Air masses and pressure systems

Air Masses Air masses take on the characteristics of the places where they originally formed. These characteristics are primarily restricted to wet/dry and hot/cold.

Continental (land) air masses may form over…….. or the tropics in which case they are dry and hot and are referred to as “Continental Tropical - cT”. The polar regions in which case they are known as “Continental Polar - cP” and are dry and cold….. cP cT cP

Maritime (ocean) air masses may form over... Polar regions in which case they are cold and wet (Maritime Polar - mP) or in the tropics in which case they are warm and wet (Maritime Tropical - mT). mP mT

Maritime Polar Continental Polar Maritime Polar Maritime Tropical Maritime Tropical Continental Tropical

Pressure Centers

Air pressure.. Is most simply thought of as the weight of the overlying column of air. This weight or pressure is affected by factors such as temperature and humidity. Cold temperatures lead to low kinetic energy and closely packed molecules (more dense = high pressure) Warm temperatures lead to high kinetic energy and widely spaced molecules (less dense = low pressure)

Air pressure (continued…) Since water vapor is a very light weight gas, air that has a lot of water vapor in it (high humidity) is less dense and lighter than dry air. Dry air = higher pressure Wet air = lower pressure

High and Low Pressure Centers

Wind patterns around “Highs and Lows” bring about different weather. L H

High pressure…. Results as cold air descends from above, reaches the ground and spirals outward in a clockwise direction. High pressure generally brings cool dry conditions we refer to as “Fair skies”.

Low Pressure Results from cool air from a neighboring “High” circulating along the ground and pushing warmer moist air spiraling upward in a counter-clockwise direction. “Lows” generally bring cloudy warm weather.

Note how the winds blow from the “High” into the “Low”. H L

The instrument used to measure air pressure is called a “barometer”. The instrument seen here is a “Mercury barometer”. Air pushing down on a dish of mercury forces the liquid metal up into the hollow glass tube. The level to which the mercury rises in the tube (in inches) indicates how heavy the air is, or how much air pressure there is. “Normal” air pressure will support a column of mercury inches high.

Lines connecting points of equal barometric pressure are often seen on weather maps and are called “Isobars”

When isobars are closely spaced, a rapid change in air pressure and a corresponding increase in wind speed is indicated.

Fronts

Fronts are the result…. of two air masses of different characteristics coming into contact with one another. The two air masses will not readily mix and a boundary forms between the two. This boundary is called a front. The type of front which develops and the weather associated with the passage of this front depends on the types of air masses involved.

Cold Front As heavy cold air moves into warm air, it forces the warm air to rise rapidly causing short periods of heavy thunderstorms.

Simplified diagram of a cold front.

Warm Front As warm air moves into heavier cold air, it slowly creeps up and over the colder air. As the warm air rises, long periods of gentle rain develop

Occluded Front These fronts develop when a cold front overtakes a warm front. The associated weather is therefore a combination of long gentle rain and heavier periods of thunder- storms.