Air Pressure

High and Low Pressure Areas High pressure causes air to sink Usually results in several days of clear sunny skies Air rises in low pressure areas and forms water droplets Usually results in rain and storms

Barometer – tool used to measure air pressure. High pressure brings mild winds, stable temperatures, and clear weather. Low pressure brings strong winds, dropping temperatures and precipitation. H L

L H Barometric Pressure – tells us when weather will change. when it falls - low pressure moving in when it rises- high pressure moving in Warm air= expanding or rising air= leaves behind L pressure Air Pressure L Air Pressure Cold Air=sinking air= leaves an area of H pressure H

Global Circulation and Wind Systems

Global Air Masses North America gets 5 types of air masses Defined by areas of origination Highly variable in terms of temperature and moisture

A jet stream forms high in the upper troposphere over the boundary between two air masses of different temperature. The greater the temperature difference between the air masses, the greater the air pressure difference, and the faster the wind blows in the jet stream. This river of air has wind speeds which often exceed 100 mph, and sometimes peak over 200 mph. Jet streams usually form in the winter, when there is a greater contrast in temperature between cold continental air masses and warm oceanic air masses. 8

Wind is caused by air flowing from high pressure to low pressure Wind is caused by air flowing from high pressure to low pressure. Since the Earth is rotating, however, the air does not flow directly from high to low pressure, but it is deflected to the right (in the Northern Hemisphere; to the left in the Southern Hemisphere), so that the wind flows around the high and low pressure areas. The closer the high and low pressure areas are together, the stronger the "pressure gradient", and the stronger the winds. On weather maps, lines of constant pressure are drawn (as in the example, above) which are called "isobars". The closer these lines are together, the stronger the wind. 9

Air Masses

Air masses are like the surface they form over. a huge “bubble” of air that shares the same temperature and pressure. (can be big enough to cover several states!) Air Masses are huge clumps of air that move around the Earth, bringing changes in… weather. Air masses are like the surface they form over.

The air mass shown above formed over land in northern Canada. Therefore the air mass will be… cold and dry.

H A polar air mass brings cold air Cold Air Mass – this is an air mass made up of cool temperatures and high pressure. Usually does not hold much moisture Forms over cold water or land. H

The pink air mass formed over the ocean near the equator. The air mass will be… warm and moist.

Warm Air Mass – this is an air mass made up of warm air that has a low pressure. Usually holds a lot of moisture. Forms over hot land or water. L A tropical air mass brings warm air

Fronts Where air masses meet and do not mix

When two air masses meet, they form a… front.

Thunderstorms pushes the warm air up quickly. During a cold front, a cold air mass catches up to a warm air mass and… pushes the warm air up quickly. Weather associated with cold fronts is… Thunderstorms

See A Cold Front Forming…. Click Here!

The map symbol for a cold front is a curved line with small… blue spikes When cold air catches up to warm air and pushes it up and away, what is formed?... cold front

From the map above, you can predict that the weather in the southern U From the map above, you can predict that the weather in the southern U.S. will be getting… colder

See A Warm Front Forming…. Click Here!

The map symbol for a warm front is a curved line with small… rounded knobs When warm air catches up to a cold air mass, what is formed?... warm front

slides over the top of the cold air, When a warm air mass catches up with a cold air mass, it forms a… warm front The warm air… slides over the top of the cold air, before … pushing it away. Weather associated with warm fronts are…. Clouds and long periods of steady rain

From the map above, you can predict that the weather over the Great Lakes will be getting … warmer

Warm or cold front stops moving. Light wind and precipitation may occur across the front boundary

One front overtakes another Cold front typically faster moving Can have warm and cold occluded fronts Creates Precipitation

Name the Weather Fronts