Meteorological Terms

Absolute humidity

The weight of water vapor per unit volume of air.

Acid Precipitation

Rain or snow with a pH value of less than 5.6; (sometimes caused by air pollutants)

Adiabatic Temperature Change

A cooling or heating of the air caused by the contraction or expansion of air molecules, as opposed to the loss or gain of heat. For example, adiabatic cooling takes place as air rises.


Horizontal movement of air, moisture, or heat.

Advection Fog

Horizontal movement of warm, humid air over colder ground or water.


The glow in the western sky after sunset.

Air Mass

A large body of air with nearly uniform temperature and moisture content.


A significant body of air flowing in the same general direction.


Height expressed as the distance above a reference point, which is normally sea level or ground level.


The point in the path of a celestial body (as a planet)that is farthest from the sun.


An instrument that measures wind speed.

Aneroid Barometer

An instrument built around a metal structure that bends with changing air pressure. These changes are recorded on a pointer that moves back and forth across a printed scale.


An isogram with the same rise of barometric pressure in a given time.


The difference between the mean of any meteorological element, and the phase of that element over the same time for all other points on the same parallel of latitude.


A closed wind circulation of high barometric pressure that rotates clockwise in the northern hemisphere and counterclockwise in the southern hemisphere. With respect to the relative direction of its rotation, it is the opposite of a cyclone.

Arctic Air

A mass of very dry, very cold air that develops over the snow-and-ice-covered regions of the Far North.


The degree to which a climate lacks effective, life promoting moisture; the opposite of humidity.


The mass of air surrounding the Earth.

Atmospheric pressure

The amount of force exerted on a unit surface area. Also called 'air pressure.'


A luminous phenomenon that consists of streamers or arches of light and is caused by electrical discharges in the atmosphere, mostly confined in the tenuous air of high altitude. It is most commonly seen in sub-Arctic and sub-Antarctic latitudes. However, observations with the spectroscope seem to indicate that a faint permanent aurora is a normal feature of the sky in all parts of the world.

Aurora Australis

An aurora that occurs in the southern hemisphere; also called southern lights.

Aviation weather forecast

A forecast of weather elements of particular interest to aviation. These elements include the ceiling, visibility, upper winds, icing, turbulence, and types of precipitation and/or storms. It can be divided into four basic categories, area forecasts, terminal forecasts, route forecasts, and flight forecasts.

Aviation weather observation

An evaluation, according to set procedure, of weather elements which are most important to aircraft operations. It includes the cloud height or vertical visibility, sky cover, visibility, obstructions to vision, certain atmospheric phenomena, and wind speed and direction that prevail at the time of the observation.


The length of the arc of the horizon intercepted between a given point and an adopted reference direction, usually true north, and measured clockwise from the reference direction. It is a horizontal direction expressed in degrees. It is sometimes synonymous with bearing, but the latter is a navigation term and can be modified in several ways. Any point on or above the horizon can be located by its angles of azimuth and elevation plus either height or distance (or slant range) data.

Aurora Borealis

An aurora that occurs in the northern hemisphere; also called northern lights.

Back-Door Cold Front

A cold front that moves in from the northeast, rather than from the normal north or northwest direction.

Backing Wind

Shifting of the wind in a counterclockwise direction, usually resulting from the approach of a low-pressure system.


An instrument that provides a continuous record of atmospheric pressure.


An instrument for measuring the pressure of the atmosphere. The two principal types are the mercury barometer and the aneroid barometer.

Barometric Tendency

The amount and direction of change in barometer readings over a three-hour period.

Beaufort Wind Scale

A system used to classify wind speed, indicated by numbers from 0 to 12. It was developed in 1805 by British Admiral Francis Beaufort.

Bishop ring

A faint, broad, reddish-brown corona occasionally seen in dust clouds, especially those which result from violent volcanic eruptions. It has been seen after certain great volcanic eruptions, especially that of Krakatoa, in 1883.


A violent, intensely strong cold wind, laden with snow.


A sudden change in the weather; usually applied to the end of an extended period of unusually hot, cold, wet, or dry weather.


A light wind with a speed ranging from 4 to 27 knots (4 to 31 mph or 6 to 50 kph).


Sea water containing a higher concentration of dissolved salt than that normally found in the ocean. Brine is produced by evaporation or freezing of sea water.

Buys Ballot's law

In the Northern Hemisphere, if you face the wind the atmospheric pressure decreases towards your right and increases towards your left. The reverse is true in the Southern Hemisphere. The law is useful in locating centers of cyclones and anticyclones.


Absence of apparent motion of wind. This condition is reported when smoke is observed to rise vertically, or the surface of the sea is smooth and mirror-like.


The height above the ground of the base of the lowest layer of clouds, when at least 60 percent of the sky is covered by clouds.


A thermometric scale on which 0° denotes the freezing point and 100° the boiling point water, both under standard atmospheric pressure.


General circulation is the flow of air of large, semi-permanent weather systems, while secondary circulation is the flow of air of more temporary weather systems.


An average portrait of weather conditions in a specific place over a long period.


The scientific study of climate.


A visible aggregate of minute water and/or ice particles in the atmosphere above the earth's surface.

Cloud banner

A cloud streaming off from the mountain peak, resembling a banner.


A sudden and extremely heavy downpour of rain; especially in mountain regions.


The point of intersection of trough and ridge in the pressure pattern of a weather map. A neck of relatively low pressure between two anticyclones; also called a saddle.

Cold Front

The forward edge of an advancing cold air mass which is displacing warmer air in its path.

Cold Wave

A rapid and marked fall of temperature during the cold season of the year over a short period of time.


The change of a substance from vapor to liquid, usually caused by a decrease in temperature of the substance; the opposite of evaporation.

Condensation Nuclei

Small particles in the air around which water vapor condenses.


The transfer of heat by molecular action within a substance or when two substances are in direct contact.

Continental Air Mass

An air mass that forms over land. it is usually dry, but may be cold or warm.

Continental Climate

The type of climate characteristic of the interior of a continent. Compared to maritime climate, it has a large annual and daily range of temperature.


A cloud-like stream in cold, clear air formed behind the engines of an airplane.


It is often used to indicate the vertical movement of warm air, as opposed to advection.


The condition that exists when the distribution of winds within a given area is such that there is a net horizontal inflow of air into the area. The removal of the resulting excess is accomplished by an upward movement of air; consequently, areas of convergent winds are regions favorable to the occurrence of precipitation.

Coriolis Effect

The curving motion of an moving object, such as air, caused by the rotation of the earth. In the northern hemisphere, moving objects deflect to the right and in the southern hemisphere, moving objects deflect to the left.


A set of one or more colored rings, concentrically surrounding the disk of the sun, moon or other luminary when veiled by a thin cloud.


A general term applied to all clouds having dome-shaped upper surfaces which exhibit protuberances, bases of such clouds being generally horizontal. Cumuliform clouds are characteristically distinct and separated from one another by clear spaces.


A low pressure system in which winds spin inward in a counterclockwise direction in the northern hemisphere.

Dangerous quadrant

The advance quadrant of the dangerous semi-circle. A vessel and all its attendants in bad weather re-curve over it.

Dangerous semi-circle

It is the half of the storm area in which rotary and progressive motions of the storm reinforce each other, and the winds are also directed in such a way as to drive a vessel running before the wind across the storm track ahead of the advancing center.


An area of low-pressure in which storm conditions occur.

Deviation of the Wind

The angle between the direction of the wind and the direction of the pressure gradient.


Atmospheric moisture condensed, in liquid form, upon objects cooler than the air, especially at night.

Dew point

The temperature at which, under ordinary conditions, condensation begins in a cooling mass of air. It varies with the specific humidity. It is a conservative air mass property.


An area of low-pressure in which storm conditions occur.


Having a daily cycle, especially pertains to actions which are completed within twenty-four hours and which recur every twenty-four hours.

Diurnal Tide

A tide in which there is only one high water level and one low water level on a lunar day.


The conditions that exists when the distribution of winds within a given area is such that there is a net horizontal flow of air outward from the region. The resulting deficit is compensated by a downward movement of air from above; consequently areas of divergent winds are regions unfavorable to the occurrence of precipitation.


A part of the ocean near the equator abounding in calms, squalls, and light shifting winds.

Doppler Radar

A sophisticated radar that can measure the speed and direction of moving objects, such as wind.


A sudden, strong, downward blast of air, usually from a thundercloud.


Precipitation featuring tiny water droplets, no more than .02 inch in diameter. Unlike fog droplets, drizzle fall to the ground.


Abnormally dry weather in a region over an extended period of time.


Solid materials suspended in the atmosphere in the form of small irregular particles, many of which are microscopic in size.

Dry Fog

A haze due to the presence of dust or smoke in the air.

Dynamic Meteorology

The branch of meteorology that studies the motions of the winds and their relation to other atmospheric phenomena.

Earth hummock

A small dome-shaped uplift of soil caused by the pressure of ground water.

Earth shadow

Any shadow projecting into a hazy atmosphere from mountain peaks during sunrise or sunset.

Easterly wave

A migratory wave-like disturbance of the tropical easterlies. It is a wave within the broad easterly current and moves from east to west, generally more slowly than the current in which it is embedded.


A more or less fully developed vortex in the atmosphere, constituting a local irregularity in a wind system. All winds near the earth's surface contains eddies, which at any given place produce gusts and lulls. Air containing numerous eddies is said to be turbulent.

El Niño

The occurrence of an unusually warm ocean current setting south along the coast of Ecuador, so-called because it generally develops just after Christmas. In exceptional years, concurrently with a southerly shift in the tropical rain belt, the current may extend along the coast of Peru to 12°S.


When a line is drawn perpendicular to the plane of the earth's axis from the sun to the earth, the sun crosses the equator and day and night everywhere are of equal length. Vernal equinox occurs around March 21 (spring in the Northern Hemisphere) and Autumnal equinox occurs around September 22 (fall in the Northern Hemisphere).


The physical process by which a liquid or solid is transformed to vapor or the gaseous state; the opposite of condensation.

Extratropical cyclone

A cyclone that forms in the mid-latitudes outside the tropics.

Eye of the storm

A calm region at the center of a tropical cyclone or a break in the clouds marking its location.


A thermometric scale on which 32° denotes the freezing point and 212° the boiling point of water, both under standard atmospheric pressure.


With respect to weather, generally descriptive of pleasant weather conditions, with regard to location and time of year.


The area in which ocean waves are generated by the wind. It is generally delineated by coast lines, fronts, or areas of wind curvature or divergence.


A bright eruption from the sun's chromosphere (outer layers). Flares may appear within minutes and fade within an hour. They cover a wide range of intensity and size, and they tend to occur between sunspots or over their penumbrae. Also called solar flare.

Flash Flood

Flooding caused by a rapid rise in the water level of rivers, streams, or lakes, usually as a result of heavy rains.


A warm, dry wind on the lee side of a mountain range, the warmth and dryness of air being due toadiabatic compression upon descending the mountain slopes.


A cloud of water droplets suspended in the air that touches the ground; also described as a cloud at the earth's surface.

Fog drip

Moisture that is deposited on terrestrial objects by fog and then falls to the ground.


The change in substance from a liquid to a solid state.

Freezing Nuclei

Particles suspended in the air around which ice crystals form.

Freezing Rain

Supercooled drops of water that turn to ice when they hit a cold surface.


The boundary between two different air masses.


The term used to describe the process which creates a front i.e., produces discontinuity in a continuous field of the meteorological elements; also applied to the process which increases the intensity of a pre-existing front. Frontogenesisis generally set up by the horizontal convergence of air currents possessing widely differing properties.


Ice crystals that form on grass and other objects when the temperature and dew point fall below freezing.

Frost smoke

A fog produced by apparent steaming of the sea in the presence of air having a temperature much below freezing. Also called Arctic sea smoke.

Fujita Scale

A scale for estimating damage caused by the winds of tornado, developed by Theodore Fujita.


An unusually strong wind, with velocities ranging from 28 to 47 knots (32 to 63 mph or 51 to 101 kph). In practice, a wind of or exceeding force 8 on the Beaufort scale is counted a gale.


Term applied to a smooth coating of ice on terrestrial objects due to the freezing of rain. In other countries such deposit is called glazed frost. A deposit of glaze on an extensive scale constitutes an ice storm.


Change of value of a certain meteorological element per unit distance. The gradients commonly discussed in meteorology are the horizontal gradient of pressure, the vertical gradient of temperature, and the vertical gradient of electrical potential. Meteorologists now prefer the term lapse-rate to vertical gradient.


Precipitation formed when water droplets freeze in layers around a falling ice crystal. Also called soft hail.

Greenhouse effect

The heating effect produced when the atmosphere absorbs and re-emits infrared radiation. The shorter wavelength of insolation are transmitted freely through the atmosphere to be absorbed at the earth's surface, then Earth re-emits this as long-wave terrestrial radiation back to space.


A sudden brief increase in the force of the wind. It is of a more transient character than a squall and is followed by a lull or slackening in the wind speed. Most winds near the earth's surface display alternate gusts and lulls.


Chunks of ice that form in layers in the updrafts of thunderstorms.


A ring or arc of light around the sun or moon caused by the refraction of light through ice crystals in the atmosphere.


Particles of fine dust suspended in the air that produce limited visibility.

Heat Index

This index is a measure of the contribution that high humidity makes with abnormally high temperatures in reducing the body's ability to cool itself.

Heat Lightning

Lightning that can be seen, but too far away to be heard.

Heat wave

A period of abnormally high temperature. It is sometimes defined as a period of three or more consecutive days during which the maximum temperature is 90°F or over.


The degree to which the air is charged with water vapor. This may be expressed as absolute humidity,relative humidity, and specific humidity.


A tropical cyclone in the western Atlantic that has sustained wind speeds of 74 miles per hour or greater.


A generic term for weather phenomena that mostly depend upon modifications in the condition of the water vapor in the atmosphere. Examples are fog, rain and hail.


The Earth's water.


An instrument that measures the water vapor content of the air.


The solid state of water; it is found in the atmosphere in different forms such as crystals, snow pellets etc.


A large mass of ice that breaks from the tongue of a glacier, runs into the sea and then floats away. Only about ten percent of it is visible.


A white, luminous appearance near the horizon caused by the reflection of light from ice.

Ice needles

Thin crystals or shafts of ice, so light that they seem to be suspended in the air.

Ice rain

A rain that causes a deposit of glaze. Falling pellets of clear ice.

Ice crystals

Frozen water vapor suspended in the air.


Solar radiation that has been received by the earth's surface.


A state of the atmosphere in which convection takes place spontaneously, leading to cloud formation and precipitation.

Intertropical convergence zone

The axis, or portion of the broad trade winds current of the tropics.This axis is the dividing line between the southeast trades and the northeast trades.


A condition in which air near the ground is cooler than air above it; a condition opposite to the concept of a normal decrease in temperature with height.


An isogram having the same amount of change in barometric pressure within a specified period.


A line drawn on a chart or diagram to show the distribution of some physical condition in space or time (or both), by connecting points corresponding to equal values of the phenomenon represented. Most of the isograms used in meteorology are drawn on geographical charts, and show the distribution of meteorological elements in space only. Also called isoline.


An isanomalous line; line of anomaly, i.e., of the departure of the local mean value of an element from the mean pertaining to the latitude.


An isogram having the same barometric pressure for a certain period.


An isogram having the same rainfall amount.


An isogram that shows the variation of an element in relation to two coordinates; one of the coordinates representing the time of the year (month), and the other usually the time of the day (hour), and sometimes space (especially altitude).


An isogram that surrounds an area with the same temperature.


A narrow band of winds blowing high in the troposphere at speeds of 57 miles per hour or greater.


An isogram having the same fall of barometric pressure in a given time.


A scale in which 0° is the point at which all molecular motion ceases (absolute zero)


A metric unit of air pressure. It is simply millibars divided by 10.

Land breeze

The breeze that, on certain coasts and under certain conditions, blows from the land by night.

Latent Heat

The energy that is stored when water evaporates. The energy is released when water vapor condenses or is turned to ice.


The forcing of air in a vertical direction when it meets with an upslope in terrain or a denser air mass.


An electrical discharge produced by a thunderstorm.

Line squal

A more or less continuous line of squalls and thunderstorms marking the position of an advancing cold front.


An area of low barometric pressure, with its attendant system of winds. An area of low barometric depression or cyclone.


The variation of any meteorological element in the course of a day, year, or other interval of time.

Maritime Air Mass

An air mass that forms over water. It is usually humid, and may be cold or warm.

Maritime Climate

A type of climate characteristic of the ocean and oceanic islands. Its most prominent feature is a small range of minimum and maximum temperatures.

Mean Temperature

The average of a series of temperatures taken over a period of time, such as a day or a month.

Mercury Barometer

An instrument that measures atmospheric pressure by measuring the level of mercury in a column.


A large, rotating column of air that forms into a violent thunderstorm and may spawn tornadoes.


The science of the atmosphere.

Meteor shower

A large concentration of falling meteors. Many meteor showers are recurring phenomena and their appearance can be predicted.


An instrument designed for recording small and rapid variations of atmospheric pressure.


An instrument used to show very small changes of atmospheric pressure.


A downburst from a thunderstorm that is confined to a small area.


The areas in the northern and southern hemispheres between the tropics and the Arctic and Antarctic circles, the temperate zones.


A metric unit of atmospheric pressure equal to 1/1000 bar or 1000 dynes (unit of force equal to the force that would give a free mass of one gram an acceleration of one centimeter per second per second) per square centimeter.


An apparent displacement or distortion of observed objects by abnormal atmospheric refraction. Sometimes the images are inverted, magnified, multiplied, raised, or brought nearer to the eye than the object. Refraction layers in the atmosphere often assume the appearance of fog.


A very thin fog in which the horizontal visibility is greater than 1 kilometer, or approximately 1,100 yards. Often used synonymously with drizzle or fine rain.


A wind that reverses its direction with the season, blowing more or less steadily from the interior of a continent toward the sea in winter, and in the opposite direction during summer.

Nautical mile

The length of one minute of arc along any great circle on the earth's surface. Since this actual distance varies slightly with altitude, a nautical mile by international agreement is defined as 1852 meters (6076.103 feet or 1.1508 statue miles).

Navigable semi-circle

A vessel situated in this semi-circle may safely run before the wind, because she is then moving away from the storm.

Neap tide

A tide of minimum range, occurring twice a month, one or two days after quadrature.


An instrument for measuring the speed and movement of clouds.


A numerical figure representing the average of conditions at a location over a period of years.

Numerical Forecasting

Forecasting the weather through digital computation carried by supercomputers.

Oblique visual range

The greatest distance at which a specific target can be perceived when viewed along a line of sight inclined to the horizon.

Occluded front

The front that is formed when and where the cold front overtakes the warm front of the cyclone. Serves as a boundary between cold and warm air masses.


The term used to denote the process whereby the air in the warm sector of a cyclone is forced from the surface to the higher levels. The process is accompanied by an increase in the intensity of the cyclone.


The study of the oceans, embracing and integrating all knowledge pertaining to the sea's physical boundaries, the chemistry and physics of sea water, and marine biology.

Orographic lifting

The upward flowing of air caused by rising terrain, such as a mountain range.


Descriptive of sky cover of 1.0 (95 percent or more) when at least a portion of this amount is attributed to clouds or obscuring phenomena aloft; that is, when the total sky cover is not due entirely to surfaced-based obscuring phenomena.


The flow of warm air over cold air in advance of a warm front.


In its free form, a colorless, tasteless, and odorless gaseous element; the second most abundant gas in the earth's atmosphere and a prerequisite of virtually all forms of animal life. It is a usually a diatomic (containing two atoms) substance.


A triatomic form of oxygen which occurs transiently in small quantities in the lower atmosphere and is supposed to be permanently present and relatively abundant at high atmospheric levels.

Ozone layer

Otherwise known as ozonosphere; an atmospheric layer where there is a maximum concentration of ozone which blocks most solar ultraviolet radiation from entry into the lower atmosphere.


The direction in which the vortex will probably travel.


The point in the path of a celestial body (as a planet) that is nearest to the sun.

Pilot balloon

A small free balloon that drift off which, as observed from the ground, indicates the movements of the air aloft.

Polar Air Mass

A mass of very cold, very dry air that forms in polar regions.

Polar continental air mass

Polar air mass that originates over land or frozen ocean areas in the polar regions. It is characterized by low temperatures, low specific humidity and a high degree of vertical visibility.

Polar maritime air mass

Polar air mass that originally came from the polar regions but has since been modified by reason of its passage over a relatively warm ocean surface. It is characterized by moderately low surface temperatures, moderately high surface specific humidity, and a considerable degree of vertical instability.


Any liquid or solid form of water that falls from the atmosphere and reaches the surface of the Earth, usually measured in millimeters.


An elliptical expression, current in meteorological literature, for atmospheric pressure, or barometric pressure.

Pressure Gradient Force

Force acting on air that causes it to move from areas of higher pressure to areas of lower pressure.

Prevailing Winds

The direction from which the wind blows frequently in any location.

Prevailing westerlies

The belts of winds lying on the poleward sides of the subtropical high-pressure belts


An instrument that measures relative humidity of the air.


An instrument that measures solar radiation by its heating effects.


A configuration in which the moon and the sun have an angular separation of 90 degrees as seen from the earth.


The transferring of energy through electromagnetic waves.


Liquid precipitation with drops larger than .02 inch in diameter.


A luminous arc formed by the refraction and reflection of light in drops of water suspended in the atmosphere.


A term sometimes synonymous with rain, but most frequently used in reference to amounts of precipitation which includes snow, hail, etc.

Rain gauge

An instrument for measuring rainfall.


The production of an image by or as if by a mirror caused by light 'bouncing'; off a certain surface.


The bending of light as it passes through areas of different density, such as from air through ice crystals.

Relative humidity

The ratio of the actual vapor pressure to the vapor pressure corresponding to saturation at the prevailing temperature, or simply the percentage of saturation.


A relatively narrow extension of an anticyclone or high-pressure area as shown on a weather chart.

Right hand and left hand semi-circles

The semi-circles are named on the assumption that the observer is at the storm's center, and, is looking along the path.


Tiny balls of ice formed when tiny drops of water freeze on contact with the surface.


The condition that exists in the atmosphere when the partial pressure exerted by the water vapor present is equal to the maximum vapor pressure possible at the prevailing temperature.

Sea breeze

The breeze that, on certain coasts and under certain conditions, blows from the water; by day.


A division of the year according to some regularly recurring phenomena, usually astronomical or climatic.


The variation of vector field along a given direction in space. The most frequent context for this concept is wind shear.


A snowfall or rainfall, of short duration but often of considerable intensity, and usually consisting of relatively large drops. Also a similar fall of sleet, or hail. Showers characteristically fall from the isolated clouds separated from one another by clear spaces. They occur typically in air masses that possess a high degree of instability.


Precipitation consisting of ice particles formed when raindrops freeze.


Air pollution caused by a mixture of smoke and fog.


Precipitation in the form of small ice crystals, falling either separately or in loosely coherent clusters (snowflakes).

Soft hail

White, opaque, round pellets of snow.

Solar constant radiation

The intensity of solar radiation outside the Earth's atmosphere at the Earth's mean distance from the sun. Findings indicate that this intensity may vary and that its mean value is 1.94 gram-calories per minute per square centimeter of area lying normal to the incident solar ray.

Solar Energy

The energy produced by the sun.


The time of year when the sun is the farthest north or the farthest south (about June 21 and December 21). The time of year when there exists the largest difference between the hours of daytime and nighttime at certain latitudes.

Sounding balloon

A free, unmanned balloon carrying a set of self-registering meteorological instruments.

Specific humidity

The mass of water vapor contained in a unit mass of moist air.

Spring tide

Tide near the time of syzygy, when ranges between high tide and low tide are greatest.


A sudden storm of brief duration; closely akin to thunderstorm but not necessarily attended by thunder and lightning. A sudden brief blast of wind, of longer duration than a gust.

Squall Line

A line of thunderstorms that forms along a front.

Stable Air

Air in which temperature and humidity at various levels discourage the formation of convection currents.

Stationary Front

The border between cold and warm air masses that is not moving

St. Elmo's Fire

A luminous brush discharge of electricity from elevated objects, such as the masts and yardarms of ships, lightning rods, steeples, etc., occurring in stormy weather.


A marked disturbance in the normal state of the atmosphere. The term is often applied to a disturbance in which strong wind is the most prominent characteristic, and specifically to a wind force of 11 on the Beaufort scale. It is also used for other types of disturbance, including thunderstorms, rainstorms, snowstorms, hailstorms, dust storms, sand storms, magnetic storms. etc.

Storm Track

The path that storms generally follow in a given time.


A general term applied to all clouds which are arranged in unbroken horizontal layers or sheets.


The change of water vapor directly into ice crystals or ice crystals directly into water vapor.


The word used to denote a slow downward motion of air over a large area. Subsidence accompanies divergence in the horizontal motion of the lower layers of the atmosphere.

Summer solstice

Occurs just a few days before aphelion. The sun is directly overhead at noon in latitude 23 1/2°N, and with regards to the amount of daylight hours, the Northern Hemisphere experiences the longest daylight hours and the Southern Hemisphere the shortest. Happens around June 21.


A general change in barometric pressure apparently superposed upon cyclonic and normal diurnal changes.

Synoptic chart

A chart, such as the ordinary weather map, which shows the distribution of meteorological conditions over an area at a given moment.

Synoptic Meteorology

The branch of meteorology that deals with the analysis of meteorological observations made simultaneously at a number of points in the atmosphere (at the ground or aloft) over the whole or a part of the earth, and the application of the analysis to weather forecasting and other problems.


The points in the moon's orbit about the earth at which the moon is new or full. At this time also the moon, earth and sun forms a nearly straight-line configuration.


An instrument for measuring temperature; in meteorology, generally the temperature of air. Maximum and minimum thermometers indicate, respectively, the highest and lowest temperatures occurring between the times of setting the instrument. A wet-bulb thermometer is used in measuring humidity.

Temperate Zone

The area of the globe between the tropics and the polar regions.


The sound emitted or produced by a lightning discharge.


Weather disturbance that produces thunder and lightning, aside from wind and rain. Thunderstorms occur locally, often as episodes of cyclones, and in common with squalls, are marked by abrupt variations in pressure, temperature, and wind.

Tidal Wave

The wave motion of the tides. Any unusually high water level along a shore. It usually refers to either a storm surge or tsunami.


A violently rotating column of air that reaches from the base of a cloud to the ground (in funnel shape). In other areas, it is called a violent thundersquall.


A precipitation amount of less than 0.005 inches. In general, an unmeasurable or insignificant small quantity.


The direction from which the vortex or storm center has already moved.

Trade wind

Two belts of wind, one on either side of the equatorial doldrums in which the winds blow almost constantly from easterly quadrants.

Tropical Cyclone

The general term for a cyclone that originates over the tropical oceans.

Tropical Depression

A tropical cyclone with winds that do not exceed 63 kph. Most common in the region of the equatorial or intertropical convergence and less frequently in the trade winds.

Tropical Storm

A tropical cyclone with winds of 64 to 118 kph.


The elongated area of low barometric pressure that generally stretches north and south. This is the line where the lowest readings of the barometer will be recorded.

True north

The direction from any point on the earth's surface toward the geographic North Pole.


An ocean wave produced by a submarine earthquake, landslide, or volcanic eruption that may reach enormous dimensions and have sufficient energy to travel across entire oceans.


Astronomical twilight is the interval between sunrise or sunset and the total darkness of night. Civil twilight is the period of time before sunrise or after sunset during which there is enough daylight for ordinary outdoor occupations.


A tropical cyclone with winds that exceed 118 kph. The name is applied to a severe tropical cyclone in the western Pacific.

Ulloa's ring

A glory. A halo (also called Bouguer's halo), surrounding a point in the sky diametrically opposite the sun; sometimes described as a 'white rainbow'.

Ultraviolet radiation

Electromagnetic radiation of shorter wavelength than visible radiation but longer than x-rays.

Unstable Air

Air with temperature differences that encourage the formation of convection currents that can produce clouds and precipitation.


An upward current of air, usually within a thundercloud.


The rising of water toward the surface from subsurface layers of a body of water.


A device that shows which way the wind blows; also called weather vane or wind vane.

Vapor pressure

The pressure exerted by a vapor when it is confined in space. In meteorology vapor pressure refers exclusively to the pressure of water vapor. When several gases or vapors are mixed together in the same space each one exerts the same pressure as it would if the others were not present; the vapor pressure is that part of the total atmospheric pressure which is due to water vapor.


Inter-diurnal variability is the average difference between successive daily averages of a meteorological element.

Veering Winds

Winds that shift in a clockwise direction, a shift caused by a high-pressure system.


An auxiliary scale for estimating fractions of a scale division when the reading to the nearest whole division on the main scale is not sufficiently accurate.


The point of re-curvature, about 20° latitude.


The maximum distance at which one can see and identify objects.


Water droplets or ice crystals that fall from high clouds but evaporate before hitting the ground.


The central area of light winds and calms, or within the ring of hurricane force winds.

Warm air mass

An air mass that is warm in relation to its neighboring air masses. The term implies that the air mass originated in latitudes lower than those in which it now finds itself and that it is, therefore, warmer than the surface over which it is moving.

Warm front

The leading edge of a mass of warm air that displaces a mass of cold air.


A message given to relate the occurrence of a threatening weather condition, as indicated by a radar or spotter; or of one that is expected to occur or strike an area very soon.


A tornado-like vortex and cloud occurring over a body of water, frequently in tropical waters.

Water vapor

The invisible gaseous form of water.


A small cyclonic circulation in the early stages of development that moves along a cold front.

Wave disturbance

A localized deformation of a front, which travels along the front as a wave-shaped formation, which generally develops into a well-marked cyclone.


The conditions in the atmosphere at any given time

Weather forecast

A forecast of the future state of the atmosphere with specific reference to one or more associated weather elements.


The dominant west-to-east motion of the atmosphere, centered over the middle latitudes of both hemispheres.


Air in motion that moves horizontally in relation to the surface of the Earth. Vertical streams of air are usually called currents.

Windchill Factor

A measure of the effect of wind in increasing the heat loss from exposed flesh.

Wind Direction

The direction from which the wind is blowing.

Wind shear

A sudden shift in wind direction.

Winter solstice

The Southern Hemisphere has its maximum exposure to the sun. It occurs just a few days before perihelion and the sun is directly overhead at noon in latitude 23 1/2°S. This occurrence brings about the shortest daylight hours in the Northern Hemisphere and the longest daylight hours in the Southern Hemisphere. Happens around December 21.

Zenith distance

The angular distance of any celestial object from a given observer's zenith, measured along the great circle of the celestial sphere from zenith to object; the compliment of the elevation angle.

Zodiacal light

A cone of faint light in the sky which is seen stretching along the zodiac from the western horizon after the twilight of sunset has faded and from the eastern horizon before the twilight of sunrise has begun.