AIR PRESSURE, AIR MASSES AND FRONTS Earth Science

Air Pressure Air Pressure: The weight of the air pushing down on the surface of earth. Barometer: The tool that measures air pressure.

High and Low Pressure Systems High pressure is caused by Cool, Dry air SINKING. Low pressure is caused by Warm, Moist air RISING. Cool, Dry Air = High Pressure = Clear Skies Warm, Moist Air = Low Pressure = Cloudy Skies

ISOTHERMS Isotherms: Lines connecting place of equal temperature

Pressure Centers Isobars: Lines on a map connecting places with equal pressure. This shows you the area of High and Low Pressure. Arrows show direction of wind.

DRAWING ISOBARS • 000 •

Prevailing Westerlies Global Winds Global Winds: the global circulation of air from HIGH pressure to LOW pressure. THE WINDS WILL BLOW FROM HIGH TO LOW The winds would go either North or South except for the Coriolis Effect causes the wind to curve to the What Global Winds affect North America? Prevailing Westerlies RIGHT

Coriolis Effect The apparent curve of the wind to the right in the Northern Hemisphere. Caused by Earth’s Rotation http://www.wiley.com/college/strahler/0471480533/animations/ch07_a nimations/animation2.html

Pressure Systems Surface winds: High = Move OUT of the center Low = Move IN toward the center Add the Coriolis effect (curve to the right) Motion around a High = Clockwise Motion around a Low = Counter Clockwise AntiCyclone Cyclone

AIR MASSES Air Mass: A large volume of air defined by its TEMPERATURE and WATER VAPOR. c = Continental = Dry m = Maritime = Moist P = Polar = Cold T = Tropical = Warm cP = Continental Polar air = mP = Maritime Polar air = cT = Continental Tropical = mT = Maritime Tropical = Cold, Dry air from Canada Cold, Moist air from N. Atlantic Warm, Dry air from Mexico Warm, Moist air from Gulf of Mexico

Frontal Boundaries Frontal Boundary = Where two different air masses collide. Cool, Dry air collides with Warm, Moist air.

Cold Front Cold Front = Where a cP air mass collides with a mT air mass. cP air is doing the pushing. mT air is rising UP creating vertical clouds. Cumulonimbus clouds Thunderstorm What is the weather at A? What is the weather at B? What is the weather at C? C B A

Warm Front Warm Front = When a mT air mass collides with a cP air mass. mT air mass is doing the pushing. As mT air pushes, it rises over the cP air creating low horizontal clouds. Stratus, nimbostratus, cirrus What is the weather at A? What is the weather at B? What is the weather at C? A C B

Weather Map Symbols cP mT Identify the weather map symbols to determine weather in your area. Cold Front = Warm Front = cP mT

Weather Forecasting cP NW mT SW

Current U.S. Weather Map May 24, 2011

Friday’s Weather Map mP mT

Mid-Latitude Cyclone

Hurricanes 3 things needed to form a hurricane Low Pressure Warm ocean temperatures High humidity

Hurricane Where are hurricanes located when at their strongest? Winds over 76 mph

Hurricanes What happens when the hurricane hits land?

Weather Station Model Used to deliver a lot of information in a small space

Weather Map