Weather Glossary

The following is a list of definitions for commonly used weather-related terms. For a more comprehensive list visit www.weather.com/glossary.

A | B | C | D | E | F | G | H | I

ABSOLUTE HUMIDITY
A type of humidity that considers the mass of water vapor present per unit volume of space. Also considered as the density of the water vapor. It is usually expressed in grams per cubic meter.

AIR
This is considered the mixture of gases that make up the earth's atmosphere. The principal gases that compose dry air are nitrogen (N2) at 78.09%, oxygen (O2) at 20.946%, argon (A) at 0.93%, and carbon dioxide (CO2) at 0.033%. One of the most important constituents of air and most important gases in meteorology is water vapor (H2O).

AIR MASS
An extensive body of air throughout which the horizontal temperature and moisture characteristics are similar.

AIR POLLUTION
The soiling of the atmosphere by contaminants to the point that may cause injury to health, property, plant, or animal life, or prevent the use and enjoyment of the outdoors.

ALTIMETER
An instrument used to determine the altitude of an object with respect to a fixed level. The type normally used by meteorologists measures the altitude with respect to sea level pressure.

ALTITUDE
In meteorology, the measure of a height of an airborne object in respect to a constant pressure surface or above mean sea level.

ANEMOMETER
An instrument that measures wind speed.

ANTARCTIC
Of or related to the area around the geographic South Pole, from 90 degrees South to the Antarctic Circle at approximately 66 1/2 degrees South latitude, including the continent of Antarctica. Along the Antarctic Circle, the sun does not set on the day of the summer solstice (approximately December 21st) and does not rise on the day of the winter solstice (approximately June 21st).

ANTARCTIC OCEAN
Although not officially recognized as a separate ocean body, it is commonly applied to those portions of the Atlantic, Pacific, and Indian Oceans that reach the Antarctic continent on their southern extremes.

ARCTIC
Of or relating to the area around the geographic North Pole, from 90 degrees North to the Arctic Circle at approximately 66 1/2 degrees North latitude.

ARID
A term used for an extremely dry climate. The degree to which a climate lacks effective, life-promoting moisture. It is considered the opposite of humid when speaking of climates.

AURORA
It is created by the radiant energy emission from the sun and its interaction with the earth's upper atmosphere over the middle and high latitudes. It is seen as a bright display of constantly changing light near the magnetic poles of each hemisphere. In the Northern Hemisphere, it is known as the Aurora Borealis or Northern Lights, and in the Southern Hemisphere, this phenomena is called the Aurora Australis.

AUTUMN
The season of the year which occurs as the sun approaches the winter solstice, and characterized by decreasing temperatures in the mid-latitudes. Customarily, this refers to the months of September, October, and November in the North Hemisphere and the months of March, April, and May in the Southern Hemisphere. Astronomically, this is the period between the autumnal equinox and the winter solstice.

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BALL LIGHTNING
A relatively rare form of lightning consisting of a luminous ball, often reddish in color, which moves rapidly along solid objects or remains floating in mid-air. Also known as globe lightning.

BAROGRAPH
An instrument that continuously records a barometer's reading of atmospheric pressure. For an example, see aneroid barometer.

BAROMETER
An instrument used to measure atmospheric pressure. Two examples are the aneroid barometer and the mercurial barometer.

BAROMETRIC PRESSURE
The pressure exerted by the atmosphere at a given point. Its measurement can be expressed in several ways. One is in millibars. Another is in inches or millimeters of mercury (Hg). Barometric pressure is a measurement that indicates the passing of low and high pressure systems across the earth's surface. They are represented as Highs "H" and Lows "L" on current and forecast conditions weather maps. Generally speaking, low pressure means that moist air is dominating an area... This is typical when the weather is rainy, snowy, stormy, etc. High pressure represents dry sinking air and typically means dry, sunny skies. Also known as atmospheric pressure.

BEAUFORT WIND SCALE
A system of estimating and reporting wind speeds. It is based on the Beaufort Force or Number, which is composed of the wind speed, a descriptive term, and the visible effects upon land objects and/or sea surfaces. The scale was devised by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.

BLACK ICE
Thin, new ice on fresh or salt water that appears dark in color because of its transparency. Also refers to thin, transparent ice on road surfaces.

BLIZZARD
A severe weather condition characterized by low temperatures, winds 35 mph or greater, and sufficient falling and/or blowing snow in the air to frequently reduce visibility to1/4 mile or less for a duration of at least 3 hours. A severe blizzard is characterized by temperatures near or below 10 degrees Fahrenheit, winds exceeding 45 mph, and visibility reduced by snow to near zero.

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CELSIUS TEMPERATURE SCALE
A temperature scale where water at sea level has a freezing point of 0 degrees C (Celsius) and a boiling point of +100 degrees C. More commonly used in areas that observe the metric system of measurement. Created by Anders Celsius in 1742. Same as Centigrade. In 1948, the Ninth General Conference on Weights and Measures replaced "degree centigrade" with "degree Celsius."

CHINOOK
A type of foehn (warm and dry, gusty) wind. Refers to the warm down slope wind in the Rocky Mountains that may occur after an intense cold spell when the temperature could rise by 20 to 40 degrees Fahrenheit in a matter of minutes. Also known as the Snow Eater.

CLEAR ICE
A glossy, clear, or translucent ice formed by the relatively slow freezing of large supercooled water droplets. The droplets spread out over an object, such as an aircraft wing's leading edge, prior to complete freezing and forms a sheet of clear ice. Often synonymous with glaze.

CLIMATE
The historical record and description of average daily and seasonal weather events that help describe a region. Statistics are generally drawn over several decades. The word is derived from the Greek "klima," meaning inclination, and reflects the importance early scholars attributed to the sun's influence.

CLOUD
A visible collection of minute particle matter, such as water droplets and/or ice crystals, in the free air. A cloud forms in the atmosphere as a result of condensation of water vapor. Condensation nuclei, such as in smoke or dust particles, form a surface upon which the water vapor can condense.

COLD FRONT
The leading edge of an advancing cold air mass that is under running and displacing the warmer air in its path. Generally, with the passage of a cold front, the temperature and humidity decrease, the pressure rises, and the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere). Precipitation is generally at and/or behind the front, and with a fast-moving system a squall line may develop ahead of the front. See occluded front and warm front.

CONDENSATION
The process by which water vapor undergoes a change in state from a gas to a liquid. It is the opposite physical process of evaporation.

CRYSTALIZATION
The process of a substance going directly from a vapor form (water vapor) to a solid (ice) at the same temperature, without going through the liquid phase (water). The opposite of sublimation.

CURRENT
A horizontal movement of water, such as the Gulf Stream off the east coast of North America, or air, such as the jet stream.

CYCLONE
An area of closed pressure circulation with rotating and converging winds, the center of which is a relative pressure minimum. The circulation is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Commonly called a low pressure system, the term is also used for a hurricane (tropical cyclone) in the Indian Ocean. Other phenomena with cyclonic flow may be referred to by this term, such as dust devils, tornadoes, and tropical and extratropical systems. The opposite of an anticyclone or a high pressure system.

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DAWN
The first appearance of light in the eastern sky before sunrise. It marks the beginning of morning twilight. The visual display is created by the scattering of light reaching the upper atmosphere prior to the sun's rise to the observer's horizon. Also known as daybreak.

DAY
Considered a basic unit of time as defined by the earth's motion. It represents the time needed for one complete revolution of the earth about its own axis. Also known as a sidereal day, it is approximately equal to 23 hours, 56 minutes, and 4.09 seconds. See night.

DEGREE
A measure of temperature difference representing a single division on a temperature scale. See Celsius, Fahrenheit, and Kelvin scales.

DENSITY ALTITUDE
Density altitude is a measure used primarily by pilots, high-performance engine mechanics and long-range shooters. The density altitude is a measure of the density of the air, given in units of distance. It is a function of temperature, relative humidity and air pressure.

DEW
Condensation in the form of small water drops that form on grass and other small objects near the ground when the temperature has fallen to the dew point, generally during the nighttime hours.

DEW POINT
The dew point is the temperature at which the air would be completely saturated with moisture, or a relative humidity of 100 percent. The same amount of moisture that would cause 100 percent humidity at 60 degrees would produce 71 percent humidity at 70 degrees, 51 percent at 80 degrees, and 37 percent at 90 degrees. At 60 degrees (the dew point), the air is saturated -- most likely, there is dense fog in that circumstance. But if the amount of moisture doesn't change as the sun comes out and warms the day up to a high of 90, the relative humidity falls all the way to 37 percent by afternoon. It would be easy to presume that the atmosphere got a lot drier during the course of the day, with the humidity falling from 100 percent to 37 percent. But actually, with the dew point staying at 60, the amount of moisture in the air did not change. Instead, the atmosphere's ability to hold moisture increased with heating, so the same amount of moisture fills a much smaller percentage of space available at 90 than it did at 60.

Think of it this way: You have enough boxes to fill a closet. If you put those same boxes in the living room, the boxes hold as much stuff as they did in the closet, but they fill a smaller percentage of space in the living room than they did in the closet. The boxes fill 100 percent of the space in the closet, but perhaps only 37 percent of the space in the living room. In the summer, we often talk of humid afternoons. You might be surprised that what would be considered an extremely sultry afternoon (temperature of 95, dew point of 72) would have a relative humidity of less than 50 percent (47 percent, in this case). It's the dew point number, not the relative humidity percentage, that reflects the humidity.

DOG DAYS
The name given to the very hot summer weather that may persist for four to six weeks between mid-July through early September in the United States. In western Europe, this period may exist from the first week in July to mid- August and is often the period of the greatest frequency of thunder. Named for Sirius, the Dog Star, which lies in conjunction with the sun during this period, it was once believed to intensify the sun's heat during the summer months.

DOLDRUMS
A nautical term for the equatorial region of light winds between the trade winds of the two hemispheres.

DROUGHT
Abnormal dry weather for a specific area that is sufficiently prolonged to cause serious hydrological imbalance for the lack of water.

DRY BULB THERMOMETER
A thermometer used to measure the ambient temperature. The temperature recorded is considered identical to air temperature. One of the two thermometers that make up a psychrometer.

DUSK
The period of waning light from the time of sunset to dark. See twilight and dawn.

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EARTHQUAKE
A sudden, transient motion or trembling of the earth's crust, resulting from waves in the earth caused by faulting of the earth's crust or by volcanic activity.

ECLIPSE
The obscuring of one celestial body by another. See lunar eclipse or solar eclipse.

EL NINO
The cyclical warming of East Pacific Ocean sea water temperatures off the western coast of South America that can result in significant changes in weather patterns in the United States and elsewhere. This occurs when warm equatorial waters move in and displace the colder waters of the Humbolt Current, cutting off the upwelling process.

EQUINOX
The point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. In the Northern Hemisphere, the vernal equinox falls on or about March 20 and the autumnal equinox on or about September 22.

EVAPORATION
The physical process by which a liquid, such as water, is transformed into a gaseous state, such as water vapor. It is the opposite physical process of condensation.

EVAPOTRANSPIRATION
Evapotranspiration is the combination of water that is evaporated and transpired by plants as part of their metabolic process. Growers use the evapotranspiration to determine when to run their irrigation systems. Farmers and those associated with crops, green houses, etc. are often very interested in both of these measurements.

EYE (HURRICANE)
The center of a tropical storm or hurricane, characterized by a roughly circular area of light winds and rain-free skies. An eye will usually develop when the maximum sustained wind speeds exceed 78 mph. It can range in size from as small as 5 miles to up to 60 miles, but the average size is 20 miles. In general, when the eye begins to shrink in size, the storm is intensifying.

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FAHRENHEIT TEMPERATURE SCALE
A temperature scale where water at sea level has a freezing point of +32 degrees F (Fahrenheit) and a boiling point of +212 degrees F. More commonly used in areas that observe the English system of measurement. Created in 1714 by Gabriel Daniel Fahrenheit (1696-1736), a German physicist, who also invented the alcohol and mercury thermometers.

FLASH FLOOD
A flood that rises and falls quite rapidly with little or no advance warning, usually as the result of intense rainfall over a relatively small area. Flash floods can be caused by situations such as a sudden excessive rainfall, the failure of a dam, or the thaw of an ice jam.

FLOOD
High water flow or an overflow of rivers or streams from their natural or artificial banks, inundating adjacent low lying areas.

FLOOD PLAIN
Level land that may be submerged by flood waters.

FOG
A visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth, reducing horizontal visibility to less than 5/8 statute miles. It is created when the temperature and the dew point of the air have become the same, or nearly the same, and sufficient condensation nuclei are present. It is reported as "FG" in an observation and on METAR.

FORECAST
A statement of expected future weather occurrences. Weather forecasting includes the use of objective models based on certain atmospheric parameters, along with the skill and experience of a meteorologist. Also called a prediction.

FREEZING POINT / FREEZE
The process of changing a liquid to a solid. The temperature at which a liquid solidifies under any given set of conditions. Pure water under atmospheric pressure freezes at 0 degrees Celsius or 32 degrees Fahrenheit. It is the opposite of fusion. In oceanography, the freezing point of water is lowered with increasing salinity.

FROST
The covering of ice crystals that forms by direct sublimation on exposed surfaces whose temperature is below freezing.

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GALE
On the Beaufort Wind Scale, a wind with speeds from 28 to 55 knots (32 to 63 miles per hour). For marine interests, it can be categorized as a moderate gale (28 to 33 knots), a fresh gale (34 to 40 knots), a strong gale (41 to 47 knots), or a whole gale (48 to 55 knots). In 1964, the World Meteorological Organization defined the categories as near gale (28 to 33 knots), gale (34 to 40 knots), strong gale (41 to 47 knots), and storm (48 to 55 knots).

GREENHOUSE EFFECT
The overall warming of the earth's lower atmosphere primarily due to carbon dioxide and water vapor which permit the sun's rays to heat the earth, but then restrict some heat-energy from escaping back into space.

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HAIL
Precipitation that originates in convective clouds, such as cumulonimbus, in the form of balls or irregular pieces of ice, which comes in different shapes and sizes. Hail is considered to have a diameter of 5 millimeters or more; smaller bits of ice are classified as ice pellets, snow pellets, or graupel. Individual lumps are called hailstones. It is reported as "GR" in an observation and on METAR. Small hail and/or snow pellets is reported as "GS" in an observation and on METAR.

HEAT
A form of energy transferred between two systems by virtue of a difference in temperature. The first law of thermodynamics demonstrated that the heat absorbed by a system may be used by the system to do work or to raise its internal energy.

HEAT EXHAUSTION
The effect of excessive heat, particularly when combined with high humidity, on a human being. Signs of heat exhaustion include a general weakness, heavy sweating and clammy skin, dizziness and/or fainting, and muscle cramps.

HEAT INDEX
The combination of air temperature and humidity that gives a description of how the temperature feels. This is not the actual air temperature. For an example, check out the National Weather Service heat index chart.

HEAT LIGHTNING
Lightning that appears as a glowing flash on the horizon. It is actually lightning occurring in distant thunderstorms, just over the horizon and too far away for thunder to be heard.

HEAT STROKE
Introduced to the body by overexposure to high temperatures, particularly when accompanied by high humidity. The signs of heat stroke include when an individual's body temperature is greater than 105 degrees Fahrenheit, the skin is hot and dry, there is a rapid and irregular pulse, perspiration has stopped, and one has lost consciousness. Seek immediate medical aid. May be called a sun-stroke when caused by direct exposure to the sun.

HEAT WAVE
A period of abnormally and uncomfortably hot weather. It could last from several days to several weeks. The Weather Channel uses the following criteria for a heat wave: a minimum of ten states must have 90 degree plus temperatures and the temperatures must be at least five degrees above normal in parts of that area for at least two days or more.

HORSE LATITIDES
Located between 30 degrees North and South in the vicinity of the equator, this area typically has calm or light and variable winds. Another name for the equatorial trough, the Intertropical Convergence Zone (ITCZ), or the doldrums.

HUMIDITY
The amount of water vapor in the air. It is often confused with relative humidity or dew point. Types of humidity include absolute humidity, relative humidity, and specific humidity.

HURRICANE
The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific Ocean. This same tropical cyclone is known as a typhoon in the western Pacific and a cyclone in the Indian Ocean.

HYDROMETEOR
Any any form of atmospheric water vapor, including those blown by the wind off the earth's surface. Liquid or solid water formation that is suspended in the air includes clouds, fog, ice fog, and mist. Drizzle and rain are example of liquid precipitation, while freezing drizzle and freezing rain are examples of freezing precipitation. Solid or frozen precipitation includes ice pellets, hail, snow, snow pellets, snow grains, and ice crystals. Water vapor that evaporates before reaching the ground is called virga. Examples of liquid or solid water particles that are lifted off the earth's surface by the wind includes drifting and blowing snow and blowing spray. Dew, frost, rime, and glaze are examples of liquid or solid water deposits on exposed objects.

HYGROMETER
An instrument that measures the water vapor content of the atmosphere. See the psychrometer as an example.

HYPOTHERMIA
Occurs when the core temperature of one's body falls below normal. It is the failure of the body to maintain adequate production of heat under conditions of extreme cold.

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ICE The solid form of water. It can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail, for example.

ICE CRYSTALS
Precipitation in the form of slowly falling, singular or unbranched ice needles, columns, or plates. They make up cirriform clouds, frost, and ice fog. Also, they produce optical phenomena such as halos, coronas, and sun pillars. May be called "diamond dust". It is reported as "IC" in an observation and on METAR.

ICE JAM
An accumulation of broken river ice caught in a narrow channel, frequently producing localized flooding. Primarily occurs during a thaw in the late winter or early spring.

ICE STORM
A severe weather condition characterized by falling freezing precipitation. Such a storm forms a glaze on objects, creating hazardous travel conditions and utility problems.

ICICLE
Ice that forms in the shape of a narrow cone hanging point down. It usually forms when liquid water from a sheltered or heated source comes in contact with below-freezing air and freezes more or less rapidly as it flows.

ICING
The forming or depositing of ice on an object. See glaze.

INCHES OF MERCURY (inHg)
The name comes from the use of mercurial barometers which equate the height of a column of mercury with air pressure. One inch of mercury is equivalent to 33.86 millibars or 25.40 millimeters. See barometric pressure. First devised in 1644 by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

INDIAN SUMMER
A period of abnormally warm weather in mid to late autumn with clear skies and cool nights. A period of freezing temperatures has to preceed this warm spell.

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J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

JET STREAK
A region of accelerated wind speed along the axis of a jet stream.

JET STREAM
A narrow band of strong winds usually found at elevations from 20000 to 50000 feet.

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KNOT
A nautical unit of speed equal to the velocity at which one nautical mile is traveled in one hour. Used primarily by marine interests and in weather observations. A knot is equivalent to 1.151 statute miles per hour or 1.852 kilometers per hour.

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LIGHTNING
A sudden and visible discharge of electricity produced in response to the build up of electrical potential between cloud and ground, between clouds, within a single cloud, or between a cloud and surrounding air. For an example, see ball lightning.

LUNAR ECLIPSE
An eclipse of the moon occurs when the earth is in a direct line between the sun and the moon. The moon does not have any light of its own, instead, it reflects the sun's light. During a lunar eclipse, the moon is in the earth's shadow. It will often look dim and sometimes copper or orange in color.

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MERCURIAL BAROMETER
An instrument used for measuring the change in atmospheric pressure. It uses a long glass tube, open at one end and closed at the other. After first filling the open end with mercury, it is then temporarily sealed and placed into a cistern of mercury. A nearly perfect vacuum is established at the closed end after the mercury descends. The height of the column of mercury in the tube is a measurement of air pressure. As atmospheric pressure increases, the mercury is forced from the cistern up the tube; when the atmospheric pressure decreases, the mercury flows back into the cistern. Measurement is taken in inches of mercury. Although mercurial barometers are very accurate, practicality has led observers to use aneroid barometers. First used by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician, to explain the fundamental principles of hydromechanics.

METEOROLOGY
The science and study of the atmosphere and atmospheric phenomena. Various areas of meteorology include agricultural, applied, astrometerology, aviation, dynamic, hydrometeorology, operational, and synoptic, to name a few. A meteorologist is a scientist who studies the atmosphere and atmospheric phenomena.

MIDDLE LATITUDES
The latitude belt roughly between 35 and 65 degrees North and South. Also referred to as the temperate region.

MIST
A collection of microscopic water droplets suspended in the atmosphere. It does not reduce visibility as much as fog and is often confused with drizzle.

MOISTURE
Refers to the water vapor content in the atmosphere, or the total water, liquid, solid or vapor, in a given volume of air.

MONSOON
The seasonal shift of winds created by the great annual temperature variation that occurs over large land areas in contrast with associated ocean surfaces. The monsoon is associated primarily with the moisture and copious rains that arrive with the southwest flow across southern India. The name is derived from the word mausim, Arabic for season. This pattern is most evident on the southern and eastern sides of Asia, although it does occur elsewhere, such as in the southwestern United States.

MUD SLIDE
Fast moving soil, rocks and water that flow down mountain slopes and canyons during a heavy a downpour of rain.

MUGGY
A subjective term for warm and excessively humid weather. Generally experienced when the dew point temperature is above 60 degrees.

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NOCTILUCENT CLOUDS
Rarely seen clouds of tiny ice particles that form approximately 46.6 to 56.9 miles (75-90 km) above the earth's surface. They have been seen only during twilight (dusk and dawn) during the summer months in the higher latitudes. They may appear bright against a dark night sky, with a blue-silver color or orange-red.

NOR'EASTER
A cyclonic storm occurring off the east coast of North America. These winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas.

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OBSERVATION
In meteorology, the evaluation of one or more meteorological elements, such as temperature, pressure, or wind, that describe the state of the atmosphere, either at the earth's surface or aloft. An observer is one who records the evaluations of the meteorological elements.

OVERCAST
The amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer amount for that layer.

OZONE (O3)
A nearly colorless gas and a form of oxygen (O2). It is composed of an oxygen molecule made up of three oxygen atoms instead of two.

OZONE LAYER
An atmospheric layer that contains a high proportion of oxygen that exists as ozone. It acts as a filtering mechanism against incoming ultraviolet radiation. It is located between the troposphere and the stratosphere, around 9.5 to 12.5 miles (15-20 km) above the earth's surface.

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PRECIPITATION
Any and all forms of water, liquid or solid, that falls from clouds and reaches the ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and snow grains. The amount of fall is usually expressed in inches of liquid water depth of the substance that has fallen at a given point over a specified time period.

PREVAILING WIND
A wind that blows from one direction more frequently than any other during a given period, such as a day, month, season, or year.

PSYCHROMETER
An instrument used to measure water vapor content of the atmosphere. It consists of two thermometers, a wet bulb and dry bulb. May also be referred to as a sling psychrometer.

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RAIN
Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely scattered, the drop size may be smaller. It is reported as "R" in an observation and on METAR. The intensity of rain is based on rate of fall. "Very light" (R--) means that the scattered drops do not completely wet a surface. "Light" (R-) means it is greater than a trace and up to 0.10 inch an hour. "Moderate" (R) means the rate of fall is between 0.11 to 0.30 inch per hour. "Heavy" (R+) means over 0.30 inch per hour.

RAINBOW
A luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue, indigo, and violet). It is created by refraction, total reflection, and the dispersion of light. It is visible when the sun is shining through air containing water spray or raindrops, which occurs during or immediately after a rain shower. The bow is always observed in the opposite side of the sky from the sun.

RELATIVE HUMIDITY
A type of humidity that considers the ratio of the actual vapor pressure of the air to the saturation vapor pressure. It is usually expressed in percentage. Relative humidity is, most simply put, how full the air is with moisture. If the humidity is 50 percent, for instance, it means that the air is holding about half the moisture it could hold. The ability of air to hold moisture changes with its temperature. The warmer it is, the more widely spaced air molecules become, which means there is more room to squeeze water molecules between them. So the same amount of moisture can produce radically different humidity readings depending on temperature. The same amount of moisture that would cause 100 percent humidity at 60 degrees would produce 71 percent humidity at 70 degrees, 51 percent at 80 degrees, and 37 percent at 90 degrees. That is why meteorologists tend to prefer the dew point as a better measure of actual moisture in the air. See dew point.

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SATURATE
To treat or charge something to the point where no more can be absorbed, dissolved, or retained. In meteorology, it is used when discussing the amount of water vapor in a volume of air.

SATURATION POINT
The point when the water vapor in the atmosphere is at its maximum level for the existing temperature.

SEA BREEZE
A diurnal coastal breeze that blows onshore, from the sea to the land. It is caused by the temperature difference when the surface of the land is warmer than the adjacent body of water. Predominate during the day, it reaches its maximum early to mid afternoon. It blows in the opposite direction of a land breeze.

SHOWER
Precipitation from a convective cloud that is characterized by its sudden beginning and ending, changes in intensity, and rapid changes in the appearance of the sky. It occurs in the form of rain (SHRA), snow (SHSN), or ice (SHPE). It is reported as "SH" in an observation and on METAR.

SKY
The vault-like apparent surface against which all aerial objects are seen from the earth.

SLEET
Also known as ice pellets, it is winter precipitation in the form of small bits or pellets of ice that rebound after striking the ground or any other hard surface. It is reported as "PE" in an observation and on METAR.

SLUSH
Snow or ice on the ground that has been reduced to a softy watery mixture by rain and/or warm temperatures.

SNOW
Frozen precipitation in the form of white or translucent ice crystals in complex branched hexagonal form. It most often falls from stratiform clouds, but can fall as snow showers from cumuliform ones. It usually appears clustered into snowflakes. It is reported as "SN" in an observation and on METAR.

SOLAR RADIATION
Solar radiation is the amount of radiant energy reaching the earth from the sun.

SPRING
The season of the year which occurs as the sun approaches the summer solstice, and characterized by increasing temperatures in the mid-latitudes. Customarily, this refers to the months of March, April, and May in the North Hemisphere, and the months of September, October, and November in the Southern Hemisphere. Astronomically, this is the period between the vernal equinox and the summer solstice.

SUMMER
Astronomically, this is the period between the summer solstice and the autumnal equinox. It is characterized as having the warmest temperatures of the year, except in some tropical regions. Customarily, this refers to the months of June, July, and August in the North Hemisphere, and the months of December, January, and February in the Southern Hemisphere.

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TEMPERATURE
The measure of molecular motion or the degree of heat of a substance. It is measured on an arbitrary scale from absolute zero, where the molecules theoretically stop moving. It is also the degree of hotness or coldness. In surface observations, it refers primarily to the free air or ambient temperature close to the surface of the earth.

THAW
A warm spell of weather when ice and snow melt. To free something from the binding action of ice by warming it to a temperature above the melting point of ice.

THERMOMETER
An instrument used for measuring temperature. The different scales used in meteorology are Celsius, Fahrenheit, and Kelvin or Absolute.

THUNDER
The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Over three-quarters of lightning's electrical discharge is used in heating the gases in the atmosphere in and immediately around the visible channel. Temperatures can rise to over 10,000 degrees Celsius in microseconds, resulting in a violent pressure wave, composed of compression and rarefaction. The rumble of thunder is created as one's ear catches other parts of the discharge, the part of the lightning flash nearest registering first, then the parts further away.

THUNDERSTORM
Produced by a cumulonimbus cloud, it is a microscale event of relatively short duration characterized by thunder, lightning, gusty surface winds, turbulence, hail, icing, precipitation, moderate to extreme up and downdrafts, and under the most severe conditions, tornadoes.

TIDE
The periodic rising and falling of the earth's oceans and atmosphere. It is the result of the tide-producing forces of the moon and the sun acting on the rotating earth. This propagates a wave through the atmosphere and along the surface of the earth's waters.

TORNADO
A violently rotating column of air in contact with and extending between a convective cloud and the surface of the earth. It is the most destructive of all storm-scale atmospheric phenomena. They can occur anywhere in the world given the right conditions, but are most frequent in the United States in an area bounded by the Rockies on the west and the Appalachians in the east.

TSUNAMI
An ocean wave with a long period that is formed by an underwater earthquake, landslide, or volcanic eruption. It may travel unnoticed across the ocean for thousands of miles from its point of origin and builds up to great heights over shallower water. Also known as a seismic sea wave, and incorrectly, as a tidal wave.

TWISTER
A slang term used in the United States for a tornado.

TYPHOON
The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the western North Pacific Ocean. This same tropical cyclone is known as a hurricane in the eastern North Pacific and North Atlantic Ocean, and as a cyclone in the Indian Ocean.

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ULTRAVIOLET
Electromagnetic radiation that has a wavelength shorter than visible light and longer than x-rays. Although it accounts for only 4 to 5 percent of the total energy of insolation, it is responsible for many complex photochemical reactions, such as fluorescence and the formation of ozone.

UPDRAFT
A small scale current of air with vertical motion. If there is enough moisture, then it may condense, forming a cumulus cloud, the first step towards thunderstorm development. Contrast with a downdraft.

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VAPOR PRESSURE
The pressure exerted by the molecules of a given vapor. In meteorology, it is considered as the part of total atmospheric pressure due to the water vapor content. It is independent of other gases or vapors.

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WARM FRONT
The leading edge of an advancing warm air mass that is replacing a retreating relatively colder air mass. Generally, with the passage of a warm front, the temperature and humidity increase, the pressure rises, and although the wind shifts (usually from the southwest to the northwest in the Northern Hemisphere), it is not as pronounced as with a cold frontal passage. Precipitation, in the form of rain, snow, or drizzle, is generally found ahead of the surface front, as well as convective showers and thunderstorms. Fog is common in the cold air ahead of the front. Although clearing usually occurs after passage, some conditions may produce fog in the warm air. See occluded front and cold front.

WARNING
A forecast issued when severe weather has developed, is already occurring and reported, or is detected on radar. Warnings state a particular hazard or imminent danger, such as tornadoes, severe thunderstorms, flash and river floods, winter storms, heavy snows, etc.

WATER
Refers to the chemical compound, H2O, as well as its liquid form. At atmospheric temperatures and pressures, it can exist in all three phases: solid (ice), liquid (water), and gaseous (water vapor). It is a vital, life- sustaining part of life on earth.

WEATHER
The state of the atmosphere at a specific time and with respect to its effect on life and human activities. It is the short term variations of the atmosphere, as opposed to the long term, or climatic, changes. It is often referred to in terms of brightness, cloudiness, humidity, precipitation, temperature, visibility, and wind.

WEATHER VANE
Originally used as a wind vane, it is an instrument that indicates the wind direction. The name developed based on observations on what kind of weather occurred with certain wind directions. Creative designs often adorn the tops of barns and houses.

WET BULB DEPRESSION
Dependent on the temperature and the humidity of the air, it is the difference between the dry bulb and the wet bulb readings.

WET BULB THERMOMETER
A thermometer used to measure the lowest temperature in the ambient atmosphere in its natural state by evaporating water from a wet muslin- covered bulb of a thermometer. The wet bulb temperature is used to compute dew point and relative humidity. One of the two thermometers that make up a psychrometer.

WIND
Air that flows in relation to the earth's surface, generally horizontally. There are four areas of wind that are measured: direction, speed, character (gusts and squalls), and shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds are detected through pilot balloons, rawin, or aircraft reports.

WIND CHILL INDEX
The calculation of temperature that takes into consideration the effects of wind and temperature on the human body. Describes the average loss of body heat and how the temperature feels. This is not the actual air temperature. For an example, check out the National Weather Service wind chill chart.

WIND DIRECTION
The direction from which the wind is blowing. For example, an easterly wind is blowing from the east, not toward the east. It is reported with reference to true north, or 360 degrees on the compass, and expressed to the nearest 10 degrees, or to one of the 16 points of the compass (N, NE, etc.).

WIND SPEED
The rate of the motion of the air on a unit of time. It can be measured in a number of ways. In observing, it is measured in knots, or nautical miles per hour. The unit most often used in the United States is miles per hour.

WINTER
Astronomically, this is the period between the winter solstice and the vernal equinox. It is characterized as having the coldest temperatures of the year, when the sun is primarily over the opposite hemisphere. Customarily, this refers to the months of December, January, and February in the North Hemisphere, and the months of June, July, and August in the Southern Hemisphere.

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YEAR
The interval required for the earth to complete one revolution around the sun. A sidereal year, which is the time it takes for the earth to make one absolute revolution around the sun, is 365 days, 6 hours, 9 minutes, and 9.5 seconds. The calendar year begins at 12 o'clock midnight local time on the night of December 31st-January 1st. Currently, we operate under the Gregorian calendar of 365 days, with 366 days every four years, a leap year. The tropical year, also called the mean solar year, is dependent on the seasons. It is the interval between two consecutive returns of the sun to the vernal equinox. In 1900, that took 365 days, 5 hours, 48 minutes, and 46 seconds, and it is decreasing at the rate of 0.53 second per century.

YELLOW SNOW
Snow that is given golden, or yellow, appearance by the presence of pine or cypress pollen in it.

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ZULU TIME
One of several names for the twenty-four hour time which is used throughout the scientific and military communities. Other names for this time measurement are Coordinated Universal Time (UTC) or Greenwich Mean Time (GMT).

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Sources:

Geer, Ira (ed). Glossary of Weather and Climate. Boston. American Meteorological Society, 1996.

Huschke, R.E. (ed.). The Glossary of Meteorology. Boston, Massachusetts. American Meteorological Society Press, 1980.

National Weather Service Observing Handbook No. 7, Surface Weather Observations and Reports. U.S. Department of Commerce, 1996.

Schneider, Stephen H. (ed.). Encyclopedia of Climate and Weather. New York. Oxford University Press, 1996.

The Weather Channel Glossary

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