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LECTURE 6: MICRO CLIMATES
Unlike regional climates as discussed above, local climates, as the term implies, are very much
controlled by factors within the locality. These may include altitude and relief in the case of
mountain climate, forest vegetation in the case of forest micro-climate, and urban structure and
land surface characteristics in the case of urban micro-climate.
Mountain climates are much less complex. Temperature decreases with height at the
environmental lapse rate. Thus for every kilometre rise the temperature would be about 6 oC
less. Other influences of mountain would include precipitation changes, humidity, and wind
characteristics, but none of these are as dominant as temperature change alone.
The rest of the discussion will deal with forest and urban microclimate. Because very often the
understanding of urban microclimate is best done through contrasting it with its rural
surroundings, which in many cases are occupied by vegetation, forest micro-climate is aften
discussed together urban micro-climate.
Forest Micro-climate
Contrasts in climate are marked between forested and the open ground.
The characteristics of forests are such that within the ecosystem a unique climate is created.
Several characteristics come to mind:
a.
Presence of vegetation, often continuous canopy layer, sometimes layered canopies,
prevents penetration of sunlight to the forest floor. This affects the radiation balance.
b.
The vertical structure of the forest conditions to a great extent the control which it will
exercise over local atmospheric conditions.
c.
Much of the climatic influence of a forest can be explained in terms of the geometry of
the forest, including morphological characteristics, size, coverage and stratification.
Morphological characteristics include the amount of branching, the periodicity of
growth (ie evergreen or deciduous) together with the size, density and texture of the
leaves.
The net effects of the above on climate are:
a.
Lower maximum temperature and higher minimum temperature in forests
b.
Lower temperature range
c.
Rainfall interception
d.
Blocks wind movement
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e.
Slows down evaporation process
f.
Moisture loss from the ground in low
g.
Calm situation in the forest makes the air moist, thus high relative humidity
Urban Micro-climate
The building of cities modify the physical and atmospheric environments. The construction of
buildings, roads, car parks, etc destroys the existing micro-climates and creates new ones. The
trend in the next century is the growth of existing cities and the creation of new ones. The major
climatic changes as a result of urban growth and expansion is summarised in Tables 1 & 2.
Urban heat island
This is a very frequently studied phenomenon of urban climates. The term indicates the higher
temperatures within cities compared with the rural surroundings. Within the urban complex, the
intensity of the heat island is not uniform but varies considerably. Generally, the highest
temperatures coincide with zones where the greatest building intensity and industrialisation
exists.
The magnitude of temperature differences between the city centre and the rural areas depend on
the location of such cities, seasons of the year and whether it is day time or at night. The effects
of higher temperatures within the city means that in addition to the fact that mean temperatures
are higher in the city, these locales also have a greater number of hot days and fewer cold days
than outlying areas. The heat island effect is most pronounced when minimum temperatures are
examined. A case of Philadelphia shows that although mean maximum temperatures are only
0.3 to 0.4 oC higher in the city, minima are from 1.2 to 1.7 oC higher. In Washington, the
suburbs experienced average minimum temperatures that were as much as 3.3 oC lower. On
clear, calm nights, however, the temperature difference between the city centre and the
countryside was considerably greater, often 11 oC or more. On overcast nights, the differentials
may approach 0 oC.
The reasons for this temperature contrast are:
a.
Tall buildings and concrete and asphalt in the city absorb and store greater quantities of
solar radiation than do the vegetation and soil in the rural areas.
b.
Higher impermeability results in greater runoff and severe reduction in evaporation rate.
Excess heat which once was used to convert liquid water to vapour now goes to increase
the surface temperature further.
c.
Waste heat from home heating, air conditioning, power generation, industry,
transportation. Energy generated through human activities can constitute a significant
proportion of the total energy received from the sun. Studies in some European cities
concluded that the amount can be as much as one third of solar energy received. In
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winter, in Manhattan, heat produced by combustion alone was 2 1/2 times greater than
the amount of solar radiation reaching the ground. In summer the figure dropped to 1/6.
d.
Pollutant blanket over cities absorbs terrestrial radiation and re-emitting some to the
surface.
e.
The complex three-dimensional structure of buildings does not readily allow radiation to
escape as readily as outlying areas.
f.
Tall buildings alter the flow of air, reducing wind speeds in the order of 25 % from rural
areas. This decreases the city's ventilation by inhibiting the movement of cooler outside
air. However, if the wind speeds exceed a critical limit, the heat island effect can no
longer be detected.
Other hypothesised causes of urban heat island have been given by other people, but the basic
elements remain the same.
Precipitation
Urban micro-climate does affect precipitation amounts and occurrence. In general the amount
tends to increase due to:
a.
The urban heat island creates thermally induced upward motions that act to increase the
atmosphere's instability.
b.
The addition of condensation nuclei from industrial discharges increases the opportunity
of precipitation formation
c.
The rougher city surface leads to low level convergence and increased upward air
motions. In addition, the rougher the urban landscape creates an obstacle effect that
impedes the progress of weather systems. Therefore when rain-producing processes are
taking place, they may linger over the urban area and increase the city's rainfall.
However, the increase of precipitation is about 10 % greater than over the nearby countryside.
However, in some instances where there is wind component, the greatest effects of increased
rainfall is experienced not in the city centre but downwind of the city.
Other effects
a.
Other effects include the effects of pollutants in blanketing the urban sky leading to a
reduction of solar radiation, as much as 15 %.
b.
Relative humidities are also lower in urban areas because temperatures are higher.
However the frequency of fogs and the amount of cloudiness are greater, due the great
abundance of hygroscopic nuclei.
c.
Another phenomenon is the country breeze. Due to the upward movement of air in the
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city particularly when the heat island is most pronounced, air from the countryside
converges in the urban area.
Discussion
1. What are the ways by which we can ameliorate the impact of urbanisation on climate?
2. Can a greater degree of urbanisation in the world cause global climate change?
3. Discuss the significance of forest types on rainfall interception, relative humidity, wind and
temperature characteristics within such ecosystems.
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