ESC-101 Chapter 8 Vocab

Definitions for Chapter 8

Air Mass

A large body of air that has similar horizontal temperature and moisture characteristics.

source regions (for air masses)

Regions where air masses originate and acquire their properties of temperature and moisture.

continental polar (air mass)

An air mass characterized by low temperatures and dry air. Not as cold as arctic air masses.

continental arctic (air mass)

An air mass characterized by extremely low temperatures and very dry air.

continental tropical

An air mass characterized by high temperatures and low humidity.

lake-effect snows

Localized snowstorms that form on the downwind side of a lake. Such storms are common in late fall and early winter near the Great Lakes as cold, dry air picks up moisture and warmth from the unfrozen bodies of water.

maritime polar (air mass)

An air mass characterized by low temperatures and high humidity.

Pacific air

Cool, moist air that originates over the Pacific Ocean, moves eastward, then descends the Rocky Mountains and moves over the plains as dry, stable, and relatively cool air.

maritime tropical (air mass)

An air mass characterized by high temperatures and high humidity.

continental tropical (air mass)

An air mass characterized by high temperatures and low humidity.

front

The transition zone between two distinct air masses.

stationary front

A front that is nearly stationary with winds blowing almost parallel and from opposite directions on each side of the front.

cold front

A transition zone where a cold air mass advances and replaces a warm air mass.

“back door” cold fronts

A cold front moving south or southwest along the Atlantic seaboard of the United States.

warm front

A front that moves in such a way that warm air replaces cold air.

dryline

A boundary that separates warm, dry air from warm, moist air. It usually represents a zone of instability along which thunderstorms form.

overrunning

A condition that occurs when air moves up and over another layer of air.

occluded front (occlusion)

A complex frontal system that ideally forms when a cold front overtakes a warm front. When the air behind the front is colder than the air ahead of it, the front is called a cold-type occlusion. When the air behind the front is milder than the air ahead of it, it is called a warm-type occlusion.

cold-type occlusion

A complex frontal system that ideally forms when a cold front overtakes a warm front. When the air behind the front is colder than the air ahead of it, the front is called a cold-type occlusion. When the air behind the front is milder than the air ahead of it, it is called a warm-type occlusion.

warm-type occlusion

A complex frontal system that ideally forms when a cold front overtakes a warm front. When the air behind the front is colder than the air ahead of it, the front is called a cold-type occlusion. When the air behind the front is milder than the air ahead of it, it is called a warm-type occlusion.

polar front theory

A theory developed by a group of Scandinavian meteorologists that explains the formation, development, and overall life history of cyclonic storms that form along the polar front.

wave cyclone

An extratropical cyclone that forms and moves along a front. The circulation of winds about the cyclone tends to produce a wavelike deformation on the front.

frontal wave

A wavelike deformation along a front in the lower levels of the atmosphere. Those that develop into storms are termed unstable waves, while those that do not are called stable waves.

open wave

The stage of development of a wave cyclone (mid-latitude cyclonic storm) where a cold front and a warm front exist, but no occluded front. The center of lowest pressure in the wave is located at the junction of the two fronts.

cyclogenesis

The development or strengthening of middle-latitude (extratropical) cyclones.

lee-side low

Storm systems (extratropical cyclones) that form on the downwind (lee) side of a mountain chain. In the United States, they frequently form on the eastern side of the Rockies and Sierra Nevada mountains.

nor’easter

A name given to a strong, steady wind from the northeast that is accompanied by rain and inclement weather. It often develops when a storm system moves northeastward along the coast of North America.

convergence

An atmospheric condition that exists when the winds cause a horizontal net inflow of air into a specified region.

divergence

An atmospheric condition that exists when the winds cause a horizontal net outflow of air from a specific region.

longwave (in westerly flow aloft)

A wave in the upper level of the westerlies characterized by a long length (thousands of kilometers) and significant amplitude. Also called Rossby waves.

shortwave (in westerly flow aloft)

A small wave that moves around longwaves in the same direction as the air flow in the middle and upper troposphere.

cold advection

The transport of cold air by the wind from a region of lower temperatures to a region of higher temperature.

warm advection

The transport of warm air by the wind from a region of higher temperatures to a region of lower temperatures.

jet streak

A region of high wind speed that moves through the axis of a jet stream. Also called jet maximum.