ATM SC 1050 Self-Paced Final Mizzou

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.

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, relatively cool air.

Maritime tropical (air mass)

An air mass characterized by high temperatures and high humidity.

Atmospheric River

A region of upper-level flow that transports large amounts of moisture, typically from the tropics and subtropics into the midlatitudes.

Continental tropical (air mass)

An air mass characterized by high temperatures and low humidity.

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.

frontolysis

the weakening or dissipation of a front.

frontogenesis

the formation, strengthening, or regeneration of a front.

"back door" cold front

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

cold air damming

A shallow layer of cold air that is trapped between the Atlantic coast and the Appalachian Mountains.

Overrunning

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

Frontal inversion

A temperature inversion encountered upon ascending through a sloping front, usually a warm front.

dryline

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

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 occlusion. When the air behind the front is milder than the air ahead of it, it is called a warm occlusion.

Upper-air front

A front that is present aloft but usually does not extend down to the ground. Also called an upper front and an upper-tropospheric front.

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.

warm sector

The region of warm air within a wave cyclone that lies between a retreating warm front and an advancing cold front.

secondary low

A low-pressure area (often an open wave) that forms near, or in association with, a main low-pressure area.

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 lee-side lows frequently form on the eastern side of the Rockies and Sierra Nevada mountains.

Northeaster

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. Also called nor'easter.

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.

shortwave

A small wave that moves around longwaves in the same direction as the air flow in the middle and upper troposphere. Shortwaves are also called shortwave troughs.

barotropic (atmosphere)

A condition in the atmosphere where surfaces of constant density parallel surfaces of constant pressure.

baroclinic (atmosphere)

The state of the atmosphere where surfaces of constant pressure intersect surfaces of constant density. On an isobaric chart, isotherms cross the contour lines, and temperature advection exists.

cold advection

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

warm advection

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

baroclinic instability

A type of instability arising from a meridional (north to south) temperature gradient, a strong vertical wind speed shear, temperature advection, and divergence in the flow aloft. Many mid-latitude cyclones develop as a result of this instability.

cut-off low

A cold upper-level low that has become displaced out of the basic westerly flow and lies to the south of this flow.

jet streak

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

conveyor belt model

(for mid-latitude storms) A three- dimensional picture of a mid-latitude cyclone and the various air streams (called conveyor belts) that interact to produce the weather associated with the storm.

dry slot

On a satellite image the dry slot represents the relatively clear region (or clear wedge) that appears just to the west of the tail of a comma cloud of a mid-latitude cyclonic storm.

comma clouds

A band of organized cumuliform clouds that looks like a comma on a satellite photograph.

Vorticity

A measure of the spin of a fluid, usually small air parcels. Absolute vorticity is the combined vorticity due to Earth's rotation (Earth's vorticity) and the vorticity due to the air's circulation relative to Earth. Relative vorticity is due to the curving of the air flow and wind shear.

Earth's vorticity

The rotation (spin) of an object about its vertical axis brought on by the rotation of Earth on its axis. Earth's vorticity is a maximum at the poles and zero at the equator.

Vorticity advection

The transport of vorticity by the wind. Positive vorticity advection occurs when the wind blows from high vorticity toward low vorticity, resulting in an increase in vorticity over time at a location. Negative vorticity advection occurs when the wind blows from low vorticity toward high vorticity, resulting in a decrease in vorticity over time at a location.

polar low

An area of low pressure that forms over polar water behind (poleward of) the main polar front.

arctic front

In northern latitudes, the semi-permanent front that separates deep cold arctic air from the more shallow, less-cold polar air.

atmospheric models

Simulation of the atmosphere's behavior by mathematical equations or by physical models.

forecast funnel

A sequence of steps used by forecasters to analyze current and projected conditions, moving from larger to smaller scales during the process.

AWIPS

Acronym for Advanced Weather Interactive Processing System. New computerized system that integrates and processes data received at a weather forecasting office from NEXRAD, ASOS, and analysis and guidance products prepared by NMC.

meteogram

A chart that shows how one or more weather variables has changed at a station over a given period of time or how the variables are likely to change with time.

very short-range forecast

Generally used to describe a weather forecast that is made for up to a few hours (usually less than 6 hours) into the future.

weather watch

A forecast indicating that atmospheric conditions are favorable for hazardous weather to occur over a particular region during a specified time period.

weather warning

A forecast indicating that hazardous weather is either imminent or actually occurring within the specified forecast area.

short-range forecast

Generally used to describe a weather forecast that extends from about 6 hours to a few days into the future.

medium range-forecast

Generally used to describe a weather forecast that extends from about 3 to 8.5 days into the future.

long-range forecast

Generally used to describe a weather forecast that extends beyond about 8.5 days into the future.

persistence forecast

A forecast that the future weather condition will be the same as the present condition.

steady-state (trend) forecast

A weather prediction based on the past movement of surface weather systems. It assumes that the systems will move in the same direction and at approximately the same speed as they have been moving. Also called trend forecasting.

analog forecasting method

A forecast made by comparison of past large-scale synoptic weather patterns that resemble a given (usually current) situation in its essential characteristics.

pattern recognition

An analogue method of forecasting where the forecaster uses prior weather events (or similar weather map conditions) to make a forecast.

statistical forecast

A forecast based on a mathematical/statistical examination of data that represents the past observed behavior of the forecasted weather element.

probability forecast

A forecast of the probability of occurrence of one or more of a mutually exclusive set of weather conditions.

weather type forecasting

A forecasting method where weather patterns are categorized into similar groups or types.

climatological forecast

A weather forecast, usually a month or more in the future, which is based upon the climate of a region rather than upon current weather conditions.

numerical weather prediction

Forecasting the weather based upon the solutions of mathematical equations by high-speed computers.

prognostic chart

A chart showing expected or forecasted conditions, such as pressure patterns, frontal positions, contour height patterns, and so on.

chaos

The property describing a system that exhibits erratic behavior in that very small changes in the initial state of the system rapidly lead to large and apparently unpredictable changes sometime in the future.

ensemble forecasting

A forecasting technique that entails running several forecast models (or different versions of a single model), each beginning with slightly different weather information. The forecaster's level of confidence is based on how well the models agree (or disagree) at the end of some specified time.

isallobars

A line of equal change in atmospheric pressure during a specified time interval.

omega high

A ridge in the middle or upper troposphere that has the shape of the Greek letter omega (Ω).

ordinary cell (air mass) thunderstorms

(also called air-mass thunderstorm) A thunderstorm produced by local convection within a conditionally unstable air mass. It often forms in a region of low wind shear and does not reach the intensity of a severe thunderstorm.

cumulus stage

The initial stage in the development of an ordinary cell thunderstorm in which rising, warm, humid air develops into a cumulus cloud.

dissipating stage

The final stage in the development of an ordinary cell thunderstorm when downdrafts exist throughout the cumulonimbus cloud.

multicell thunderstorm

A convective storm system composed of a cluster of convective cells, each one in a different stage of its life cycle.

overshooting top

A situation in a mature thunderstorm where rising air, associated with strong convection, penetrates into a stable layer (usually the stratosphere), forcing the upper part of the cloud to rise above its relatively flat anvil top.

gust front

A boundary that separates a cold downdraft of a thunderstorm from warm, humid surface air. On the surface its passage resembles that of a cold front.

shelf cloud

A dense, arch-shaped, ominous-looking cloud that often forms along the leading edge of a thunderstorm's gust front, especially when stable air rises up and over cooler air at the surface. Also called an arcus cloud.

roll cloud

A dense, roll-shaped, elongated cloud that appears to slowly spin about a horizontal axis behind the leading edge of a thunderstorm's gust front.

outflow boundary

A surface boundary formed by the horizontal spreading of cool air that originated inside a thunderstorm.

downburst

A severe localized downdraft that can be experienced beneath a severe thunderstorm. (Compare microburst and macroburst.)

microburst

A strong localized downdraft (downburst) less than 4 km wide that occurs beneath thunderstorms. A strong downburst greater than 4 km across is called a macroburst.

heat burst

A sudden increase in surface air temperature often accompanied by extreme drying. A heat burst is associated with the downdraft of a thunderstorm, or a cluster of thunderstorms.

squall line

A line of thunderstorms that form along a cold front or out ahead of it.

bow echo

A line of thunderstorms on a radar screen that appears in the shape of a bow. Bow echoes are often associated with damaging straight-line winds and small tornadoes.

mesoscale convective vortex

A counterclockwise circulation, usually less than 240 km (150 mi) in diameter, about an area of low pressure that forms in the mid-levels of the atmosphere in association with a mesoscale convective system.

derecho

Strong, damaging, straight-line winds associated with a cluster of severe thunderstorms that most often form in the evening or at night.

mesoscale convective complexes (MCC)

A large, organized convective weather system comprised of a number of individual thunderstorms. An MCC can span 1000 times more area than an individual ordinary cell thunderstorm. An MCC is a particular type of mesoscale convective system.

supercell

A severe thunderstorm that consists primarily of a single rotating updraft. Its organized internal structure allows the storm to maintain itself for several hours. Supercell storms can produce large hail and dangerous tornadoes.

mesocyclone

A vertical column of cyclonically rotating air within a supercell thunderstorm.

wall cloud

An area of rotating clouds that extends beneath a supercell thunderstorm and from which a funnel cloud may appear. Also called a collar cloud and pedestal cloud.

cap

A layer of warm, dry, stable air that inhibits warm, moist, unstable air from rising through it to produce thunderstorms.

sonic boom

A loud explosive-like sound caused by a shock wave emanating from an aircraft (or any object) traveling at or above the speed of sound.

stepped leader

An initial discharge of electrons that proceeds intermittently toward the ground in a series of steps in a cloud-to-ground lightning stroke.

return stroke

The luminous lightning stroke that propagates upward from Earth to the base of a cloud.

dart leader

The discharge of electrons that proceeds intermittently toward the ground along the same ionized channel taken by the initial lightning stroke.

ball lighting

A rare form of lightning that may consist of a reddish, luminous ball of electricity or charged air.

sheet lightning

Occurs when the lightning flash is not seen but the flash causes the cloud (or clouds) to appear as a diffuse luminous white sheet.

heat lightning

Distant lightning that illuminates the sky but is too far away for its thunder to be heard.

dry lightning

Lightning that occurs with thunderstorms that produce little, if any, appreciable precipitation that reaches the surface.