Copy of `Weather Doctor - Weather Terms`

The wordlist doesn't exist anymore, or, the website doesn't exist anymore. On this page you can find a copy of the original information. The information may have been taken offline because it is outdated.

Weather Doctor - Weather Terms Category: Meteorology and astronomy > Weather Glossary Date & country: 05/10/2008, USA Words: 198

Winter Storm
A large-scale disturbance, often hundreds of kilometres across, associated with a low-pressure system, or cyclone, that develops along a front during the cooler part of the year. Winter storms can produce strong winds, heavy precipitation (rain, freezing rain, ice pellets or snow) and cold temperatures.

Wind Direction
The direction from which the wind blows. For example, a wind blowing from south to north is a southerly wind.

Wind-Chill Factor
The accelerated heat loss from exposed skin due to increased wind speed. A non-physical value which combines the effect of air temperature and the wind speed to illustrate how the air 'feels.' Wind chill is calculated by combining air temperature and wind speed and reported as a wind-chill temperature in degrees Celsius (degrees Fahrenheit) or as a cooling rate in Watts per square metre.

Weather Satellite
An instrument platform in earth orbit which views the atmosphere with a variety of sensors including visible and infrared cameras. A weather satellite may produce data used to form any of a number of satellite images or raw data used in research and forecasting.

Wind
Air in motion relative to the surface of the earth. Because the vertical (up/down) component of wind is generally small compared to the horizontal component, wind generally refers to the horizontal wind. When the vertical component is discussed, vertical wind is commonly used.

Weather
The state of the atmosphere and its short term variations as defined by various weather elements, most notably, temperature, wind, humidity, cloudiness, visibility and precipitation, and storminess.

Water Vapour
The substance water (H2O) in its vapour, or gaseous, phase. Water vapour, a strong greenhouse gas, varies, vertically and horizontally, in concentration in the atmosphere from 0 to 4 percent. Highest concentrations are found in the troposphere.

Water Barometer
(1) A form of liquid barometer designed on the principle that the downward pressure of the air's weight will support a column of liquid water in an inverted, evacuated glass tube. Since a true water barometer would be over 10 m (33 ft) in length, they are not practical for weather observations. (2) A form of barometer aslo known as a 'storm glass' or 'thunder glass.' This instrument dates bac...

Virga
Virga is falling precipitation (rain, snow or ice crystals) that evaporates before reaching the ground, appearing as streaks or wisps of precipitation terminating in mid-air. Typically, it falls from altocumulus, altostratus, or high-based cumulonimbus. Virga can be detected by radar and is reported as precipitation aloft.

Vapourization
The movement of liquid water (or any other liquid) into the gaseous or vapour state. Also called evaporation.

Vernal
Pertaining to the Spring season. The Vernal Equinox occurs on or about March 21 in the Northern Hemisphere and on or about September 21 in the Southern Hemisphere.

UTC
The currently accepted name of the twenty-four hour time scale used throughout the scientific and military communities to coordinate and standardize time using an atomic clock rather than solar observations. Coordinated Universal Time is denoted by the acronym UTC or by the letter Z UTC is the modern successor of Greenwich Mean Time (GMT) which was used when the unit of time was the mean solar day...

Unstable Air
An atmospheric state where strong vertical motions are produced by displacement of a parcel of air that accelerates upward when the parcel is moved upward, or downward when the parcel is moved downward. Usually found in regions of the atmosphere where a convective temperature profile is found.

Ultraviolet Radiation
Ultraviolet radiation (UV) is one form of radiant energy coming from the sun. The sun emits a range of energy known as the electromagnetic spectrum. The various forms of energy, or radiation, are classified according to wavelength (measured in nanometres (nm) where one nm is a millionth of a millimetre). The shorter the wavelength. the more energetic the radiation. In order of decreasing energy, t...

Trough
An elongated area of relatively low atmospheric pressure. Also the lowest portion of a wave found between the wave crests.

Troposphere
The layer of the atmosphere from the Earth's surface up to the tropopause, characterized by decreasing temperature with height, appreciable vertical wind motion and considerable water vapour content. Most of what we sense as weather (most clouds, rain, etc.) occurs in the troposphere. Meteorologists subdivide the troposphere into the boundary layer, Ekman layerand free atmosphere.

Tropical Storm
A tropical cyclone in which the 1-minute sustained surface wind ranges 62 to 117 kilometres per hour (39 to 73 mph) .

Tropopause
The boundary between the troposphere and stratosphere found between 8 to 20 km in altitude. The tropopause is higher over the tropics and lowest in the polar regions. It is characterized by an abrupt increase in temperature (the stratospheric temperature inversion). The tropopause generally marks the vertical limit of most clouds and storms.

Tropical Disturbance
A discrete system of apparently organized convection originating in the tropics or subtropics, having a non-frontal migratory character and maintaining its identity for 24 hours or more.

Tropical Depression
A tropical cyclone in which the maximum 1-minute sustained surface wind is 61 kilometres per hour (38 mph) or less. They form from a tropical wave or tropical disturbance.

Tornado Outbreak
A tornado outbreak occurs when a large number (six or more) of tornadoes are formed in groups or individual storms within a 24-48-hour period over a specific geographical area and spawned from the same general weather system. Tornado outbreaks are classified into three categories: local outbreaks , line outbreaks and progressive outbreaks .

Tornado Alley
An area of the United States where tornadoes are the most frequent encompassing the low-lands of the Mississippi, Ohio and lower Missouri River valleys. Tornado Alley is generally considered to be the states of Texas (northern part), Oklahoma and Kansas.

Tornado
A violently rotating column of air, a vortex, formed in a thunderstorm which is in contact with both the parent cloud and the ground with a diameter between tens and hundreds of metres. The lifetime of a tornado is usually less than an hour, but some have been observed to last several hours. Wind speeds are usually above 64 km/h (40 mph) and can reach over 500 km/h (300 mph). The intensity of torn...

Total Column Ozone
The total amount of ozone present in a column of the Earth's atmosphere from the surface to the top of the atmosphere. It includes both surface level ozone and ozone found in the ozone layer. Measurement is either in Dobson Units or atmosphere-cm.

Thunderstorm/Thundershower
A local storm produced by a cumulonimbus cloud that contains thunder and lightning and often strong wind gusts, heavy rain showers and, at times, hail. A thundershower is a popular term usually applied to a mild thunderstorm. There are three main types of thunderstorms, classed by the mechanism that triggers them: advection thunderstorm, air-mass thunderstorm, frontal thunderstorm.

Thunderheads
A popular term referring to the anvil top of cumulonimbus clouds.

Thunder
The sound emitted from a lightning channel during a lightning discharge caused by rapidly expanding gases. The sound can have a number of different characters: roll, rumble, crack, crash, series of short bursts.

Thunder Bolt
Popularly a lightning bolt. In mythology the thunderbolt was usually hurled by the god of thunder (for example, Thor) as a bolt or dart.

Thermal
A rising parcel of warm, and less dense, air generally produced when the Earth's surface is heated or when cold air moves over a warmer surface such as warm water.

Temperature Profiles
The change of air temperature with height above the ground. When the temperature increases with height, the profile is called an inversion profile, or simply an inversion. When the temperature decreases strongly with height, a convective profile may be established

Temperature Inversion
A layer of air in which the temperature increases with height. Meteorological convention considers temperature decreasing with height as the norm, thus when temperature decreases with height, it is inverted. There are four common causes of a temperature inversion: radiational cooling, advection of warm air over cold air as in frontal situations, advection of warm air over a cold surface such as sn...

Surface Analysis
The weather map depicting surface weather conditions plotted from reported data or generated by computer models. Surface Analysis maps generally display isobars, fronts and centres of High and Low pressure.

Sun Pillar
A pillar forming from sunlight. See Pillar

Supercooled
A state when the temperature of liquid water falls below 0oC (32oF) without freezing. Although we commonly speak of 0oC as the freezing point of water, water, especially in the droplet form, rarely freezes at this temperature. (Pure ice, on the other hand, melts at 0oC, thus, this temperature should be more correctly called the melting point of ice.) The temperature of freezing varies with the siz...

Sun Dog
A popular term used to describe the parhelion, a bright portion of a solar halo which forms on points 22° either side of the sun and at the same elevation. Sun dogs are colored, luminous spots caused by the refraction of light by ice crystals. The term mock sun is also used to describe this optical phenomenon.

Summer Solstice
The date on which the sun reaches the greatest distance north (in the Northern Hemisphere) or south (in the Southern Hemisphere) of the celestial equator. In the Northern Hemisphere, the summer solstice occurs when the sun in overhead at the Tropic of Cancer (latitude 23° 27' N) about June 21. In the Southern Hemisphere, the summer solstice occurs when the sun in overhead at the Tropic of Caprico...

Summer
1) The warmest season of the year (except in some tropical regions) and the high sun season. 2) The period between the Summer Solstice and the Autumnal Equinox (June, July and August in the Northern Hemisphere; December, January, February in the Southern Hemisphere).

Storm Track
The path followed by the centre of a low pressure system or cyclone over a given period of time, usually its lifetime.

Sublimation
The transition of a substance from the solid phase directly to the vapour phase without being in the liquid phase. The opposite of sublimation is deposition.

Storm Surge
An abnormal rise local rise in sea level accompanying an intense storm system, either tropical or extratropical caused by the storm pushing a wall of water ahead of it. Storm surge is often the most damaging and deadly part of a tropical hurricane or cyclone, particularly if it arrives at high tide.

Stratosphere
The region of the atmosphere extending from the tropopause (8 to 15 km altitude) to about 50 km. The temperature of the stratosphere is warmer than the upper troposphere thus making it a region of high stability and low humidity.

Stratus
The lowest clouds, generally found below 3000 metres altitude and often appearing as an overcast deck. Can also be found as scattered patches. Individual cloud elements have very ill-defined edges. Stratus is comes from the Latin word for 'layer.'

Stable Air
An air mass or portion thereof in which vertical motions are inhibited. Usually found in regions of the atmosphere where an inversion temperature profile is found.

State
The form of a substance: solid, liquid, or gas/vapour.

Stream fog
A form of fog caused by the advection of cold air over a warm water surface or a very saturated soil surface, giving the impression of rising steam. Common over large lakes and near-shore ocean areas during the cold season.

Squalls
1) A strong wind which arrives suddenly, lasts minutes and ends with a sudden decrease in speed. 2) A severe local storm with strong. gusty winds and usually precipitation, may be accompanied by thunder and lightning.

Specific Humidity
The ratio of the mass of water vapour to the total mass of air in a given volume.

Speed of Sound
The speed that a sound wave travels through a given medium. The speed of sound depends on several properties of the medium, most importantly its density. In the atmosphere, the speed changes with air temperature and pressure. At standard sea-level pressure (101.3 kPa), the speed of sound is 340 metres per second (760 mph).

Southern Oscillation (ENSO)
A periodic reversing of the surface air pressure pattern across the tropical Pacific Ocean. The Southern Oscillation is associated with El Niño events so that the two are often referred together by the acronym ENSO.

Solstice
Either of two days when the sun's position overhead at noon is farthest north or south of the equator. These dates are June 21 for the Northern Hemisphere summer solstice, and about December 22 for the Northern Hemisphere winter solstice. The solstices are reversed for the Southern Hemisphere, with the winter solstice on June 21.

Solar Winter
The three-month period when day length is shortest and solar radiation above the clouds is the least. In the Northern Hemisphere, the period from approximately November 1 to February 1.

Solar Summer
The three-month period when day length is longest and solar radiation above the clouds is the greatest. In the Northern Hemisphere, the period from approximately May 1 to August 1.

Solar Seasons
A division of the year for non-tropical latitudes into four three-month periods based on the daylength or amount of potential solar radiation. During solar winter and solar summer, the day length changes most slowly from day to day, with the least variation around the solstice. During solar spring and fall -- the transistion seasons -- the day to day change in daylength is very rapid, peaking at t...

Solar Radiation
That portion of the electromagnetic spectrum less than 4,000 nanometres which is received from the sun.

Snowflake
A single ice crystal or an aggregate of ice crystals falling from a cloud.

Snow Squall
Short, intense snow showers accompanied by strong, gusty winds. Short- term snow accumulations may be significant and visibility greatly reduced. Snow squalls are common along the shores of the Great Lakes and other large lakes. See also, lake-effect snowfall.

Snowburst
An intense convectively generated snow squall with large accumulations of snow at a rate of 2.5 to 7.5 cm per hour (1 to 3 inches per hour). May be accompanied by thunder and lightning; such storms are called thundersnows.

Snow Crystal
One of several types of ice crystal found in snow. A snow crystal is an individual ice crystal whereas a snowflake is usually an aggregate of many single crystals. See Ice Crystals

Snowbelt
In general, any area where topographical influences favour increased accumulation of snow over the surrounding region. Usually refers to the areas around large lakes such as the Great Lakes where lake-effect snows produce significantly enhanced winter accumulations.

Snow
A type of frozen precipitation composed white translucent ice crystals in a variety of complex hexagonal forms.

Sleet
Precipitation composed of already frozen droplets — ice pellets — that bounce on impact. Outside the United States, sleet also refers to precipitation with a mixture of rain and snow or rain and ice pellets or quickly melting snow.

Singularity
A characteristic meteorological condition that tends to occur on or near a specific calendar date more frequently than chance would indicate.

Shower
Precipitation falling from a convective cloud, characterized by the suddenness with which it starts and stops, by rapid changes of intensity and usually rapid changes in the appearance of the sky.

Saturation
The condition when the partial pressure of water vapour in the atmosphere is at its maximum level for the existing temperature and pressure. For example, at saturation an equilibrium exists between water vapour and liquid water and there is no net evaporation or condensation.

Season
A division of the year according to some regularly recurring event or phenomena, usually astronomical or climatological. The four astronomical seasons: Spring, Summer, Autumn or Fall, and Winter are generally defined by the position of the sun with respect to the Earth's equatorial plane. The four climatological seasons known to most are tied to the astronomical seasons. However, in areas where c...

Satellite Imagery
Images formed from data collected by a weather satellite that reveal visual information on the nature of the atmosphere such as the flow of water vapour, the development and movement of weather systems, or the distribution of the temperatures of cloud tops or the earth's surface.

Santa Ana Wind
A hot, dry desert, foehn-type wind blowing from the northeast or east in the pass and river valley around Santa Ana, California. The wind may blow with great force and carry large amounts of dust. It most frequently occurs in the winter months. However, when it blows in the autumn, it can dessicate vegetation and increase the hazards from wild fires in canyons of Southern California.

Runoff
The movement of water across the earth's surface or just beneath the surface coming from liquid precipitation or the melting of ice/snow. Runoff ultimately reaches stream channels, rivers, lakes, or seas unless it is evaporated or taken up by the biosphere.

Ridge
When an extensive area of high pressure is elongated, it is called a high pressure ridge, or simply a ridge.

Rime
Ice that is opaque, milky and crystalline ice forming on vertical and horizontal surfaces by the freezing of supercooled water. Rime ice is less dense than glaze ice ( 0.2 to 0.3 grams per cubic centimetre) and clings less tenaciously. Factors favouring rime formation include: small water drop size, slow accretion, high supercooling of water in the drop, and rapid dissipation of the heat of fusion...

Relative Humidity
The ratio of the actual water vapour content of a given mass of air to that it would hold at saturation, usually expressed as a percentage. Also, the ratio of the water vapour pressure to the its saturation vapour pressure.

Residence Time
The length of time that some substance remains in a reservoir. For example, the typical residence time of carbon dioxide in the atmosphere is the average length of time that a molecule of the gas will remain in the air before chemically transforming or moving into the hydrosphere, biosphere, or lithosphere.

Reflectivity
A measure of the fraction of incident radiation falling on a surface that is turned back from it by reflection. Reflectivity also refers to the degree by which precipitation is able to reflect a radar beam.

Rain Shadow
The region on the lee side of a mountain or mountain range, where precipitation is decreased compared to the windward side. The rain shadow results when moisture-laden air is forced to ascend the encircling mountains and thus drops most of that moisture on the windward side of the range summits. As the air descends from the ridge, it is drier and warmer. Not only does the compression of descent wa...

Rainforest
Lush forests growing in regions of high precipitation. Most well known are the tropical rainforests or jungles such as in the Amazon basin. Rainforests also grow along the northern Pacific coast of North America. These great northern and temperate rainforests covered much of coastal regions of southern Alaska, British Columbia, and Washington State.

Radiosonde
An instrument package connected to a weather balloon that collects, and transmits by radio, meteorological data as it ascends through the atmosphere.

Quasi-Stationary Front
A weather front which is nearly stationary, having speeds of less than 8 km/h (5 mph).

Radiation fog
A form of fog caused by the cooling of moist air to below its dew point by the process of heat radiation. Radiation fog usually forms near the surface at night under clear skies and light wind conditions. Also known as ground fog.

Pyrocumulus clouds
Cumulus cloud forms formed by fire, usually large wildfires. Similarly, pyrocumulonimbus, cumulonimbus clouds formed by fire.

Psychrometer
An instrument used to measure water vapour content of the atmosphere. A common type consists of two liquid-in-glass thermometers, one a dry bulb and another, with a muslin sock over the tip which is saturated with water, called the wet bulb. The thermometers are ventilated to cause evaporation off the wet bulb thermometer. The atmospheric humidity is determined from the readings of these two therm...

Probability of Precipitation
The POP expressed as some percentage X means that forecasters have determined that in a 100 similar weather situations, rain or snow will have fallen X times in the forecast area. Note that POP is for any point in the forecast area, not the whole area. With a POP of 100% in the forecast, rain or snow is almost a certainty somewhere in the forecast area.

Polar Stratospheric Clouds
Clouds which form in the stratosphere when the temperature falls to below -80oC. The clouds are composed of ice which provides a surface for chemical reactions that result in the release and storage of reactive chlorine and bromine. In the spring when the sun warms the clouds, they release the chlorine and bromine which then may destroy ozone.

Polar Front
The boundary separating air masses of tropical origin from those of polar origin. The front is usually only present in the far-northern polar regions in summer but dips down to the more southerly temperate latitudes during the winter. Many cyclonic storm systems form along the polar front.

Pillar
A vertical shaft of light extending above or below the sun, moon, or other strong light source. Also, sun pillar.

Phase
1: The aggregate state of a substance: solid, liquid, or vapour/gas; 2: A particular shape for the appearance of the Moon: i.e., Full, New, Crescent; 3: A point of reference on a periodic wave.

Photochemical Reactions
The chemical reaction of two or more substances which is caused or hastened by the interaction of light on the reactants. In the atmosphere, the source of this light is the sun.

Persistence Forecast
A weather forecast that the future weather will be the same as the current conditions, e.g. tomorrow's weather will be the same as today's. In the tropics where weather doesn't vary much, a persistence forecast can be quite accurate. In areas along the polar front where storm waves rule and conditions change rapidly, it can be very unreliable. However, it is often used by meteorologists against...

Parhelion
A bright, colored portion of a solar halo which forms on points 22° either side of the sun and at the same elevation. These luminous spots are caused by the refraction of light by ice crystals. The terms mock sun and sun dog are also used to describe this optical phenomenon.

Ozone Layer
About 90% of the ozone in the Earth's atmosphere is found in a layer between 22 and 30 km above the surface. This layer is within a region of the atmosphere called the stratosphere. The stratosphere is a stable layer of the atmosphere where exchanges with the lower atmosphere or troposphere are minimal. Ozone is naturally formed here through the action of sunlight on molecular oxygen (O2). The in...

Ozone
A molecule made up of three oxygen atoms. It absorbs both ultraviolet-B and heat radiation. It is formed in the upper atmosphere through the dissociation of oxygen molecules by solar radiation. In the lower troposphere, it is a pollutant which characterizes photochemical or Los Angeles-type smog. It is formed by the photochemical reactions of nitrogen oxides and hydrocarbon gases.

Outflow
The outward flow of air from a weather system. From a thunderstorm, it is the result of cold downdrafts, and its passage includes a wind shift and temperature drop. On the Canadian Pacific Coast, an outflow usually describes the movement of cold air off the continent through mountain passes and over coastal waters.

Numerical Models
Computer models using the fundamental equations of hydrodynamics and thermodynamics with observed initial conditions to forecast the weather or describe the state of the atmosphere or ocean.

Orographic Clouds
Clouds formed when air forced to rise when flowing over mountains or large hills reaches the condensation level.

Nimbus
The Latin word for 'rain' used to describe a cloud or group of clouds from which rain is falling, e.g., nimbostratus.

Nor'easter
Common contraction for northeaster, a northeast wind often blowing at gale or storm speeds. Also refers to a type of storm moving up the North American Atlantic coast which is first felt as a northeast wind and it moves northward.

Monsoon
Very persistent winds which may blow for months at a time from one general wind direction during one season and then a quite different, often opposing direction for another season. Originally, the name for the seasonal winds that blow across the Arabian Sea. For six months the winds blow consistently from the northeast and for six months from the southwest. The name has been extended to other area...

Mizzling
A term used by Henry David Thoreau to describe weather combining thick mist and drizzle: a mizzling rain. North American usage may be peculiar to New England. Term is also found in Jane Austen's novel Northanger Abbey. (Thanks to Hal Noakes for the Austen information.)

Midnight Sun
A high latitude phenomenon observed around midsummer when the sun does not sink below the horizon throughout the 24 hour period; therefore the sun may be seen at midnight. At the poles, the midnight sun occurs from the Spring to Autumnal Equinoxes.

Millibar
An older metric unit of atmospheric pressure still widely used. One millibar is equal to one hectopascal.

Microburst
The microburst is a downburst which is confined to a small area, less than 4 km (2.5 miles) in diameter from the initial point of downdraft impact and lasting for less than 10 minutes. An intense microburst can result in damaging winds near 280 km/hr (170 mph).