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Date: ___________________
High and Low Pressure Systems
Directions: Read the following information and then answer the questions on the back.
What is a High Pressure System?
A high pressure system is a whirling mass of cool, dry air that generally brings fair weather and
light winds. When viewed from above, winds spiral out of a high-pressure center in a clockwise
rotation in the Northern Hemisphere. These bring sunny skies. A high pressure system is
represented as a big, blue H.
What is a Low Pressure System?
A low pressure system is a whirling mass of warm, moist air that generally brings stormy
weather with strong winds. When viewed from above, winds spiral into a low-pressure center
in a counterclockwise rotation in the Northern Hemisphere. A low pressure system is
represented as a big, red L.
What is a storm? Air covers the surface of our planet and has mass, that is, it has weight and
volume, and it can be made to move. That's fairly obvious. But what causes it to move? Our
atmosphere is dynamic, and it's temperature constantly changes in response to rotation of the
planet, changes in seasons and earth's orbit around the sun. Why is temperature important?
Because hot air is less dense than cold air, and when hot and cold air collide, the hot air is
forced to rise over the colder air. Cold air typically is dryer than warm air and originates from
our planet's poles. Warm air comes from the tropics/equator. Whenever cold dry air moves
away from the poles, it eventually encounters warm wet air moving away from the equator.
The warm wet air is forced up and over the cold air. When the warm air is forced up, it causes
surface air pressure to drop, sort of like having a small vacuum develop at the earth's surface at
the boundary between the two air masses. Cold air rushes in to fill the area of lower air
pressure, which causes more warm air to be displaced upward, and more cold air moves in,
forcing more warm air upward, and a cycle starts to develop. The low-pressure area (also
known as a low pressure center or system) starts to rotate, and all this moving air creates wind,
and lots of it. In the north hemisphere, wind rotates counter-clockwise around a storm center,
and clockwise in the south hemisphere (this driven by the Coriolis effect).
So a storm forms in response to an extreme difference in air pressure, driven by the movement
of cold and warm air. Eventually either the cold or warm air dissipates, and equilibrium
becomes re-established and calm, less-windy weather prevails. A storm can last as short as a
few days to over a week. Our atmosphere is covered with areas of relatively high and low
pressure, all driven by collisions between cold polar and warm equatorial air masses (for the
most part).
1. In your own words describe a high pressure system.
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2. In your own words describe a low pressure system.
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3. When the two pressure systems collide what happens?
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