Skip Navigation   
NOAA logo - Click to go to the NOAA homepage National Weather Service Forecast Office   NWS logo - Click to go to the NWS homepage
National Weather Service Binghamton, New York
Left nav bar  

Local forecast by
"City, St" or Zip Code

Weather in Action: Downslope Winds

The image below shows a field of springtime stratocumulus clouds that developed across New York and Pennsylvania with a northwest wind flow.

satellite image

Why were there no clouds over New Jersey or southeastern portions of New York and Pennsylvania? Take a look at the topography (below) of the region and compare it to the field of stratocumulus.

topography of area.

As the northwest wind crossed the Appalachian Mountains and began descending toward the east coast, the air underwent a basic meteorological process called adiabatic compression.

As a parcel of air rises, there is less and less air above it so the air parcel expands due to the decreasing surrounding air pressure. Likewise, as an air parcel descends there is more and more air above it so the parcel becomes compressed. The Ideal Gas Law (PV=nRT) tells us that if the pressure of an air parcel changes but all other aspects of the parcel remain the same, there must be a corresponding change in temperature to keep the equation balanced. Thus, rising air expands and cools while descending air compresses and warms. Since the amount of moisture in the parcel does not change but the temperature increases, the humidity of the parcel decreases. As the humidity decreases, the clouds evaporate.

National Weather Service
Binghamton Weather Forecast Office
32 Dawes Drive
Johnson City, NY 13790
(607) 729-1597
Page last modified: July 12, 2007
Information Quality
Privacy Policy
Freedom of Information Act (FOIA)
About Us
Career Opportunities