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Air moves into a Low pressure system. It pushes any air that was there upwards.
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Low Pressure Systems

You may have seen a weather map with a red L on it. This red L denotes a low pressure system in that area of the map. Just what does that mean?

The H for high pressure systems and alternatively the L for low pressure systems are relative measures of the pressure in that particular system compared to the pressure around the system. There are no exact measurements that would make a particular system a High and another a Low. Itís all relative!

The differences in pressure from one place to another on Earth are what cause wind. You see, air wants to move from areas of high pressure to areas of low pressure. Away from areas of high pressure and into areas of low pressure...

The Coriolis Force also adds rotation to the movement of air around pressure systems. In the Northern Hemisphere, air moves clockwise around a High and counter-clockwise around a Low. In summary, air moves away and clockwise from a High and into and counter-clockwise from a Low.

Air moves into a Low pressure system. It pushes any air that was there upwards. As the air rises, it experiences lower pressures and lower temperatures. These lower temperatures force water vapor to condense into clouds that can yield precipitation. Thatís why many people equate seeing a red L on weather chart with bad weather, cloudy skies and possibilities of rain or snow.

Last modified April 19, 2009 by Jennifer Bergman.

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Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA