On Jan 23, 5:00*pm, Weatherlawyer wrote:
http://earthobservatory.nasa.gov/Fea...n/printall.php

Cities impact rainfall and can create their own rain and storms.
More rain falls downwind of some major urban areas than in the
surrounding countryside. These local processes tend to be
hidden by large weather fronts crossing continents.

During summer, weather tends to be generated by local processes:
hot, humid air piles up along the face of a mountain, triggering a
thunderstorm; cool moist air blows off a lake, collides with hot air
over land and rain clouds form.

For more than a century, scientists had suspected that cities impact
or create rain and influence summer weather.

Shepherd, Pierce and Negri, at NASA focused on five cities in
south-central and southeastern USA and established
the urban environment affecting the regions.
They found 20 percent greater rainfall per hour, downwind of cities
compared upwind.

Air is unstable when it is warmer than the air that surrounds it.
Once the air is lifted, it will continue to rise. Cities are a source
of lift.

Cities are made with heat-absorbing building materialswhich provide a
source of lift to push warm, moist, surface air into the cooler air
above. They have machines that pump heat into the atmosphere and they
lack cooling vegetation. Average city temperatures can be six to eight
degrees Fahrenheit higher than surrounding rural and suburban
landscapes. This may disrupt the flow of air over the Earth’s surface.

Storms approaching Atlanta or Baltimore from the west split around the
cities because of the urban heat island. When they reconnect the two
halves of the storm come back together downwind of the city
(convergence) the rising air forms rain clouds.

City pollution also impacts cloud formation. All rain needs aerosols
to form. But aerosols of city pollution are smaller and more numerous
than natural aerosols. With lots of particles to collect on, water
coalesces as many tiny droplets instead of larger rain-sized drops
Some urban aerosols suppress rain but in others they increase it.

Temperature difference between the air near the ground and the
atmosphere above may be one key difference. More rain can occur when a
bubble of heated air forms over a very warm area. It rises faster and
climbs higher droplets that would normally fall out at a lower
elevation are smaller and go higher turn to ice and release heat.

In an heat island -with no lateral wind sheer, the extra heat may take
the bubble of air higher and faster.

Isn't this exactly what happens with tropical storms?
They too are heat islands.

And with moist air continually below, more moist air is
sucked up. The effect only happens when the system has "thunder-storm
potential" (hot, humid and little or no wind sheer) and when the air
near the surface is moist.