Western Regional Climate Center

Providing Climate Services Since 1986

Climate & Weather Glossary

The products, agencies, programs, and models referenced in this Web site encompass a potentially overwhelming number of specialized terms and acronyms. The following glossary defines some of these terms as a handy reference. A separate list of acronyms is included here.

If you find a term or acronym referenced in this Web site which is not defined here, please drop a note to our Webmaster.

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Absolute Humidity
The mass of water vapor in a given volume of air. It represents the density of water vapor in the air.

Absolute Zero
A temperature of -273°C, -460°F, or 0°K. Theoretically, there is no molecular motion at this temperature.

The efficiency of radiation absorption.

The gradual adjustment of the body to new climatic or other environmental conditions, for example, the adjustment to low levels of oxygen at high altitudes.

The growth of a precipitation particle by the collision of an ice crystal or snowflake with a supercooled liquid droplet that freezes upon impact.

Actual Evapotranspiraton
The rate of water lost from vegetation and soil, ordinarily at a slower rate than the potential rate.

Actual Vapor Pressure
See vapor pressure.

Adiabatic Process
A process that takes place without a transfer of heat between the system (such as an air parcel) and its surroundings. In an adiabatic process compression always results in warming, and expansion results in cooling.

The horizontal transfer of any atmospheric property by the wind.

Advection Fog
Occurs when warm, moist air moves over a cold surface and the air cools to below its dew point.

A device that resembles a wind vane with a propeller at one end. Used to indicate wind speed and direction.

Air Density
Mass per unit volume of air; about 1.275 km per cubic meter at 0°C and 1000 millibars.

Air Mass
A large expanse of air having similar temperature and humidity at any given height.

Air Pressure
The cumulative force exerted on any surface by the molecules composing air.

The percent of radiation returning from a surface compared to that which strikes it.

An instrument that indicates the altitude of an object above a fixed level. Pressure altimeters use an aneroid barometer with a scale graduated in altitude instead of pressure.

A middle cloud, usually white or gray. Often occurs in layers or patches with wavy, rounded masses or rolls.

Altocumulus Castellanus
An altocumulus showing vertical development, individual cloud elements have towerlike tops, often in the shape of tiny castles.

Altocumulus Lenticularis
A lens-shaped altocumulus cloud; a mountain-wave cloud generated by the disturbance of horizontal airflow caused by a prominent mountain range.

A middle cloud composed of gray or bluish sheets or layers of uniform appearance. In the thinner regions, the sun or moon usually appears dimly visible.

Ambient Air
The air surrounding a cloud, or the air surrounding rising or sinking air parcels.

Ambient Temperature
Temperature of the surrounding (ambient) air.

An instrument designed to measure wind speed.

Aneroid Barometer
An instrument designed to measure atmospheric pressure. It contains no liquid.

Annual Range of Temperature
The difference between the warmest and coldest months at any given location.

Departures of temperature, precipitation, or other weather elements from long-term averages.

Arctic Air
A very cold and dry air mass that forms primarily in winter and the northern interior of North America.

Atmospheric Window
A region of the electromagnetic spectrum from 8 to 12 µm where the atmosphere is transparent to radiation.

Autumnal Equinox
The equinox at which the sun approaches the Southern Hemisphere and passes directly over the equator. Occurs around September 23.

A recording instrument that provides a continuous trace of air pressure variation with time.

An instrument that measures atmospheric pressure. The two most common barometers are the mercury barometer and the aneroid barometer.

Beaufort Scale
A scale of wind strength based on visual assessment of the effects of wind on seas and vegetation.

Black Body
A hypothetical object that absorbs all of the radiation that strikes it. It also emits radiation at a maximum rate for its given temperature.

A severe weather condition characterized by low temperatures and strong winds (greater than 32 mi/hr) bearing a great amount of snow. When these conditions continue after the falling snow has ended, it is termed a ground blizzard.

A cold katabatic wind that originates in Yugoslavia and flows onto the coastal plain of the Adriatic Sea.

Bowen Ratio
The ratio of energy available for sensible heating to energy available for latent heating.

Boyle's Law
When the temperature is held constant, the pressure and density of an ideal gas are directly proportional.

An instrument that automatically records cloud height.

Centrifugal Force
A force directed outward, away from the center of a rotating object; equal in magnitude to the centripetal force but in the opposite direction.

Centripetal Force
An inward-directed force that confines an object to a circular path; equal in magnitude to the centrifugal force but in the opposite direction.

Charles's Law
With constant pressure, the temperature of an ideal gas is inversely proportional to the density of the gas.

A warm, dry wind on the eastern side of the Rocky Mountains. In the Alps, the wind is called a Foehn.

A high cloud that appears as a white patch of cloud without shadows. It consists of very small elements in the form of grains or ripples.

A high cloud appearing as a whitish veil that may totally cover the sky. Often produces halo phenomena.

A high cloud composed of ice crystals in the form of thin, white, featherlike clouds in patches, filaments, or narrow bands.

The accumulation of daily and seasonal weather events over a long period of time. A description of aggregate weather conditions; the sum of all statistical weather information that helps describe a place or region.

Cloud Base
The lowest portion of a cloud.

Any sudden and heavy rain shower.

Cloud Cover
The amount of the sky obscured by clouds when observed at a particular location.

Cloud Deck
The top of a cloud layer, usually viewed from an aircraft.

Cloud Seeding
The introduction of artificial substances (usually silver iodide or dry ice) into a cloud for the purpose of either modifying its development or increasing its precipitation.

The merging of cloud droplets into a single larger droplet.

Cold Fog
See Supercooled cloud.

Cold Front
The leading edge of a cold air mass.

Process by which water changes phase from a vapor to a liquid.

Condensation Nuclei
Small particles in the atmosphere that serve as the core of tiny condensing cloud droplets. These may be dust, salt, or other material.

The transfer of heat by molecular activity from one substance to another, or through a substance. Transfer is always from warmer to colder regions.

Continental Air Mass
An air mass that forms over land; it is normally relatively dry.

Continental Climate
A climate lacking marine influence and characterized by more extreme temperatures than in marine climates: therefore, it has a relatively high annual temperature range for its latitude.

Continental Polar Air
Relatively dry air mass that develops over the northern interior of North America; very cold in winter and mild in summer.

Continental Tropical Air
Warm, dry air mass that forms over the subtropical deserts of the south-western United States.

Contrail (CONDensation TRAIL)
A cloudlike streamer frequently seen forming behind aircraft flying in clear, cold, humid air.

Motions in a fluid that result in the transport and mixing of the fluid's properties. In meteorology, convection usually refers to atmospheric motions that are predominantly vertical, such as rising air currents due to surface heating. The rising of heated surface air and the sinking of cooler air aloft is often called free convection. (Compare with forced convection.)

Convective Condensation Level (CCL)
The level above the surface marking the base of a cumiliform cloud that is forming due to surface heating and rising thermals.

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

Cooling Degree-Day
A form of degree-day used in estimating the amount of energy necessary to reduce the effective temperature of warm air. A cooling degree-day is a day on which the average temperature is one degree above a desired base temperature. Also see this page.

Coriolis Effect
A deflective force arising from the rotation of the earth on its axis; affects principally synoptic-scale and global-scale winds. Winds are deflected to the right of the initial direction in the Northern Hemisphere, and to the left in the Southern Hemisphere.

Crepuscular Rays
Alternating light and dark bands of light that appear to fan out from the sun's position, usually at twilight.

An exceptionally dense and vertically developed cloud, often with a top in the shape of an anvil. The cloud is frequently accompanied by heavy showers, lightning, thunder, and sometimes hail. It is also known as a thunderstorm cloud.

A cloud in the form of individual, detached domes or towers that are usually dense and well defined. It has a flat base with a bulging upper part that often resembles cauliflower. Cumulus clouds of fair weather are called cumulus humilis. Those that exhibit much vertical growth are called cumulus congestur or towering cumulus.

Cumulus Congestus
An upward building convective cloud with vertical development between that of a cumulus cloud and a cumulonimbus.

Cup Anemometer
An instrument used to monitor wind-speed. Wind rotation of cups generates and electric current calibrated in wind speed.

Cutoff High
Anticyclonic circulation system that separates from the prevailing westerly airflow and therefore remains stationary.

Cutoff Low
Cyclonic circulation system that separates from the prevailing westerly airflow and therefore remains stationary.

Daily Range of Temperature
The difference between the maximum and minimum temperatures for any given day.

Degree Days
Computed from each day's mean temperature (max+min/2). For each degree that a day's mean temperature is below or above a reference temperature is counted as one degree day. Also see this page.

The ratio of the mass of a substance to the volume occupied by it.

A process that occurs in subfreezing air when water vapor changes directly to ice without becoming a liquid first. (Also called sublimation in meteorology.)

Deposition Nuclei
Tiny particles in the atmosphere that serve as the core of tiny ice crystals as water vapor changes to the solid form. These are also called ice nuclei.

One of two types of dry climate – the driest of the dry climates.

Water that has condensed onto objects near the ground when their temperatures have fallen below the dew point of the surface air.

Dew Point (Dew-Point Temperature)
The temperature to which air must be cooled (at constant pressure and constant water vapor content) for saturation to occur. When the dew point falls below freezing it is called the frost point.

The bending of light around objects, such as cloud and fog droplets, producing fringes of light and dark or colored bands.

Diffuse Insolation
Solar radiation that is scattered or reflected by atmospheric components (clouds, for example) to the earth's surface.

Direct Insolation
Solar radiation that is transmitted directly through the atmosphere to the earth's surface without interacting with atmospheric components.

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

A severe localized downdraft that can be experienced beneath a severe thunderstorm. (Compare Microburst)

Downward moving air, usually within a thunderstorm cell.

Drainage Basin
A fixed geographical region from which a river and its tributaries drain water.

Small drops between 0.2 and 0.5 mm in diameter that fall slowly and reduce visibility more than light rain.

A period of abnormally dry weather sufficiently long enough to cause serious effects on agriculture and other activities in the affected area.

Dry Adiabatic Rate
The rate of change of temperature in a rising or descending unsaturated air parcel. The rate of adiabatic cooling or warming is 10°C per 1000 m (5.5°F per 1000 ft).

Dry Climate
A climate in which yearly precipitation is not as great as the potential loss of water by evaporation.

Dust Devil (or Whirlwind)
A small but rapidly rotating wind made visible by the dust, sand, and debris it picks up from the surface. It develops best on clear, dry, hot afternoons.

A small volume of air (or any fluid) that behaves differently from the larger flow in which it exists.

Effective Emissivity
A correction factor, dependent on the radiational characteristics of the earth -atmosphere system, that permits application of black body radiation laws to the earth-atmosphere system

The fractional amount of radiation emitted by a given object or substance in comparison to the amount emitted by a perfect emitter.

The rate at which a black body radiates energy across all wave-lengths.

The mixing of environmental air into a preexisting air current or cloud so that the environmental air becomes part of the current or cloud.

Environmental Lapse Rate
The rate of decrease of temperature with elevation. It is most often measured with a radiosonde.

Equilibrium Vapor Pressure
The necessary vapor pressure around liquid water that allows the water to remain in equilibrium with its environment. Also called saturation vapor pressure.

The time when the sun crosses the plane of the earth's equator occurring about March 21 and September 22.

The process by which a liquid changes into a gas.

Vaporization of water through direct evaporation from wet surfaces and the release of water vapor by vegetation.

Evaporation Fog
Fog produced when sufficient water vapor is added to the air by evaporation. The two common types are steam fog, which forms when cold air moves over warm water, and frontal fog, which forms as warm raindrops evaporate in a cool air mass.

The outermost portion of the atmosphere.

Fall Freeze Date
The date of occurrence in the fall of the first minimum at or below a temperature threshold. See also this page.

Fall Streaks
Falling ice crystals that evaporate before reaching the ground.

See Chinook.

A cloud with its base at the earth's surface. It reduces visibility to below 1 km.

Forced Convection
On a small scale, a form of mechanical stirring taking place when twisting eddies of air are able to mix.

Free Convection
Convection triggered by intense solar heating of the earth's surface.

A condition occurring over a widespread area when the surface air temperature remains below freezing for a sufficient time to damage certain agricultural crops. A freeze most often occurs as cold air is advected into a region, causing freezing conditions to exist in a deep layer of surface air. Also called advection frost. See also this page.

Freeze Free Season
The number of days between the last spring freeze date and the first fall freeze date. See also this page.

Freezing Rain and Freezing Drizzle
Rain or drizzle that falls in liquid form and then freezes upon striking a cold object or ground. Both can produce a coating of ice on objects which is called glaze.

The transition zone between two distinct air masses.

Frontal Fog
See Evaporation fog.

Frost (also called Hoarfrost)
A covering of ice produced by deposition (sublimation) on exposed surfaces when the air temperature falls below the frost point (the dew point is below freezing).

Frost Point
See Dew point.

Frozen Dew
The transformation of liquid dew into tiny beads of ice when the air temperature drops below freezing.

Funnel Cloud
A rotating conelike cloud that extends down-ward from the base of a thunderstorm. When it reaches the surface it is called a tornado.

Geostrophic Wind
A theoretical horizontal wind blowing in a straight path, parallel to the isobars or contours, at a constant speed. The geostrophic wind results when the Coriolis force exactly balances the horizontal pressure gradient force.

The conversion of all the supercooled liquid water in a cloud into ice crystals, thus reducing the growth rate of ice crystals and hail.

Glaciated Cloud
A cloud or portion of a cloud where only ice crystals exist.

A coating of ice on objects formed when supercooled rain freezes on contact. A storm that produces glaze is called an icing storm.

Colored rings that appear around the shadow of an object.

See Snow pellets

Green Flash
A small, green color that occasionally appears on the upper part of the sun as it rises or sets.

Ground Fog
See Radiation fog.

Growing Degree-Day
A form of the degree-day used as a guide for crop planting and for estimating crop maturity dates. See also this page.

Growing Season
The number of days between the last spring freeze date and the first fall freeze date. See also this page.

A dust or sandstorm that forms as cold downdrafts from a thunderstorm turbulently lift dust and sand into the air.

Solid precipitation in the form of chunks or balls of ice with diameters greater than 5 mm. The stones fall from cumulonimbus clouds.

Transparent or partially opaque particles of ice that range in size from that of a pea to that of golf balls.

Hair Hygrometer
An instrument used to monitor relative humidity by measuring the changes in the length of human hair that accompany humidity variations.

Rings or arcs that encircle the sun or moon when seen through an ice crystal cloud or a sky filled with falling ice crystals. Halos are produced by refraction of light.

Fine dry or wet dust or salt particles dispersed through a portion of the atmosphere. Individually these are not visible but cumulatively they will diminish visibility.

A form of energy transferred between systems by virtue of their temperature differences.

Heat Capacity
The ratio of the heat absorbed (or released) by a system to the corresponding temperature rise (or fall).

Heat Index (HI)
An index that combines air temperature and relative humidity to determine an apparent temperature-how hot it actually feels.

Heat of Fusion
Heat released when water changes phase from liquid to solid; 80 calories per gram

Heat of Melting
Heat required to change the phase of water from solid to liquid; 80 calories per gram.

Heating Degree-Day
A form of the degree-day used as an index for fuel consumption. Needed on days when average air temperature falls below 69 °F (18 °C); computed by subtracting the day's average temperature from 65 °F. See also this page.

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

A faint white ring surrounding the shadow of an observer's head on a dew-covered lawn.

The atmosphere above 80 km (50 mi) where gases are stratified, with concentrations of the heavier gases decreasing more rapidly with altitude than concentrations of the lighter gases.

High Inversion Fog
A fog that lifts above the surface but does not completely dissipate because of a strong inversion (usually subsidence) that exists above the fog layer.

Highland Climate
Complex pattern of climate conditions associated with mountains. Highland climates are characterized by large differences that occur over short distances.

Fernlike crystals of ice that form by deposition of water vapor on twigs, tree branches, and other vegetation.

The atmosphere up to 80 km (50 mi) in which the proportionality of principal gaseous constituents, such as oxygen and nitrogen, is constant.

Humid Continental Climate
A relatively severe climate characteristic of broad continents in the middle latitudes between approximately 40 and 50º north latitude. This climate is not found in the southern hemisphere, where the middle latitudes are dominated by the oceans.

Humid Subtropical Climate
A climate generally located on the eastern side of a continent and characterized by hot, sultry summers and cool winters.

A severe tropical cyclone having winds in excess of 64 knots (74 mi/hr).

An instrument that provides a continuous trace of relative humidity with time.

An instrument designed to measure the air's water vapor content. The sensing part of the instrument can be hair (hair hygrometer), a plate coated with carbon (electrical hygrometer), or an infrared sensor (infrared hygrometer).

The deterioration in one's mental and physical condition brought on by a rapid lowering of human body temperature.

Ice Cap Climate
A climate that has no monthly means above freezing and supports no vegetative cover except in a few scattered high mountain areas. This climate, with its perpetual ice and snow, is confined largely to the ice sheets of Greenland and Antarctica.

Ice Fog
A type of fog composed of tiny suspended ice particles that forms at very low temperatures.

Ice Nuclei
Particles that act as nuclei for the formation of ice crystals in the atmosphere.

Ice Pellets
See Sleet

Indian Summer
An unseasonably warm spell with clear skies near the middle of autumn. Usually follows a substantial period of cool weather.

Infrared Radiation
Electromagnetic radiation with wavelengths between about 0.7 and 1000 µm. This radiation is longer than visible radiation but shorter than microwave radiation.

The incoming solar radiation that reaches the earth and the atmosphere.

Intertropical Convergence Zone (ITCZ)
The boundary zone separating the northeast trade winds of the Northern Hemisphere from the southeast trade winds of the Southern Hemisphere.

An increase in air temperature with height.

An electrically charged atom, molecule, or particle.

An electrified region of the upper atmosphere where fairly large concentrations of ions and free electrons exist.

Brilliant spots or borders of colors, most often red and green, observed in clouds up to about 30° from the sun.

A line connecting points of equal pressure.

A line connecting points of equal wind speed.

A line connecting points of equal wind temperature.

January Thaw
A period of relatively mild weather around January 20 to 23 that occurs primarily in New England; an example of a singularity in the climatic record.

Jet Stream
Relatively strong winds concentrated within a narrow band in the atmosphere.

Katabatic Wind
Any wind blowing downslope. Usually cold.

Lake Breeze
A wind blowing onshore from the surface of a lake.

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.

Land Breeze
A coastal breeze that blows from land to sea, usually at night.

Lapse Rate
The rate at which an atmospheric variable (usually temperature) decreases with height. (See Environmental Lapse Rate)

Latent Heat
The heat that is either released or absorbed by a unit mass of a substance when it undergoes a change of state, such as during evaporation, condensation, or sublimation.

Lenticular Cloud
A cloud in the shape of a lens.

A visible electrical discharge produced by thunderstorms.

Longwave Radiation
A term most often used to describe the infrared energy emitted by the earth and the atmosphere.

The region around the earth in which the earth's magnetic field plays a dominant part in controlling the physical processes that take place.

Mammatus Clouds
Clouds that look like pouches hanging from the underside of a cloud.

Marine Climate
A climate dominated by the ocean, because of the moderating effect of water, sites having this climate are considered relatively mild.

Maritime Air Mass
An air mass that originates over the ocean. These air masses are relatively humid.

Maritime Polar Air
Cool, humid air mass that forms over the cold ocean waters of the North Pacific and North Atlantic.

Maritime Tropical Air
Warm, humid air mass that forms over tropical and subtropical oceans.

Mean Annual Temperature
The average temperature at any given location for the entire year.

The scale of meteorological phenomena that ranges in size from a few km to about 100 km. It includes local winds, thunderstorms, and tornadoes.

The atmospheric layer between the stratosphere and the thermosphere. Located at an average elevation between 50 and 80 km above the earth's surface.

The study of the atmosphere and atmospheric phenomena as well as the atmosphere's interaction with the earth's surface, oceans, and life in general.

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

The climate structure of the air space near the surface of the earth.

The smallest scale of atmospheric motions.

A unit for expressing atmospheric pressure. Sea level pressure is normally close to 1013 mb.

A refraction phenomenon that makes an object appear to be displaced from its true position. When an object appears higher than it actually is, it is called a superior image. When an object appears lower than it actually is, it is an inferior mirage.

Very thin fog in which visibility is greater than 1.0 km (0.62 mi).

A katabatic wind that flows from the Alps down the Rhone River Valley of France to the Mediterranean coast.

Mixing Ratio
The ratio of the mass of water vapor in a given volume of air to the mass of dry air.

Moist Adiabatic Rate
The rate of change of temperature in a rising or descending saturated air parcel. The rate of cooling or warming varies but a common value of 6°C per 1000 m (3.3° per 1000 ft) is used.

Molecular Viscosity
The small-scale internal fluid friction that is due to the random motion of the molecules within a smooth-flowing fluid, such as air.

Mountain and Valley Breeze
A local wind system of a mountain valley that blows downhill (mountain breeze) at night and uphill (valley breeze) during the day.

Nacreous Clouds
Clouds of unknown composition that have a soft, pearly luster and that form at altitudes about 25 to 30 km above the earth's surface. They are also called mother-of-pearl clouds.

A dark, gray cloud characterized by more or less continuously falling precipitation. It is not accompanied by lightning, thunder, or hail.

Noctilucent Clouds
Wavy, thin, bluish-white clouds that are best seen at twilight in polar latitudes. They form at altitudes about 80 to 90 km above the surface.

Nocturnal Inversion
See Radiation Inversion.

Offshore Breeze
A breeze that blows from the land out over the water. Opposite of an onshore breeze.

Onshore Breeze
A breeze that blows from the water onto the land. Opposite of an offshore breeze.

Orographic Uplift
The lifting of air over a topographic barrier. Clouds that form in this lifting process are called orographic clouds.

Orographic Precipitation
Rainfall or snowfall from clouds, induced by topographic uplift.

A layer of soil beneath the earth's surface that remains frozen throughout the year.

The splitting of a molecule by a photon.

A discrete quantity of energy that can be thought of as a packet of electromagnetic radiation traveling at the speed of light.

Pileus Cloud
A smooth cloud in the form of a cap. Occurs above, or is attached to, the top of a cumuliform cloud.

Polar Air Mass
A cold air mass that forms in a high-latitude source region.

Polar Climates
Climates in which the mean temperature of the warmest month is below 10°C; climates that are too cold to support the growth of trees.

Potential Energy
The energy that a body possesses by virtue of its position with respect to other bodies in the field of gravity.

Potential Evapotranspiration (PE)
The amount of moisture that, if it were available, would be removed from a given land area by evaporation and transpiration.

Potential Temperature
The temperature that a parcel of dry air would have if it were brought dry adiabatically from its original position to a pressure of 1000 mb.

Precipitable Water Vapor
The depth of water that would result if all the vapor in the atmosphere above a location were condensed into liquid water.

Any form of water particles-liquid or solid-that falls from the atmosphere and reaches the ground.

Prevailing Wind
The wind direction most frequently observed during a given period.

Probability Forecast
A forecast of the probability of occurrence of one or more of a mutually exclusive set of weather conditions. See also this page.

An instrument used to measure the water vapor content of the air. It consists of two thermometers (dry bulb and wet bulb). After whirling the instrument, the dew point and relative humidity can be obtained with the aid of tables.

An instrument that measures the amount of radiation.

An instrument useful for remote sensing of meteorological phenomena. It operates by sending radio waves and monitoring those returned by such reflecting objects as raindrops within clouds.

Radiant Energy (Radiation)
Energy propagated in the form of electromagnetic waves. These waves do not need molecules to propagate them, and in a vacuum they travel at nearly 300,000 km per sec.

Radiation Fog
Fog produced over land when radiational cooling reduces the air temperature to or below its dew point. It is also known as ground fog and valley fog.

Radiation Inversion
An increase in temperature with height due to radiational cooling of the earth's surface. Also called a nocturnal inversion.

A balloon-borne instrument that measures and transmits pressure, temperature, and humidity to a ground-based receiving station.

Precipitation in the form of liquid water drops that have diameters greater than that of drizzle.

Rain Gauge
A device-usually a cylindrical container-for measuring rain-fall.

Rain Shadow
The region on the leeside of a mountain where the precipitation is noticeable less than on the windward side.

An instrument carried by weather balloons to measure the temperature, humidity, pressure, and winds of the atmosphere.

The process whereby a surface turns back a portion of the radiation that strikes it.

The bending of light as it passes from one medium to another

Refractive Index
The ratio of the speed of light in a vacuum to its speed in a transparent medium.

Relative Humidity
The ratio of the amount of water vapor actually in the air compared to the amount of water vapor the air can hold at the particular temperature and pressure. The ratio of the air's actual vapor pressure to its saturation vapor pressure.

Rime Ice
A white, granular deposit of ice formed by the freezing of water drops when they come in contact with an object.

Santa Ana
The local name given a foehn wind in southern California.

Saturation Vapor Pressure
The maximum amount of water vapor necessary to keep moist air in equilibrium with a surface of pure water or ice. It represents the maximum amount of water vapor that the air can hold at any given temperature and pressure. (See Equilibrium vapor pressure.)

The process by which small particles in the atmosphere deflect radiation from its path into different directions.

The apparent twinkling of a star due to its light passing through regions of differing air densities in the atmosphere.

Sea Breeze
A coastal local wind that blows from the ocean onto the land. The leading edge of the breeze is termed a sea breeze front.

Sea Level Pressure
The atmospheric pressure at mean sea level.

See Steppe.

Sensible Heat Transfer
Movement of heat from one place to another as a consequence of conduction or convection or both.

Sensible Temperature
The sensation of temperature that the human body feels in contrast to the actual temperature of the environment as measured with a thermometer.

See wind shear.

Sheet Lightning
A fairly bright lightning flash from distant thunderstorms that illuminates a portion of the cloud.

Shortwave Radiation
A term most often used to describe the radiant energy emitted from the sun, in the visible and near ultraviolet wavelengths.

Intermittent precipitation from a cumuliform cloud, usually of short duration but often heavy.

A type of precipitation consisting of transparent pellets of ice 5 mm or less in diameter. Same as ice pellets.

Originally smog meant a mixture of smoke and fog. Today, smog means air that has restricted visibility due to pollution, or pollution formed in the presence of sunlight-photochemical smog.

Solid precipitation in the form of minute ice flakes that occur below 0°C.

An aggregate of ice crystals that falls from a cloud.

Snow Flurries
Light showers of snow that fall intermittently.

Snow Grains
Precipitation in the form of very small, opaque grains of ice. The solid equivalent of drizzle.

Snow Pellets
White, opaque, approximately round ice particles between 2 and 5 mm in diameter that form in a cloud either from the sticking together of ice crystals or from the process of accretion.

Snow Rollers
A cylindrical spiral of snow shaped somewhat like a child's muff and produced by the wind.

Snow Squall (Shower)
An intermittent heavy shower of snow that greatly reduces visibility.

Either of the two times of the year when the sun is the greatest distance from the celestial equator, occurring about June 22 and December 22. See winter solstice and summer solstice.

Southern Oscillation
The reversal of surface air pressure at opposite ends of the tropical Pacific Ocean that occur during El Niño events.

Specific Heat
The ratio of the heat absorbed (or released) by the unit mass of the system to the corresponding temperature rise (or fall).

Specific Humidity
The ratio of the mass of water vapor in a given parcel to the total mass of air in the parcel.

Spontaneous Nucleation (Freezing)
The freezing of pure water without the benefit of any nuclei.

Spring Freeze Date
The date of occurrence in the spring of the last minimum at or below a temperature threshold. See also this page.

Squall Line
Any nonfrontal line or band of active thunderstorms.

Station Pressure
The actual air pressure computed at the observing station.

Steam Fog
See Evaporation Fog.

One of the two types of dry climate. A marginal and more humid variant of the desert that separates it from bordering humid climates. Steppe also refers to the short-grass vegetation associated with this semiarid climate.

Storm Surge
An abnormal rise of the sea along a shore. Primarily due to the winds of a storm, especially a hurricane.

A low cloud, predominantly stratiform with low, lumpy, rounded masses, often with blue sky between them.

The boundary between the stratosphere and the mesosphere.

The layer of the atmosphere above the troposphere and below the mesosphere (between 10 km and 50 km), generally characterized by an increase in temperature with height.

A low, gray cloud layer with a rather uniform base whose precipitation is most commonly drizzle.

Subarctic Climate
A climate found north of the humid continental climate and south of the polar climate and characterized by bitterly cold winters and short cool summers. Places within this climatic realm experience the highest annual temperature ranges on earth.

The process whereby ice changes directly into water vapor without melting. In meteorology, sublimation can also mean the transformation of water vapor into ice. (See Deposition)

The slow sinking of air, usually associated wit high-pressure areas.

Subsidence Inversion
A temperature inversion produced by the adiabatic warming of a layer of sinking air.

Summer Solstice
Approximately June 22 in the Northern Hemisphere when the sun is highest in the sky and directly overhead at latitude 23.5° N, the Tropic of Cancer.

A colored luminous spot produced by refraction of light through ice crystals that appears on either side of the sun. Also called parhelion.

Sun Pillar
A vertical streak of light extending above (or below) the sun. It is produced by the reflection of sunlight of ice crystals.

Supersaturated Air
A condition that occurs in the atmosphere when the relative humidity is greater that 100 percent.

Surface Inversion
See Radiation Inversion.

Synoptic Scale
The typical weather map scale that shows features such as high- and low-pressure areas and fronts over a distance spanning a continent. Also called the cyclonic scale.

The northern coniferous forest; also a name applied to the subarctic climate.

The degree of hotness or coldness of a substance as measured by a thermometer. It is also a measure of the average speed or kinetic energy of the atoms and molecules in a substance.

Temperature Inversion
An extremely stable air layer in which temperature increases with altitude, the inverse of the usual temperature profile in the troposphere.

Terminal Velocity
The constant speed obtained by a falling object when the upward drag on the object balances the downward force of gravity.

A small, rising parcel of warm air produced when the earth's surface is heated unevenly.

A recording instrument that gives a continuous trace of temperature with time.

An instrument used to measure temperature.

The atmospheric layer above the mesosphere. It extends from 90 km to outer space.

The sound due to rapidly expanding gases along the channel of a lightning discharge.

Tipping Bucket Rain Gauge
A device that accumulates rainfall in increments of 0.01 in. by containers that alternately fill and empty (tip).

An intense, rotating column of air that protrudes from a cumulonimbus cloud in the shape of a funnel or a rope and touches the ground. (See Funnel Cloud)

Trade Winds
The winds that occupy most of the tropics and blow from the subtropical highs to the equatorial low.

The release of water vapor to the atmosphere by plants.

Tropical Air Mass
A warm-to-hot air mass that forms in the subtropics.

Tropical Depression
A mass of thunderstorms and clouds generally with a cyclonic wind circulation of between 20 and 34 knots.

Tropical Disturbance
An organized mass of thunderstorms with a slight cyclonic wind circulation of less than 20 knots.

Tropical Storm
Organized thunderstorms with a cyclonic wind circulation between 35 and 64 knots.

The boundary between the troposphere and the stratosphere.

The layer of the atmosphere extending from the earth's surface up to the tropopause (about 10 km above the ground).

Tundra Climate
Found almost exclusively in the northern hemisphere or at high altitudes in many mountainous regions. A treeless climatic realm of sedges, grasses, mosses, and lichens that is dominated by a long, bitterly cold winter.

Any irregular or disturbed flow in the atmosphere that produces gusts and eddies.

The time immediately before sunrise and after sunset when the sky remain illuminated.

A hurricane that forms in the western Pacific Ocean.

Ultraviolet Radiation
Electromagnetic radiation with wave-lengths longer than x-rays but shorter than visible light.

Upslope Fog
Fog formed as moist, stable air flows upward over a topographic barrier.

Upslope Precipitation
Precipitation that forms due to moist, stable air gradually rising along an elevated plain. Upslope precipitation is common over the western Great Plains, especially east of the Rock Mountains.

The rising of water (usually cold) toward the surface from the deeper regions of a body of water.

Urban Heat Island
The increased air temperatures in urban areas as contrasted to the cooler surrounding rural areas.

Valley Breeze
See Mountain Breeze.

Valley Fog
See Radiation Fog.

Vapor Pressure
The pressure exerted by the water vapor molecules in a given volume of air.

Vernal Equinox
The equinox at which the sun approaches the Northern Hemisphere and passes directly over the equator. Occurs around March 20.

Precipitation that falls from a cloud but evaporates before reaching the ground. (See Fall streaks.)

Virtual temperature
An adjustment applied to the real air temperature to account for a reduction in air density due to the presence of water vapor.

The resistance of fluid flow.

The greatest distance an observer can see and identify prominent objects.

Visible Light
That portion of the electromagnetic spectrum from 0.4 to 0.7 µm wavelengths that is visible.

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

Wall Cloud
A localized, persistent, often abrupt lowering from a rain-free base. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. "Wall cloud" also is used occasionally in tropical meteorology to describe the inner cloud wall surrounding the eye of a tropical cyclone, but the proper term for this feature is eyewall.

Warm Front
The leading edge of a warm air mass.

Water Balance
The comparison of actual and potential evapotranspiration with the amount of precipitation, usually on a monthly basis.

Water Budget
Balance sheet for the inputs and outputs of water to and from the various global water reservoirs.

Water Equivalent
The depth of water that would result from the melting of a snow sample. Typically about 10 inches of snow will melt to 1 inch of water, producing a water equivalent of 10 to 1.

The state of the atmosphere in terms of such variables as temperature, cloudiness, precipitation, and radiation.

Weighing Bucket Rain Gauge
A device that is calibrated so that the weight of rainfall is recorded directly in terms of rainfall in millimeters or in inches.

Wet-Bulb Depression
The difference in degrees between the air temperature (dry-bulb temperature) and the wet-bulb temperature.

Wet-Bulb Temperature
The lowest temperature that can be obtained by evaporating water into the air.

White Frost
Ice crystals that form on surfaces instead of dew when the dew point is below freezing.

Wind Chill Equivalent Temperature
A theoretical air temperature at which the heat loss from exposed skin under calm conditions is equivalent to the heat loss at the actual air temperature and under the actual wind speeds.

Wind-Chill Factor
The cooling effect of any combination of temperature and wind, expressed as the loss of body heat. Also called wind-chill index.

Wind Shear
A difference in wind speed or direction between two wind currents in the atmosphere.

Wind Vane
An instrument used to determine wind direction.

A large, conical, open bag designed to indicate wind direction and relative speed; usually used at small airports.

Winter Solstice
Approximately December 22 in the Northern Hemisphere when the sun is lowest in the sky and directly overhead at latitude 23.5°S, the Tropic of Capricorn.


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