through shade, transpiration.
Ex: on a summer day, ground temperature inside forest may be 25o cooler than
tree top canopy, which is the “effective radiative surface”.
c. Deciduous trees screen hot sun of summer, while allowing winter rays pass.
d. Vine-covered wall cooler than bare wall.
e. Plants may completely block sunlight or filter rays.
f. Albedo: light, smooth surface reflects more than coarse, dark surface (plants),
thus less reflection. Leaves reflect and diffuse direct solar radiation.
g. Vegetation also creates dead air space, therefore provide insulation for
buildings and earth.
2. Plants absorb more sun’s energy during the day and release it slowly in evening, so
the daytime temperatures cooler and moderates temperatures.
3. Plants cut glare, making environment pleasant for pedestrian and drivers alike.
B. Wind control
1. Wind may be intercepted, diverted or lessened by obstacles such as buildings, walls,
fences, earth forms or plants.
2. 3 types of windflow:
a. laminar, where windflow layers or streams of air flow one on top of another;
b. turbulent, where windflow is more random in pattern; and
c. separated, where windflow layers vary in momentum (where separated,
turbulence may occur).
3. Change from laminar to turbulent governed by turbulence within airstream and
surface roughness.
a. Building surfaces will always produce turbulence and separation occurs around
sharp corners.
b. Moving air exerts pressure against any surface that tends to inhibit its flow.
c. Flow around streamlined object, wind speeds up and follows contour of shape,
with minimum turbulence; with ‘bluff’ object leads to turbulence, separation.
d. Wind breaks, shelter belts used to reduce velocities, though depends on
penetrability of wind. Moderate penetrable shelterbelt becomes more penetrable in
high winds.
e. Seasonal winds may have different directions, complicating control.
f. Highways can be protected against wind gusts, cross-winds by shelterbelts, snow
fences.
4. Downslope cool winds can be stopped using dense evergreens, which trap and hold
cold winds. Deciduous or ‘loose’ plants create cool spaces on downwind side by
filtering the air. Also provide shade.
5. Ideal ventilation would prevent funneling effects by favoring a country breeze, ie,
winds blowing from relatively cleaner, cooler country, suburban or green area. Can
be done by properly spaced system of green areas cutting through entire builtup
area.
C. Precipitation and plants
1. Interception of rain is a function of rain intensity, canopy charteristics, duration,
tree type.
a. In light rain, conifers retain as much as 5 times quantity intercepted by broad-leaf
trees.
b. Rain penetrates more in hardwood than pine forest.
c. As intensity, duration increases, penetration also increases.
2. Tree leaf drip, trunk flow also provides water to ground with less impact.
3. Transpiration and evaporation. Trees and shrubs tend to cause increased
precipitation above them. Also they tend to prevent evaporation of soil moisture,
thus retaining water in soil. Moisture is also retained longer in forest than open
space.
a. Protection from sun reduced wind both also reduce evaporation.
b. Plants slow down runoff, prevent soil erosion and reduce soil evaporation.
4. Dew collects on crown of trees and not under it, while in open spaces, dew occurs
just above ground level. Fog condenses on needles of conifers and drips to ground.
5. Trees intercept snowflakes, control winds to scour area clean or control snow drift
location, by determining conformation and depth, provide shade for snow retention,
less melt and by causing variation in frost depth to slow down melt.
a. snow fence (wind break) can leave snow drift in desired locations (skiing), rather
than undesireable places (roads).
b. Plants insulate ground so that frost line is deeper, snow stays longer.
D. Air Pollution
1. Air cleaned by photosynthesis, where polluted air diluted by oxygen-rich air.
2. Vegetation absorbs vapors and filters dust. Air is cleaner inside forests, parks.
3. A study of forests and ozone showed that if air contained 150 ppm ozone over
forest for 8 hours, vegetation removes about 80%. Taller trees remove more than
shorter trees. Larger and more numerous leaf stomata openings are more effective
ozone removers.
4. Particulates collected on leaves, stems, branches and later are washed to ground.
Ex: Central Park, NY has about 50% particulate levels of surrounding city.
5. Vegetation is also effective in reducing noise.