Environmental Engineering Reference
In-Depth Information
Note
: Temperature is expressed as a difference only, not as a percentage.
There appear to be multiple causes of the increases in cloud cover and precipitation.
Added heating of the air crossing the city, increases in pollutants, the frictional and
turbulent effects on air flow and altered moisture all appear to play a role. The confluence
zones induced by these urban effects may lead to the preferential development of clouds
and rain. Which factors become dominant in a particular storm varies according to the
nature of the air circulation over the city on that day. As the effects are less noticeable in
winter than in summer, it follows that it is the natural, not the artificial, heating effects
which are most important, though the way in which the summer atmosphere responds to
the urban surface is also significant.
As the degree of urbanization has increased so an ever greater number of people are
affected by an urban climate. Apart from the more obvious effects of pollution, wind and
warmth, few people may realize that their urban area has changed other aspects of the
climate. The nature of the urban area represents an extreme example of the way in which
human modification can change the climate near the ground.
THE MICROCLIMATE OF SLOPES
So far, all examples quoted have assumed that the ground surface is almost flat. In reality
few areas of the world are so level that the effect of topography can be ignored. The
reason we need to know more about the topography is that slopes modify how much
short-wave radiation reaches the surface. We saw earlier that the maximum intensity of
radiation is received when the angle between the surface and the sun's rays is 90°. If a
horizontal surface is tilted so that it is at right-angles to the sun's rays the amount of
radiation received increases. This factor is exploited by sunbathers, who can tilt the angle
of their reclining seats to achieve maximum heat input. If it were the only factor,
calculating the new input for a slope would be easy. However, while the slope remains
constant, the sun is continuously changing its position in the sky throughout the day and
throughout the year. Slopes, unlike sunbathers, cannot adjust their position. Consequently
a slope that receives maximum intensity at one time on a certain day of the year may be
in shadow at other times.
EFFECTS ON THE RADIATION BALANCE
As the movement of the sun across the sky is known, it is possible to calculate the
intensity of short-wave radiation falling on a slope of any combination of gradient and
orientation (azimuth) for clear skies. More frequently we are interested in the total
radiation rather than the intensity but even this problem has been overcome using
computers. A computer program can be devised to calculate the intensity of radiation on
the surface for any particular time and slope. So, for the start of the program, radiation
intensity is determined for sunrise, depending upon such factors as latitude, time of year,
altitude and atmospheric transmission. Then the computer calculates the sun's position in
the sky, say ten minutes later, works out the new radiation intensity and adds its value to
the previous total. This is continued until sunset or until the sun drops below the horizon
