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Major City Heat Islands and Rainfall

August 18, 2014

Temperatures in a city can be from 5 to 10 degrees higher
than that of the surrounding rural areas.
This is the urban heat island (UHI) effect.

In a recent study, NASA employed the TRMM
(Tropical Rainfall Measuring Mission) satellite to analyze
the rainfall distribution over major cities in the U.S.
The remote sensing data showed that the heat island effect
increased summer rain – over and downwind of major cities.
The rainfall in those two primary areas
exceeded rainfall in the downwind locations
by 48 percent and to as much as 116 percent.

As the air in the city warms two important factors occur,
the warmer air over the city rises faster
and a thermal gradient is created
between the city and the surrounding rural areas.
These two factors combine to create an area of convergence
and lead to an enhanced lifting of the airmass over the city.
The higher level of pollutants contained in this
rising urban airmass in turn leads to an increase
in the number of condensation nuclei in atmosphere.
All of these factors combine to create an increase
in clouds and under the right circumstances
an increase in the amount of rainfall.

Because the air over the city is warmer
it has the capacity to hold more moisture.
In order to produce the rain an ample supply
of moisture must also be available.
Because the air over the city is warmer
it has the capacity to hold more moisture
and it must be lifted higher in order to condense.
As you reach the outer edges of the city
that capacity decreases due to the
gradual decrease in temperature.
The rain or thunderstorm that develops downwind
is a result of the moisture rich airmass
that was once in place over the city
has now drifted into an airmass of cooler temperatures
thus increasing the relative humidity
of this incoming parcel of air
which in turn increases its ability to produce precipitation.

Areas most likely to receive precipitation
will be in and around the city
or in the downwind location.
The most likely time for precipitation will be
near the peak of the daytime maximum temperature.
Monitoring temperatures, surface winds, area dew points,
and water vapor imagery will help with identifying
moisture rich areas thus giving the forecaster
an idea of areas to target for potential
rain or thunderstorm development.
Research has shown that the heaviest areas of precipitation
will occur in the city or in the downwind locations.

Learn more at :

StormWater and WasteWater Treatment Plants around Metro-Detroit.

Waste Water Treatment Plants Map

Waste Water Treatment Plants Map

SouthEast Michigan Sewage Districts

SouthEast Michigan Sewage Districts


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  1. Cities pay for rain ? | Red Run

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