A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

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.

Nimbostratus 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.

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

Photodissociation The splitting of a molecule by a photon.

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.

Precipitation 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.

Psychrometer 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.

Pyranometer An instrument that measures the amount of radiation.

Radar 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.

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.

Rain gage 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.

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

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

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.)

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 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.

Semiarid 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.

Shear 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.

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

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 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.

Solstice 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 Nino events.

Specifc 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.

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.

Steppe 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.

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

Stratopause The boundary between the stratosphere and the mesosphere.

Stratosphere 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.

Stratus 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.

Sublimation 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.)

Subsidence 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.

Sundog 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.

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

Temperature 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.

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

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

Thermometer An instrument used to measure temperature.

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

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

Tipping bucket rain gage A device that accumulates rainfall in increments of 0.01 in. by containers that alternately fill and empty (tip).

Tornado 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.

Transpiration 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.

Tropopause The boundary between the troposphere and the stratosphere.

Troposphere 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.

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

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

Typhoon 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.

Upwelling 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.

Virga 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.

Viscosity The resistance of fluid flow.

Visibility 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 ?m wavelengths that is visible.

Vorticity 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.

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.

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

Weighing bucket rain gage 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.

Windsock 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.

Refrences:

C. Donald Ahrens, 1985. Meteorology Today. West Publishing Company, 497-509.

Frederick K. Lutgens, and Edward J. Tarbuck, 1995. The Atmosphere. Prentice-Hall Inc., 443-456.

Joe R. Eagleman, 1980. Meteorology The Atmosphere in Action. Litton Educational Publishing Inc., 353-364.

Joseph M. Moran, and Michael D. Morgan, 1986. Meteorology. Burgess Publishing, 486-496.