Unit 4: Air Masses, Front, Air Pressure, Wind

Air Mass

a body of air that has about the same temperature and moisture throughout

air pressure

is the force that air exerts per unit area and is caused by the weight of the air above a surface

anemometer

an instrument that measures wind speed.

aneroid barometer

An instrument used for measuring air pressure that contains an empty metal chamber that compresses or expands as pressure changes.

anticyclone

a high pressure system characterized by an outward, clockwise flow of air in the northern hemisphere

arctic

added to the name of an air mass originating in the arctic, this air mass will be extra cold

Barometer

an instrument that measures air pressure.

Cold Front

a boundary between a cold and warm air mass when the cold air mass takes over an area. The cold air pushes up the less dense warm air. Temperature goes from higher to lower as front passes. Cumulonimbus clouds, heavy rain and severe weather are common along the steep front boundary.

Continental

added to the name of an air mass that forms over the land. The air mass will be dry.

Coriolis effect

the rotation of the earth which causes the wind to bend to the right of its intended flow from high to low perpendicular to isobars

cyclone

a low pressure system characterized by an inward and counterclockwise flow of air in the northern hemisphere. Tornadoes, hurricanes and winter storms like Nor'easters are all types of cyclones.

Front

a boundary between two air masses of different density, moisture, or temperature

isobar

a line on a weather map that joins points having the same barometric pressure. The closer they are the faster the wind.

Land breeze

local wind blowing from the land toward the ocean. Forms during the night due to land losing more heat making air above land higher pressure than over water.

Maritime

added to the name of an air mass that forms over the ocean. The air mass will be humid.

Mercury Barometer

An instrument that measures changes in air pressure, consisting of a glass tube partially filled with mercury, with its open end resting in a dish of mercury. Air pressure pushing on the mercury in the dish forces the mercury in the tube to be higher if air pressure is high. Gravity allows mercury to flow out into dish if air pressure is low.

Mid-latitude cyclone

an area of low pressure that has counter-clockwise rotating winds and undergoes cyclogenesis giving it a warm and cold front coming out the south south side and ending with an occluded front. Brings us most of our precipitation and weather changes. Nor'easter, bomb cyclones are an examples of a mid-latitude cyclone.

millibar

unit of pressure shown on weather maps and recorded by aneroid barometers. 1013 is average air pressure in millibars

monsoon

A seasonal reversal of wind direction associated with large continents (especially Asia) caused by differential heating of land vs. water

Occluded Front

a front where a warm air mass is caught between and above two colder air masses, which occurs when a cold front catches up to a warm front. It brings cool temperatures and long periods of rain and snow

Polar

added to the name of an air mass that forms over northern latitudes. The air mass will be cold.

pressure gradient force

Drives air from areas of higher barometric pressure to areas of lower barometric pressure, causing winds.

sea breeze

A local wind blowing from the ocean toward the land during the afternoon in costal areas formed due to differential heating of land vs. water and associated pressure gradient

Stationary Front

A boundary between two air masses that is not moving, no change in temperature because neither is moving in. Usually results in cloudy weather and mild temperatures.

Tropical

Part of air mass classification scheme that denotes an air mass that forms over warm regions to our south. Suggests the air mass will be warm. There are both maritime and continental tropical air masses

warm front

a front between a warm and cold air mass in which the warm air mass is replacing the cold at the surface. As warm air lifts in a gentle slope over cold air it brings a typical pattern of cirrus, cirrostratus/cirrocumulus, altostratus, stratus and finally nimbostratus drizzly rain followed by warm and clear weather. Sleet and freezing rain associated with this front in the winter.

Wind

Horizontal movement of air from high to low in an attempt to equalize air pressure