>> Land Use <<

 



This lecture includes an overview of the various atmospheric impacts of land use.  Concepts such as the urban heat island are discussed throughout this lecture.

 
 

 >> Lecture Outline <<

 


Land Use Changes

>> Primary effect is on how water is cycled
>> Forests retain water

The way land is used has a drastic impact on how water is cycled in our atmosphere.  For example, a forested area of the planet retains water fairly well.  This water retention in these areas leads to increased evaporation and transpiration.  In addition to the increase in evaporation and transpiration, less runoff occurs throughout forested areas and, therefore, more water is put back into the local atmosphere.

Urban Heat Island

>> The inner city is almost always warmer than the surrounding countryside
>> The difference in temperature between the two areas usually increases as the population of the city increases
>> Surface Characteristics

Figure 1: The image above illustrates the concept of the urban heat island. 

 

 

 

The urban heat island effect is due in part to some of the characteristics of the various surfaces found throughout a city.  In general these surfaces have a much higher heat capacity which allows them to absorb and store more solar energy.  In addition to the increased absorption of solar radiation by these materials, the surface area that these materials covered is also much greater.  The large amount of surface area allows for a greater opportunity to absorb reflected solar radiation and it causes infrared radiation to be emitted from a much larger area.  Finally, rooftops and road surfaces are usually dark in color, resulting in additional absorption of solar radiation.  The combination of all these factors leads to the creation of an urban heat island.  Figure 1, found to the right, illustrates the urban heat island effect.



>> Emissions

In addition to the surface characteristics mentioned above, the large increase in urban emissions also plays a part in the creation of an urban heat island.  The abundance of automobiles, factories, and people throughout urban areas allows for massive heat production and release.

>> Water Cycling

Probably the most important factor that helps to create an urban heat island is the way water is cycled, as mentioned above.  Unlike rural areas, the precipitation that occurs in cities isn't readily absorbed by the ground as it is mostly made up of man-made materials.  This allows the precipitation that falls in urban areas to runoff at a rapid pace.  Since little water is absorbed into the ground or remains on the surface throughout the city, less of the incoming solar energy is used to evaporate the water.  Because evaporation is a cooling process, this allows a good majority of the solar energy to work to heat the various surfaces throughout the city.

Enhanced Precipitation

>> This usually occurs only in the largest cities
>> Convection

Precipitation can be enhanced in the largest cities due to the urban heat island effect.  This enhanced precipitation is a direct result of the hotter air in the city being more buoyant than the surrounding rural air.  This causes the air in the city to rise.

>> Convergence

The convective motions that occur as a result of the warmer urban air cause the surrounding rural air to flow into the city.  In addition, once the air reaches the rougher urban surfaces, the incoming winds slow.  This frictional convergence also leads to rising air.

>> Aerosols

The increased pollution found in urban areas also may play a role in the enhanced precipitation found in these areas.  Since there is more pollution in these urban areas, there are many more aerosols found throughout the air.  These aerosols can be easily used as more cloud condensation nuclei once convection has begun.

The concept of enhanced urban precipitation can be seen in Figure 2 below.

Figure 2: The image above shows how the urban heat island effect enhances the precipitation in and around cities.  Notice that the warm air in the city rises, cools and condenses using the aerosols from the increased emissions in the city as cloud condensation nuclei.


 

 

© 2004-2017 by Thomas P. Harrington