Weather Words

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 snow or ice, and subsidence, the sinking of air which is then warmed by compression.

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

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.

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.

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

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.

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 tornado damage is reported using the Fujita Scale.

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 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 .

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.

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 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.

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.

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

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, the principal forms of radiation are gamma rays, X-rays, UV (ultraviolet radiation). visible light, infrared radiation, microwaves, and radio waves. Ultraviolet, which is invisible, is so named because it occurs next to violet in the visible light spectrum.

Ultraviolet radiation contains the wavelengths between 30 and 400 nm. The UV band is divided into three sub-bands in regard to its biological impacts:

UVA - 315 to 400 nm

UVB - 280 to 315 nm

UVC - 30 to 280 nm

The sun radiates short-wave energy which includes the UV bands. The earth's atmosphere absorbs all UVC and much of the UVB bands. Most is absorbed in the ozone layer. UV radiation is generally hazardous to life on Earth, especially in the UVC and UVB bands which cause damage to DNA. However, nearly all wavelengths less than 295 nm are completely absorbed by atmospheric gases mostly ozone and oxygen.

UTC or Coordinated Universal Time: 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.

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.

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.

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 back to early 17th Century Netherlands and was brought to America by the Pilgrims. Water in a partially filled glass chamber rises and falls due to changes in air pressure. These are amplified by a constrictive spout. A low-water level in the spout indicates high pressure and generally fair weather. The water rises in stormy weather as the pressure falls.

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.

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.

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.

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.

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

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.

Xlokk: The name of a hot, dry wind that blows regularly in Malta.

Yellow Snow: 1) Snow which has a yellow or golden colour due to the presence of pine or cypress pollen in or on it. 2) Snow patches with a yellow colour which has been contaminated by urine (usually from dogs). (The city folk made me add this second one.)

Zephyr: Any soft, gentle breeze. The name derives from the ancient Greek name for the west wind Zephyros, which is light and beneficial in Greece. On the Tower of the Winds in Athens, Zephyros is represented by a lightly clad youth whose skirt is filled with flowers.

Zonal Flow: Large-scale atmospheric flow in which the east-west component, i.e., flow parallel to the latitude lines, is much greater than the meridional (north-south) component. Compare with meridional flow.

The Weather Doctor's Weather Glossary ©2006, Keith C. Heidorn, PhD. All Rights Reserved.
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