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Urban Impact on Weather
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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Update: June 2012
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