ATMO336 - Glossary

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 reading of -273°C, -460°F, or OK. Theoretically, there is no molecular motion at this temperature.

Absolutely stable air

An atmospheric condition that exists when a lifted parcel of air is colder than the air around it.

Absolutely unstable air

An atmospheric condition that exists when a lifted parcel of air is warmer than the air around it.

Accretion

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

Acid deposition

The depositing of acidic particles (usually sulfuric acid and nitric acid) at the earth's surface. Acid deposition occurs in dry form (dry deposition) or wet form (wet deposition). Acid rain and acid precipitation often denote wet deposition. (See Acid rain.)

Acid fog

See Acid rain.

Acid rain

Cloud droplets or raindrops combining with gaseous pollutants, such as oxides of sulfur and nitrogen, to make falling rain (or snow) acidic - pH less than about 5.0. If fog droplets combine with such pollutants it becomes acid fog.

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.

Advection

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.

Aerosols

Tiny suspended solid particles (dust, smoke, etc.) or liquid droplets that enter the atmosphere from either natural or human (anthropogenic) sources, such as the burning of fossil fuels.

Aerovane

A wind instrument that indicates or records both wind speed and wind direction.

Air density

See Density.

Air mass

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

Air mass (ordinary) thunderstorm

A thunderstorm produced by local convection within an unstable air mass.

Air mass weather

A persistent type of weather that may last for several days (up to a week or more). It occurs when an area comes under the influence of a particular air mass.

Air parcel

See Parcel of air.

Air pollutants

Solid, liquid, or gaseous airborne substances that occur in concentrations high enough to threaten the health of people and animals, to harm vegetation and structures, or to toxify a given environment.

Air pressure (atmospheric pressure)

The pressure exerted by the weight of air above a given point, usually expressed in millibars (mb) or inches of mercury (Hg).

Albedo

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

Aleutian low

The subpolar low-pressure area that is centered near the Aleutian Islands on charts that show mean sea level pressure.

Altimeter

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.

Altocumulus

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

Altostratus

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.

Analogue method of forecasting

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

Analysis

The drawing and interpretation of the patterns of various weather elements on a surface or upper-air chart.

Anemometer

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.

Anticyclone

An area of high pressure around which the wind blows clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

Anthropogenic

of, relating to, or resulting from the influence of human beings on nature

Apparent temperature

What the air temperature "feels like" for various combinations of air temperature and relative humidity.

Arena cloud

See Roll cloud.

Arid climate

An extremely dry climate - drier than the semi-arid climate. Often referred to as a "true desert" climate.

Atmosphere

The envelope of gases that surround a planet and are held to it by the planet's gravitational attraction. The earth's atmosphere is mainly nitrogen and oxygen.

Atmospheric greenhouse effect

The warming of an atmosphere by its absorbing and emitting infrared radiation while allowing shortwave radiation to pass on through. The gases mainly responsible for the earth's atmospheric greenhouse effect are water vapor and carbon dioxide. Also called the greenhouse effect.

Atmospheric models

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

Atmospheric stagnation

A condition of light winds and poor vertical mixing that can lead to a high concentration of pollutants. Air stagnations are most often associated with fair weather, an inversion, and the sinking air of a high-pressure area.

Atmospheric window

The wavelength range between 8 and 11 micrometers in which little absorption of infrared radiation takes place.

Aurora

Glowing light display in the nighttime sky caused by excited gases in the upper atmosphere giving off light. In the Northern Hemisphere it is called the aurora borealis (northern lights); in the Southern Hemisphere, the aurora australis (southern lights).

Autumnal equinox

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

Absolute Humidity

Measurement of atmospheric humidity. Absolute humidity is the mass of water vapor in a given volume of air (this measurement is not influenced by the mass of the air). Normally expressed in grams of water vapor per cubic meter of atmosphere.

Acid

Substance having a pH less than 7.

Acid Deposition

Atmospheric deposition of acids in solid or liquid form on the Earth's surface. Also see acid precipitation.

Acid Precipitation

Atmospheric precipitation with a pH less than 5.6. Normal pH of precipitation is 5.6.

Air Pollution

Toxification of the atmosphere through the addition of one or more harmful substances in the air. Substance must be in concentrations high enough to be hazardous to humans, other animals, vegetation, or materials.

Albedo

The reflective quality of a surface, expressed as the percentage of reflected insolation to incoming insolation; a function of surface color, angle of incidence, and surface texture.

Atmosphere

The thin veil of gases surrounding the Earth, which forms a protective boundary between outer space and the biosphere; generally considered to extend about 480 km (300 mi) elevation from the Earth's surface.

Ball lightning

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

Barograph

A recording barometer.

Barometer

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

Billow clouds

Broad, nearly parallel lines of clouds oriented at right angles to the wind.

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.

Blizzard

A severe weather condition characterized by low temperatures and strong winds (greater than 35 mi/hr) bearing a great amount of snow either falling or blowing. When these conditions continue after the falling snow has ended, it is termed a ground blizzard.

California current

The ocean current that flows southward along the west coast of the United States from about Washington to Baja California.

Cap cloud

See Pileus cloud.

Carbon dioxide (CO2)

A colorless, odorless gas whose concentration is about 0.035 percent (355 ppm) in a volume of air near sea level. It is a selective absorber of infrared radiation and, consequently, it is important in the earth's atmospheric greenhouse effect. Solid CO2 is called dry ice.

Carbon monoxide (CO)

A colorless, odorless, toxic gas that forms during the incomplete combustion of carbon-containing fuels.

Celsius scale

A temperature scale where zero is assigned to the temperature where water freezes and 100 to the temperature where water boils (at sea level).

Centripetal acceleration

The inward-directed acceleration on a particle moving in a curved path.

Centripetal force

The radial force required to keep an object moving in a circular path. It is directed toward the center of that curved path.

Chinook wall cloud

A bank of clouds over the Rocky Mountains that signifies the approach of a chinook.

Chinook wind

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

Chlorofluorocarbon (CFC)

a compound consisting of chlorine,fluorine, and carbon.

CFCs are very stable in the troposphere. They are broken down by strong ultraviolet light in the stratosphere and release chlorine atoms that then deplete the ozone layer. CFCs are commonly used as refrigerants, solvents, and foam blowing agents. The most common CFCs are CFC-11, CFC-12, CFC-113, CFC-114, and CFC-115. The ozone depletion potential (ODP) for each CFC is, respectively, 1, 1, 0.8, 1, and 0.6.

Chlorophyll

A light-sensitive pigment that resides within the chloroplast bodies of plants in leaf cells; the basis of photosynthesis.

Cirrocumulus

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

Cirrostratus

High, thin, sheetlike clouds, composed of ice crystals. They frequently cover the entire sky and often produce a halo.

Cirrus

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

Clear air turbulence (CAT)

Turbulence encountered by aircraft flying through cloudless skies. Thermals, wind shear, and jet streams can each be a factor in producing CAT.

Clear ice

A layer of ice that appears transparent because of its homogeneous structure and small number and size of air pockets.

Climate

The accumulation of daily and seasonal weather events over a long period of time.

Climatic controls

The relatively permanent factors that govern the general nature of the climate of a region.

Climatic optimum

A period in geological history (about 7000 to 5000 years ago) when temperatures were warmer than at present.

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.

Cloudburst

Any sudden and heavy rain shower.

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.

Coalescence

The merging of cloud droplets into a single larger droplet.

Cold front

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

Cold occlusion

See Occluded front.

Cold wave

A rapid fall in temperature within 24 hours that often requires increased protection for agriculture, industry, commerce, and human activities.

Computer enhancement

A process where the temperatures of radiating surfaces are assigned different shades of gray (or different colors) on an infrared picture. This allows specific features to be more clearly delineated.

Condensation

The process by which water vapor becomes a liquid.

Condensation level

The level above the surface marking the base of a cumuliform cloud.

Condensation nuclei

Tiny particles upon whose surfaces condensation of water vapor begins in the atmosphere.

Conditionally unstable air

An atmospheric condition that exists when the environmental lapse rate is between the dry adiabatic rate and the moist adiabatic rate. Also called conditional instability.

Conduction

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 arctic air mass

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

Continental polar air mass

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

Continental tropical air mass

An air mass characterized by high temperatures and low humidity.

Contour line

A line that connects points of equal elevation above a reference level, most often sea level.

Contrail (condensation trail)

A cloudlike streamer frequently seen forming behind aircraft flying in clear, cold, humid air.

Controls of temperature

The main factors that cause variations in temperature from one place to another.

Convection

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.)

Convergence

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.

Coriolis force

An apparent force observed on any free moving object in a rotating system. On the earth this deflective force results from the earth's rotation and causes moving particles (including the wind) to deflect to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Corona (optic)

A series of colored rings concentrically surrounding the disk of the sun or moon. Smaller than the halo, the corona is caused by the diffraction of light around small water droplets of uniform size.

Country breeze

A light breeze that blows into a city from the surrounding countryside. It is best observed on clear nights when the urban heat island is most pronounced.

Crepuscular rays

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

Cumulonimbus

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.

Cumulus

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 congestus or towering cumulus.

Cumulus stage

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

Cyclogenesis

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

Cyclone

An area of low pressure around which the winds blow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.

Doppler radar

Weather radar that measures direction and speed of a moving object, such as drops of precipitation, by determining whether atmospheric motion is horizontally toward or away from the radar. Using the Doppler effect, it measures the velocity of particles. Named for J. Christian Doppler, an Austrian physicist, who in 1842 explained why the whistle of an approaching train had a higher pitch than the same whistle when the train was going away.

Electromagnetic radiation

All objects above the temperature of absolute zero (-273.15 degrees Celsius) radiate energy to their surrounding environment. This energy, or radiation, is emitted as electromagnetic waves that travel at the speed of light. Many different types of radiation have been identified. Each of these types is defined by its wavelength. The wavelength of electromagnetic radiation can vary from being infinitely short to infinitely long.

Fossil fuel

any of a class of materials of biological origin occurring within the Earth's crust that can be used as a source of energy.

Fossil fuels include coal, natural gas, petroleum, shale oil, and bitumen. They all contain carbon and were formed as a result of geologic processes from the remains of organic matter produced by photosynthesis hundreds of millions of years ago.

Greenhouse effect

The greenhouse effect is the name applied to the process which causes the surface of the Earth to be warmer than it would have been in the absence of an atmosphere.

Greenhouse gas

Gases such as water vapor, carbon dioxide, and methane, which are relatively transparent to sunlight but absorb and emit longwave radiation.

Heat Capacity

Is the ratio of the amount of heat energy absorbed by a substance compared to its corresponding temperature rise.

Infrared Radiation

Electromagnetic radiation with a wavelength between 0.7 and 1000 micrometers. Also called longwave radiation.

Insolation

Solar radiation that is intercepted by Earth.

Ionosphere

A layer in the atmosphere above 80 km (50 mi), where gamma, X-ray, and some ultraviolet radiation is absorbed and converted into infrared, and where the solar winds stimulates the auroras.

Isotherm

An isoline connecting all points of equal temperature.

Latent heat

Heat energy is stored in one of three states- ice, water, or water vapor. The energy is absorbed or released in each phase change from one state to another. Heat energy is absorbed as the latent heat of melting, vaporization, or evaporation. Heat energy is released as the latent heat of condensation and freezing.

Latitude

The angular distance measured north or south of the equator from a point at the center of the Earth. A line connecting all points of the same latitudinal angle is called a parallel.

Lithosphere

Earth's crust and that portion of the uppermost mantle directly below the crust, extending down to about 70 km (45 miles). Some use this term to refer to the entire Earth.

Longitude

The angular distance measured east or west of a prime meridian from a point at the center of the Earth. A line connecting all points of the same longitude is called a meridian.

Longwave Radiation

See infrared radiation.

Mesosphere

The upper region of the homosphere from 50 to 80 km (30 to 50 mi) above the ground; designated by temperature criteria; has very low pressures.

Meteorology

The scientific study of the atmosphere, including its physical characteristics and motions; related chemical, physical, and geological processes; the complex linkages of atmospheric systems; and weather forecasting.

Methane

A radiatively active gas that participates in the greenhouse effect; derived from the organic processes of burning, digesting, and rotting in the presence of oxygen; CH₄.

Nitrogen dioxide

A noxious reddish-brown gas produced in combustion engines; can be damaging to human respiratory tracts and to plants; participates in photochemical reactions and acid deposition.

Ozone

Ozone is a bluish gas that is harmful to breathe. Nearly 90% of the Earth's ozone is in the stratosphere and is referred to as the ozone layer. Ozone absorbs a band of ultraviolet radiation called UVB that is particularly harmful to living organisms. The ozone layer prevents most UVB from reaching the ground.

Ozone layer

the region of the stratosphere containing the bulk of atmospheric ozone

The ozone layer lies approximately 15-40 kilometers (10-25 miles) above the Earth's surface, in the stratosphere. Depletion of this layer by ODS will lead to higher UVB levels, which in turn will cause increased skin cancers and cataracts and potential damage to some marine organisms, plants, and plastics.

Peroxycetyl nitrate (PAN)

A pollutant formed from photochemical reactions involving nitric oxide (NO) and hydrocarbons (HC). PAN produces no known human health effect, but it is particularly damaging to plants.

Photochemical smog

Air pollution produced by the interaction of ultraviolet light, nitrogen dioxide, and hydrocarbons; produces ozone and PAN through a series of complex photochemical reactions. Automobiles are the major source of the contributive gases.

Photosynthesis

the process by which green plants and certain other organisms transform light energy into chemical energy. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds.

Radar

Acronym for RAdio Detection And Ranging. An electronic instrument used to detect distant objects and measure their range by how they scatter or reflect radio energy. Precipitation and clouds are detected by measuring the strength of the electromagnetic signal reflected back.

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 kilometers per second.

Radiational cooling

The process by which the earth's surface and adjacent air cool by emitting infrared radiation.

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.

Radiative equilibrium temperature

The temperature achieved when an object, behaving as a black body, is absorbing and emitting radiation at equal rates.

Radiometer

An instrument designed to measure the intensity of radiation (usually infrared) emitted by an object.

Radiosonde

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

Rain

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

Rainbow

An arc of concentric colored bands that spans a section of the sky when rain is present and the sun is positioned at the observer's back.

Rain gauge

An instrument designed to measure the amount of rain that falls during a given time interval.

Rain shadow

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

Rawinsonde observation

A radiosonde observation that includes wind data.

Reflection

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

Refraction

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

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 that particular temperature and pressure. The ratio of the air's actual vapor pressure to its saturation vapor pressure.

Respiration

The process by which plants use their food to derive energy for their operations; essentially, the reverse of the photosynthetic process; releases carbon dioxide, water, and heat into the environment.

Return stroke

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

Ridge

An elongated area of high atmospheric pressure.

Rime ice

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

Riming

See Accretion.

Roll cloud

A dense, roll-shaped cloud attached to the lower front part of the main cloud. It often forms with thunderstorms along the leading edge of a gust front. Also called an arcus cloud.

Rotor cloud

A turbulent cumuliform type of cloud that forms on the leeward side of large mountain ranges. The air in the cloud rotates about an axis parallel to the range.

Rotors

Turbulent eddies that form downwind of a mountain chain, creating hazardous flying conditions.

Saflir-Simpson scale

A scale relating a hurricane's central pressure and winds to the possible damage it is capable of inflicting.

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.

Shower

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

Sleet

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

Smog

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.

Snow

A solid form of precipitation composed of ice crystals in complex hexagonal form.

Snowflake

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 millimeters in diameter that form in a cloud either from the sticking together of ice crystals or from the process of accretion.

Snow squall (shower)

An intermittent heavy shower of snow that greatly reduces visibility.

Solar constant

The rate at which solar energy is received on a surface at the outer edge of the atmosphere perpendicular to the sun's rays when the earth is at a mean distance from the sun. The value of the solar constant is about two calories per square centimeter per minute or about 1376 W/m2 in the SI system of measurement.

Sounding

An upper-air observation, such as a radiosonde observation. A vertical profile of an atmospheric variable such as temperature or winds.

Source regions

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

Southern oscillation

The reversal of surface air pressure at opposite ends of the tropical Pacific Ocean that occur during major El Nino 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).

St. Elmo's fire

A bright electric discharge that is projected from objects (usually pointed) when they are in a strong electric field, such as during a thunderstorm.

Santa Ana wind

A warm, dry wind that blows into southern California from the east off the elevated desert plateau. Its warmth is derived from compressional heating.

Saturation (of air)

An atmospheric condition whereby the level of water vapor is the maximum possible at the existing temperature and pressure.

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.

Savanna

A tropical or subtropical region of grassland and drought-resistant vegetation. Typically found in tropical wet- and-dry climates.

Scales of motion

The hierarchy of atmospheric circulations from tiny gusts to giant storms.

Scattering

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

Scintillation

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 at the surface blows from the ocean onto the land.

Sea level pressure

The atmospheric pressure at mean sea level.

Secondary air pollutants

Pollutants that form when a chemical reaction occurs between a primary air pollutant and some other component of air.

Selective absorbers

Substances such as water vapor, carbon dioxide, clouds, and snow that absorb radiation only at particular wavelengths.

Semi-arid climate

A dry climate where potential evaporation and transpiration exceed precipitation. Not as dry as the arid climate. Typical vegetation is short grass.

Sensible heat

The heat we can feel and measure with a thermometer.

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.

Severe thunderstorms

Intense thunderstorms capable of producing heavy showers, flash floods, hail, strong and gusty surface winds, and tornadoes.

Specific humidity

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

Squall line

Any nonfrontal line or band of active thunderstorms.

Stable air

See Absolutely stable air.

Standard atmosphere

A hypothetical vertical distribution of atmospheric temperature, pressure, and density in which the air is assumed to obey the gas law and the hydrostatic equation. The lapse rate of temperature in the troposphere is taken as 6.5°C/1000 m or 3.6°F/1000 ft.

Standard atmospheric pressure

A pressure of 1013.26 millibars (mb), 29.92 inches of mercury (Hg), 760 millimeters (mm) of mercury, 14.7 pounds per square inch (lb/in.2), 101,326 pascals (Pa).

Stationary front

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

Station pressure

The actual air pressure computed at the observing station.

Steady-state 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.

Steam fog

See Evaporation (mixing) fog.

Steppe

An area of grass-covered, treeless plains that has a semi-arid climate.

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.

Storm surge

An abnormal rise of the sea along a shore; primarily due to the winds of a storm, especially a hurricane.

Stratocumulus

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

Stratosphere

The layer of the atmosphere above the troposphere and below the mesosphere (between 10 km and 60 km), generally characterized by an increase in temperature with height.

Stratospheric polar night jet

A jet stream that forms near the top of the stratosphere over polar latitudes during the winter months.

Stratus

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

Streamline

A line that shows the wind flow pattern.

Sublimation

The process whereby ice changes directly into water vapor without melting.

Subpolar climate

A climate observed in the Northern Hemisphere that borders the polar climate. It is characterized by severely cold winters and short, cool summers. Also known as taiga climate and boreal climate.

Subpolar low

A belt of low pressure located between 60° and 70° latitude. In the Northern Hemisphere, this "belt" consists of the Aleutian low in the North Pacific and the Icelandic low in the North Atlantic. In the Southern Hemisphere, it exists around the periphery of the Antarctic continent.

Subsidence

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

Subsidence inversion

A temperature inversion produced by compressional warming - the adiabatic warming of a layer of sinking air.

Subtropical high

A semipermanent high in the subtropical high-pressure belt centered near 30° latitude. The Bermuda high is located over the Atlantic Ocean off the east coast of North America. The Pacific high is located off the west coast of North America.

Subtropical Jet stream

The jet stream typically found between 20° and 30° latitude at altitudes between 12 and 14 km.

Suction vortices

Small, rapidly rotating whirls perhaps 10 meters in diameter that are found within large tornadoes.

Sulfur dioxide (SO2)

A colorless gas that forms primarily in the burning of sulfur-containing fossil fuels.

Summer solstice

Approximately June 22 in the Northern Hemisphere when the sun is highest in the sky and directly overhead at latitude 23M"N, the Tropic of Cancer.

Sundog

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

Sun pillar

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

Sunspots

Relatively cooler areas on the sun's surface. They represent regions of an extremely high magnetic field.

Supercell storm

An enormous severe thunderstorm whose updrafts and downdrafts are nearly in balance, allowing it to maintain itself for several hours. It can produce large hail and tornadoes.

Supercooled cloud droplets

Liquid cloud droplets observed at temperatures below freezing.

Superior mirage

See Mirage.

Supersaturated air

A condition that occurs in the atmosphere when the relative humidity is greater than 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.

Sensible heat

Heat that can be measured with a thermometer; a measure of the concentration of kinetic energy from molecular motion.

Specific Heat

Is the heat capacity of a unit mass of a substance or heat needed to raise the temperature of 1 gram (g) of a substance 1 degree Celsius.

Stomata

Small openings on the undersides of leaves, through which water and gases pass.

Stratosphere

That portion of the homosphere that ranges from 20 to 50 km (12.5 to 30 mi) above the Earth's surface, with temperatures ranging from -70 degrees (F) at the tropopause to 32 degrees (F) at the stratopause. The functional ozonosphere is within the stratosphere.

Sulfur dioxide

A colorless gas detected by its pugent odor; produced by the combustion of fossil fuels that contain sulfur as an impurity; can react in the atmosphere to form sulfuric acid, a component of acid deposition.

Thermopause

A zone approximately 300 mi. in altitude that serves conceptually as the top of the atmosphere; an altitude used for the determination of the solar constant.

Thermosphere

A region of the heterosphere extending from 50 to 300 mi. in altitude; contains the functional ionosphere layer.

Total runoff

Surplus water that flows across a surface toward stream channels; formed by sheet flow, combined with precipitation and subsurface flows into those channels.

Transpiration

The movement of water vapor out through the pores in leaves; the water is drawn by their roots from soil moisture storage.

Troposphere

The home of the biosphere; the lowest layer of the homosphere, containing approximately 90 percent of the total mass of the atmosphere; extends up to the tropopause, defined by a temperature of -70 degrees (F); occurring at an altitude of 11 mi. at the equator, 8 mi. in the middle latitudes, and at lower altitudes near the poles.

Ultraviolet radiation

Electromagnetic radiation with a wavelength between 0.1 and 0.4 micrometers.

Wall cloud

An area of rotating clouds that extends beneath a severe thunderstorm and from which a funnel cloud may appear.

Warm-core low

A low-pressure area that is warmer at its center than at its periphery. Tropical cyclones exhibit this temperature pattern.

Warm front

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

Warm occlusion

See Occluded front.

Warm sector

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

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.

Waterspout

A column of rotating wind over water that has characteristics of a dust devil and tornado.

Water vapor

Water in a vapor (gaseous) form. Also called moisture.

Wave cyclone

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

Wavelength

The distance between successive crests, troughs, or identical parts of a wave.

Weather

The condition of the atmosphere at any particular time and place.

Weather elements

The elements of air temperature, air pressure, humidity, clouds, precipitation, visibility, and wind that determine the present state of the atmosphere, the weather.

Westerlies

The dominant westerly winds that blow in the middle latitudes on the poleward side of the subtropical high pressure areas.

Wet-bulb temperature

The lowest temperature that can be obtained by evaporating water into the air.

Whirlwinds

See Dust devils.

WHO

World Health Organization.

Wind

Air in motion relative to the earth's surface.

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 direction

The direction from which the wind is blowing.

Wind machines

Fans placed in orchards for the purpose of mixing cold surface air with warmer air above.

Wind profiler

A Doppler radar capable of measuring the turbulent eddies that move with the wind. Because of this, it is able to provide a vertical picture of wind speed and wind direction.

Wind rose

A diagram that shows the percent of time that the wind blows from different directions at a given location over a given time.

Wind shear

The rate of change of wind speed or wind direction over a given distance. Most often we are concerned with vertical wind shear, which is a measure of how the horizontal winds at different altitudes change as you move up or down.

Wind vane

An instrument used to indicate wind direction.

Windward side

The side of an object facing into the wind.

Winter solstice

Approximately December 22 in the Northern Hemisphere when the sun is lowest in the sky and directly overhead at latitude 23 deg S,the Tropic of Capricorn.

Zonal wind flow

A wind that has a predominate west-to-east component.