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Name: Melanie
Status: student
Grade: n/a
Location: AZ
Country: USA
Date: N/A 


Question:
Can the temperature of hot cities like Phoenix, change the flow of the jet stream, or alter the amount of rain the clouds would normally have for other areas? Because heat causes the clouds to form and rise and cause rain clouds which would indicate a change in the pattern in which the rain would naturally flow over a particular area.


Replies:
Urban areas with their cover of concrete and/or asphalt are believed to cause local changes in cloud cover, relative humidity, and low level air temperature and pressure. That the effect is so large that it would affect the jet stream is a unlikely extrapolation. There are just too many other things also happening. In addition, the "natural landscape" is not inert. It too is exchanging heat energy, and light (electromagnetic radiation) with the atmosphere.

Vince Calder


Melanie,

The urban heat island primarily affects only the lowest level of the atmosphere. It is unlikely that it has any affect on the jet stream, which is often tens of thousands of feet above the ground, at the tropopause (the dividing line between the troposphere and stratosphere).

The urban heat island can affect rainfall patterns, but not just because it is warmer than the surrounding areas. Urban heat islands are often drier than the surrounding areas, as concrete and pavement cause rainwater to run off into rivers, lakes, and drains instead of being stored in the soil where it can evaporate later. It is not unusual for large city areas to alter local meteorological patterns, particularly convection leading to thunderstorms. The reduced amount of water available for evaporation can actually reduce the buoyancy of the air, reduce the intensity of convection, and thereby reduce the amount of rainfall in the upwind areas of a very large city. However, rainfall is often enhanced in the downwind areas of the city, as the effect of warmer temperatures increases convection over a larger area and results in smaller rainfall rates over a larger area (but leading to a slightly greater rainfall total over the city as a whole) than would be expected if the city were not there. The dynamics of these effects can be quite complicated and therefore computer models are used to predict the probable distribution of rainfall.

The urban heat island effect has been studied for a number of cities. One of the first, and most intensely studied cities was St. Louis, during a mid 1970s field experiment called METROMEX. If you type in "metromex" in a Google search, the first link that you will come across is a report on METROMEX. You will find lots of interesting information about the effects of the St. Louis urban heat island on weather.

David R. Cook
Meteorologist
Climate Research Section
Environmental Science Division
Argonne National Laboratory



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