Glossary of Terms

Essentials of Meteorology: An Invitation to the Atmosphere

C. Donald Ahrens

1998 Wadsworth Publishing Company, Belmont, CA

ISBN: 0-534-53766-9

absolute humidity – The mass of water vapor in a given volume of air. It represents the density of water vapor in the air.

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 5.0. If fog droplets combine with such pollutants it becomes acid fog.

aerosols – Tiny suspended solid particles (dust, smoke, etc.) or liquid droplets that enter the atmosphere from either natural or human sources, such as the burning of fossil fuels. Sulfur-containing fossil fuels, such as coal, produce sulfate aerosols.

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

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.

anemometer – An instrument designed to measure wind speed

aneroid barometer – An instrument designed to measure atmospheric pressure. It contains no liquid.

apparent temperature – What the air temperature “feels like” for various combinations of air temperature and relative humidity

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 reemitting 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.

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

Chlorofluorocarbons (CFCs) – Compounds consisting of methane (CH₄) or ethane (C₂H₆) with some or all of the hydrogen replaced by chlorine or fluorine. Used in fire extinguishers, as refrigerants, as solvents for cleaning microcircuits, and as propellants. CFCs contribute to the atmospheric greenhouse effect and they release chlorine in the stratosphere that destroys ozone.

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

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.

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

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.

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

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.

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

daily range of temperature – The difference between the maximum and minimum temperatures for any given day.

density – The ratio of the mass of a substance to the volume occupied by it. Air density is usually expressed as grams per cubic centimeter or kilograms per cubic meter.

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.

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

Doppler radar – A radar that determines the velocity of falling precipitation either toward or away from the radar unit by taking into account the Doppler shift.

Doppler shift (effect) – The change in the frequency of waves that occurs when the emitter or the observer is moving toward or away from the other.

El Niño – An extensive ocean warming that begins along the coast of Peru and Ecuador. Major El Niño events occur once every 3 to 7 years as a current of nutrient-poor tropical water moves southward along the west coast of South America.

energy – The property of a system that generally enables it to do work. Some forms of energy are kinetic, radiant, potential, chemical, electric, and magnetic.

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

ENSO (El Niño/Southern Oscillation) – A condition in the tropical Pacific whereby the reversal of surface air pressure at opposite ends of the Pacific Ocean induces westerly winds, a strengthening of the equatorial countercurrent, and extensive ocean warming.

front – The transition zone between two distinct air masses.

gas law – The thermodynamic law applied to a perfect gas that relates the pressure of the gas to its density and absolute temperature.

Gulf Stream – A warm, swift, narrow ocean current flowing along the east coast of the United States.

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

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

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

heterosphere – The region of the atmosphere above about 85 kilometers where the composition of the air varies with height.

homosphere – The region of the atmosphere below about 85 kilometers where the composition of the air remains fairly constant.

humidity – A general term that refers to the air’s water vapor content.

hurricane – A severe tropical cyclone having winds in excess of 64 knots (74 miles per hour)

hydrostatic equilibrium – The state of the atmosphere when there is a balance between the vertical pressure gradient force and the downward pull of gravity.

hygrometer – An instrument designed to measure the air’s water vapor content.

infrared radiation – Electromagnetic radiation with wavelengths between about 0.7 and 1000 micrometers. This radiation is longer than visible radiation but shorter than microwave radiation.

inversion – An increase in air temperature with height.

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

isobar – A line connecting points of equal pressure.

isotach – A line connecting points of equal wind speed.

isotherm – A line connecting points of equal temperature.

jet stream – Relatively strong winds concentrated within a narrow band in the atmosphere.

knot – A unit of speed equal to 1 nautical mile per hour. 1 knot equals 1.15 miles per hour.

La Niña – A condition where the central and eastern tropical Pacific Oceans turns colder than normal.

lightning – A visible electrical discharge produced by thunderstorms.

mean annual temperature – The average temperature at any given location for the entire year.

mean daily temperature – The average of the highest and lowest temperature for a 24-hour period.

mesoclimate – The climate of an area ranging in size from a few acres to several square kilometers.

mesohigh – A relatively small area of high atmospheric pressure that forms beneath a thunderstorm

mesoscale – The scale of meteorological phenomena that range in size from a few kilometers to about 100 kilometers. It includes local winds, thunderstorms, and tornadoes.

mesosphere – The atmospheric layer between the stratosphere and the thermosphere. Located at an average elevation between 50 and 80nkilometers above the earth’s surface.

meteorology – 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.

microscale – The smallest scale of atmospheric motions.

millibar (mb) – A unit for expressing atmospheric pressure. Sea level pressure is normally close to 1013 millibars.

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

obliquity (of the earth’s axis) – The tilt of the earth’s axis. It represents the angle from the perpendicular to the plane of the earth’s orbit.

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

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

ozone hole – A sharp drop in stratospheric ozone concentration observed over the Antarctic during the Antarctic spring.

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

pollutants – Any gaseous, chemical, or organic matter that contaminates the atmosphere, soil, or water.

pollutant standards index (PSI) – An index of air quality that provides daily air pollution concentrations. Intervals on the scale relate to potential health effects.

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

pressure gradient – The rate of decrease of pressure per unit of horizontal distance. On the same chart, when the isobars are close together, the pressure gradient is steep. When the isobars are far apart, the pressure gradient is weak.

pressure tendency – The rate of change of atmospheric pressure within a specified period of time most often 3 hours. Same as barometric tendency.

prevailing westerlies – The dominant westerly winds that blow in middle latitude on the poleward side of the subtropical high pressure areas. Also called westerlies.

prevailing wind – the wind direction most frequently observed during a given period.

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.

radar – An electronic instrument used to detect object (such as falling precipitation) by their ability to reflect and scatter microwaves back to a receiver.

radiation inversion – An increase in temperature with height due to radiational cooling of the earth’s surface. Also called nocturnal inversion.

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

rain shadow – The region on the lee side of a mountain where the precipitation in noticeably less than on the windward side.

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 required for saturation (at a particular temperature and pressure). The ration of the air’s actual vapor pressure to its saturation vapor pressure.

semipermanent highs and lows – Areas of high pressure (anticyclones) and low pressure (extratropical cyclones) that tend to persist at a particular latitude belt throughout the year. In the Northern Hemisphere, typically they shift slightly northward in summer and slightly southward in winter.

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.

solar constant – The rate at which solar energy is received on a surface at the outer edge of the atmospheric 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 2 calories per square centimeter per minute or about 1376 watts per square meter in the SI system of measurement.

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.

stable air (absolutely stable atmosphere) – An atmospheric condition that exists when a lifted parcel of air is colder than the air around it.

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.

standard atmospheric pressure – A pressure of 1013.25 millibars (mb), 29.92 inches of mercury (Hg), 760 millimeters (mm) of mercury, 14.7 pounds per square inch (lb./in.2), 101,325 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.

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

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

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.

supersaturation – A condition whereby the atmosphere contains more water vapor than is needed to produce saturation with respect to a flat surface of pure water or ice, and the relative humidity is greater than 100 percent.

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 cyclonic scales.

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 increase in air temperature with height. Often simply called an inversion.

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

thermograph – An instrument that measures and records air temperature.

thermometer – An instrument for measuring temperature. The most common are liquid-in-glass, which have a sealed glass tube attached to a glass bulb filled with liquid.

thermosphere – The atmospheric layer above the mesosphere (above about 85 kilometers) where the temperature increases rapidly with height.

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

thunderstorm – A local storm produced by cumulonimbus clouds. Always accompanied by lightning and thunder.

tornado – An intense, rotating column of air that produces form a cumulonimbus cloud in the shape of a funnel of a rope whose circulation is present on the ground.

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

tropical depression – A mass of thunderstorms and clouds generally with a cyclonic wind circulation of less than 20 knots.

tropical disturbance – An organized mass of thunderstorms and clouds generally with a cyclonic wind circulation of between 20 and 34 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 kilometers above the ground).

trough – An elongated area of low atmospheric pressure.

turbulence – Any irregularity or disturbed flow in the atmosphere that produces gusts and eddies.

typhoon – A hurricane that forms in the western Pacific Ocean.

ultraviolet radiation – Electromagnetic radiation with wavelengths longer than X-rays but shorter than visible light.

unstable air (absolutely unstable air) – An atmospheric condition that exists when a lifted parcel of air is warmer than the air around it.

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

virga – Precipitation that falls from a cloud but evaporates before reaching the ground.

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

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

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 e atmosphere, the weather.

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

weather types – Certain weather patterns categorized into similar groups. Used as an aid in weather prediction.

westerlies – The dominant westerly winds that blow in the middle latitudes on the poleward side of the subtropical high-pressure as.

wet-bulb temperature – The lowest temperature that can be obtained by evaporation water into the air.

wind – Air in motion relative to the earth’s surface.

wind-chill factor – The cooling effect of any combination of temperature and wind, expressed loss of body heat. Also called the wind chill index.

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.