G LOSSARY
   

AIR MASS - A large expanse of air having similar temperature and humidity at any given height.

ANEMOMETER - An instrument used to measure wind speed

ANTICYCLONE - A region of high atmospheric pressure around which winds move in a clockwise direction in the northern hemisphere. Generally associated with warm and dry weather.

ANVIL - The cirrus outflow forming the top of a large thunderstorm giving it (the storm) an anvil shape.

ATMOSPHERE - The gaseous fluid surrounding a planet. The earth's atmosphere consists primarily of oxygen and nitrogen; however it is the water vapor content that is responsible for most of the weather.

ATMOSPHERIC PRESSURE - The downward force of the air per unit area, averaging 101.3 kpa at sea level (1013.25 millibars or 29.92 inches of mercury).

AWIPS - An acronym for Advanced Weather Interactive Processing System. This computer system is now the backbone of the NWS; it replaces the old AFOS computer network. It consists of powerful minicomputers linked in a nationwide network to weather service offices and national centers. AWIPS workstations are able to simultaneously display real-time radar and satellite imagery as well as high-resolution model output graphics - all on the same screen!

BACKING - A counterclockwise shift in wind direction (for example, south winds shifting to the east).

BLIZZARD - A very cold wind with blowing snow. Sustained winds (or frequent gusts) reach or exceed 40 mph, and visibilities are restricted to 1/4 mile or less by the blowing snow.

CAPE - Convective Available Potential Energy - The amount of buoyant energy available to accelerate an air parcel vertically; it is a measure of the potential intensity of deep,moist convection. An atmosphere with high CAPE values has the potential to develop strong, tall and violent thunderstorms, whereas an environment with little or no CAPE has little chance of developing a thunderstorm - or if they do form, they will be weak and low topped..

CELSIUS SCALE - The standard temperature scale in the metric system which sets the freezing point at 0 and the boiling point at 100 (compared to 32 and 212 degrees respectively in the more familiar Fahrenheit scale).

CIRRUS - High clouds (generally above 20,000 feet) which are composed of ice crystals.

CLIMATE - This refers to the average state of the atmosphere over a long period of time, taking into account temperature, wind, humidity and rainfall. To put it succinctly, "Climate is what we expect - weather is what we get".

CLIMATOLOGY - A branch of meteorology that deals with the long-term characteristics of weather.

COLD ADVECTION - This occurs as a cold air mass replaces a warm air mass.

COLD FRONT - The leading edge of a cold air mass. Cold frontal passages are usually associated with sharp wind shifts, pressure rises, showery or stormy conditions, and rapid temperature drops.

CONTRAILS - A trail of clouds, usually cirrus, that forms behind aircraft as they travel through the atmosphere.

CONVECTION - Rising air currents due to heating as sunlight warms the ground and the lower atmosphere. Warm air rises because it is less dense after it expands. Convection is the process that leads to the formation of thunderstorms.

CORIOLIS ACCELERATION - The earth's rotation causes moving objects, unattached to the earth, to accelerate to the right of their path of motion in the northern hemisphere.

CUMULONIMBUS CLOUD - A cloud with considerable vertical growth and precipitation; a thunderstorm. Generally contains an anvil cloud at the top.

CUMULUS - Clouds that exhibit vertical growth and are caused by convective processes.

CYCLOGENESIS - Deepening of a low pressure system associated with the continuing development of a mid-latitude cyclone (storm).

CYCLONE - An area of low pressure around which winds blow in a counter-clockwise fashion in the northern hemisphere. Generally associated with cool, wet and unsettled weather.

CYCLONIC - Counter-clockwise airflow around a low pressure system in the northern hemisphere (clockwise in the southern).

DEW POINT TEMPERATURE - The temperature of the air when it is cooled to saturation. (When saturation is reached, the air cannot hold any additional moisture, and any excess moisture condenses as dew on surface features such as grass or plants).

DIURNAL TEMPERATURE RANGE - The temperature difference between the minimum at night (low) and the maximum during the day (high).

DOPPLER RADAR - Radar that employs the shift in frequency of radio waves returned from moving objects in order to give the objects' speed. Thus, the radar can detect the motion of individual air parcels, raindrops, fronts, or storms.

DRY ADIABATIC LAPSE RATE - The rate of cooling (by expansion) as an air parcel rises, if no other additions of heat occur. It is approximately 10 degrees Celsius per kilometer, or 5 degrees Fahrenheit per thousand feet. It is also the rate of heating (by compression) as an air parcel descends.

DRY LINE - The leading edge of an advancing hot dry air mass of continental tropical origin, the Desert Southwest for the United States. The dry line usually acts as a focusing mechanism for strong thunderstorms during the summer convective season.

DUST DEVIL - A small surface generated vortex storm (or whirlwind) that occurs in deserts and other locations where the ground is very hot. Dust devils resemble tiny tornados, but are caused by convective processes, and they produce significantly less damage than tornados - in many cases they cause no damage of consequence.

DYNAMIC LIFTING - The forced uplifting of air from various atmospheric processes, such as weather fronts, and cyclones.

ENVIRONMENTAL LAPSE RATE - the measured lapse rate (rate of decrease in temperature with height) in the atmosphere surrounding a rising bubble of air. It usually is around 3-4 degrees Fahrenheit per thousand feet.

EQUINOX - The time when the sun crosses the plane of the Earth's equator occurring about March 21 and September 22. During the Equinox, the day will have equal number (12) of daylight and nighttime hours.

FOG - Suspension of liquid water droplets in the air near the earth's surface that reduces visibility to less than 1 kilometer (5/8 mile). This occurs when the air is saturated, or when the temperature is the same as the dewpoint. It also generally occurs with light winds.

FREEZING RAIN - Rain that turns to ice as it strikes cold objects or the cold ground.

FUNNEL CLOUD - A ROTATING vortex extending from the base of a thunderstorm (or developing convective cloud) that would become a tornado IF it touched the ground.

GEOSTROPHIC WIND - The calculated or measured wind that occurs when the pressure gradient acceleration equals the Coriolis acceleration. The geostrophic wind is seen above the boundary layer, in the upper layers of the atmosphere.

GRADIENT WIND - Wind blowing along curved isobars with a balance of centrifugal, Coriolis and pressure gradient accelerations.

GRAUPEL - A type of winter precipitation formed by packed snow flakes or ice crystals after riming has given it the appearance of small soft hail. Otherwise known as snow pellets.

GREENWICH MEAN TIME (GMT) - Since the prime meridian of 0 degrees longitude passes through Greenwich, England, this location is used for a universal time throughout the world. Each of the time zones are measured outward 15 degrees from the prime meridian. Thus, 1400 (2 PM) GMT is 0900 EST, 0800 CST, 0700 MST etc. GMT minus 7 hours equals MST. GMT is also called "Z" time and most NWS products, especially model guidance products, use this time.

GUST FRONT - A miniature cold front created by the downdraft of a large thunderstorm as the dense, rain-cooled air spreads outward beneath the thunderstorm. Gust fronts can produce strong, gusty, damaging surface winds, and often serve as a triggering mechanism for new thunderstorms.

HAIL - Solid precipitation in the form of chunks or balls of ice that occurs with strong convection, generally in the form of thunderstorms.

HOOK ECHO - The shape of a severe thunderstorm echo on a radar screen which denotes the presence of a tornado. The echo is in the form of a hook, or appendage extending off of the main storm echo. Most often seen in the radar imagery of Midwest or great plains storms - very rarely seen in Arizona radar imagery.

HORIZONTAL CONVERGENCE - A net flow of air towards a given location, that occurs from converging air currents or changes in wind speed. Convergence in the lower atmosphere results in rising air, and the formation of clouds and weather.

HORIZONTAL DIVERGENCE - A net flow of air outward from a given location, that occurs from diverging air currents or changes in wind speed. Divergence in the lower atmosphere results in sinking air, and generally warm and dry weather.

HURRICANE - An atmospheric vortex storm of tropical origin that is intermediate in size between a tornado and a mid-latitude cyclone. A tropical storm is classified as a hurricane when its sustained winds reach or exceed 74 mph.

HYGROMETER - An instrument used to measure atmospheric humidity.

INVERSION - A region in the atmosphere where the temperature increases with height. The presence of an inversion creates a very stable atmosphere; when it occurs at the surface it leads to very little mixing and a trapping of pollutants in the lower atmosphere.

INSTABILITY - The atmosphere is in an unstable state when a displaced parcel of air continues to move either up or down, away from its initial location. Instability is necessary for convection to occur, and for the formation of thunderstorms.

ISOBARS - Lines of constant pressure. On surface meteorological charts, these contours are used to delineate areas of low and high pressure.

ISOTHERMS - Lines of constant temperature on a weather map.

JET STREAM - A stream of air, about 12 km (8 miles) high, with an average speed of 100 km/hr (67 mph) at 30 to 50 degrees north latitude. Jet stream winds are from a general westerly direction, and since mid latitude storm systems follow the jet stream, it is also known as the "storm track".

LAPSE RATE - The change in temperature with altitude. A positive lapse rate occurs when the temperature decreases with altitude (the normal occurrence in the lower atmosphere).

LIFTED INDEX - A calculated index used to measure atmospheric stability. Positive lifted indices denote a stable atmosphere, negative numbers an unstable atmosphere; the greater the negative number the more unstable the atmosphere, and the greater the chances for thunderstorm development.

LOW-LEVEL JET - A stream of air moving northward from the Gulf of Mexico, generally lower in altitude (~ 2 km or 1.3 miles) than the jet stream. Wind speeds are lower than the polar jet stream, and the low level jet generally transports significant moisture from the gulf into the central and northern United States.

LOW PRESSURE SYSTEM - A region of relatively lower atmospheric pressure in the midlatitudes, generally accompanied by cloudy, windy and wet weather.

MESOCYCLONE - The cyclonic, rotating part of a large thunderstorm. Tornados are usually formed in association with the mesocyclone.

MESOSCALE - A scale used in meteorology that extends from approximately 1 to 100 km (.7 to 67 miles). Mesoscale features include such things as thunderstorms, sea breezes, gust fronts and macrobursts.

MICROSCALE - A scale used in meteorology that extends from about 1 centimeter to a kilometer (.67 miles). Microscale processes include such things as dust devils, tornados and thunderstorm updrafts/downdrafts.

MILLIBAR - A unit of pressure in the metric system. One atmosphere equals 1013 millibars. The 500 millibar level, a standard level used in meteorology and in weather forecast maps, is located about 18000 feet above sea level; approximately half of the earth's atmosphere is located below this height!

MIXING RATIO - The mass of water vapor per unit mass of dry air, in grams/kilogram. Higher values of mixing ratio imply a more humid atmosphere.

MOIST ADIABATIC LAPSE RATE - The expansion cooling rate of rising saturated air (from 4 to 10 degrees Celsius per kilometer - or 2 to 5 degrees Fahrenheit per thousand feet). As air rises it expands, and cools. However, as saturated air rises, the cooling forces moisture to condense out of the air. As the vapor condenses it releases heat into the atmosphere, which partially offsets the expansional cooling.

MONSOON - A seasonal shift in the winds. For Arizona, the monsoon results in westerly winds shifting to the south or southeast; this shift brings considerable moisture into the state from the Gulf of California and the Gulf of Mexico.

OCCLUDED FRONT - A front that is formed as a cold front overtakes a warm front.

POLAR JET STREAM - A stream of air blowing from a westerly direction at a height of about 12 km (8 miles) over midlatitude locations. It generally separates polar air masses from warmer air; storms generally move with the jet stream and thus it is also known as the "storm track".

PRECIPITABLE WATER - The depth, or amount, of water that would result if all of the water vapor in the atmosphere above a certain location were condensed into liquid water. NWS graphics show precipitable water in terms of inches and percentage of normal (for the given day).

PRECIPITATION PROBABILITY - the degree of certainty, or chance, of rainfall at a given location (from 0 to 100 percent). In the NWS, the percent figure represents the chance of getting measurable rainfall (0.01 inch or more) in the official rain gauge at a site.

PRESSURE GRADIENT FORCE - The force exerted on air due to a difference in pressure between two points. One of the forces that causes wind; as the TV meteorologists often say "wind blows from the high pressure to the low pressure area".

RADAR - An instrument useful for remote sensing of meteorological phenomena. It operates by sending radio waves and monitoring those returned by reflecting objects such as raindrops, cloud droplets, dust particles, blowing dirt and sand grains, or birds. When the reflecting particles are raindrops, the higher the energy returned to the radar, the larger the drops and the heavier the rainfall.

RADIATION - Electromagnetic energy emitted by all objects (including the sun, of course) in amounts related to the temperature of the object.

RADIATION FOG - Fog formed as the ground (and objects on the ground) cool by losing long- wave infrared radiation. Conditions favorable for formation of radiation fog include light winds, clear skies, and high humidity at the surface (near or at 100 percent).

RADIOSONDE - The instrument package carried by weather balloons to measure the temperature, humidity and pressure of the atmosphere at multiple levels (up to at least 30000 feet in altitude).

REFLECTION - Light that bounces off an object at the same angle and wavelength as that of the incident (initial) light ray.

REFRACTION - The bending of light at a particular angle as it passes through a transparent medium (such as glass or water). Rainbows are causes by light refracting and reflecting (twice) inside raindrops.

RELATIVE HUMIDITY - The amount of water vapor in the air divided by the amount of water vapor at saturation, expressed as a percentage. 100 percent humidity denotes saturated air. Warm air can hold more moisture than cold air, so for a given amount of water vapor, the higher the temperature, the lower the relative humidity.

RIDGE - A region of anticyclonic curvature of winds in the atmosphere located around a high pressure area. A ridge in the upper atmosphere is generally associated with dry and warm weather.

SANTA ANA WIND - Strong down slope winds that blow through the mountain passes in southern California. These winds, which can easily exceed 40 mph, are warm and dry and can severely exacerbate brush or forest fires, especially under drought conditions.

SATURATION - The condition when the atmosphere is holding the maximum amount of water vapor that it can for a given temperature. When the air is saturated, the temperature will equal the dew point temperature.

SEVERE THUNDERSTORM - Thunderstorms with damaging winds or hail. The NWS defines a severe storm as one having winds greater than 57 mph or hail 3/4 inch in diameter or larger (dime size or bigger).

SNOW PELLETS - Precipitation of white or opaque spherical ice, usually 5mm in diameter or less. Snow pellets, also known as graupel, are softer than hail or ice pellets.

SOLSTICE - Either of the two times of the year when the sun is the greatest distance from the celestial equator, occurring about June 22 and December 22. The summer solstice marks the day containing the maximum hours of daylight, whereas the winter solstice has the fewest hours of daylight (the longest night).

STABLE ATMOSPHERE (STABILITY) - The condition when little or no vertical mixing occurs due to the nature of the temperature change with height (the temperature either becomes warmer with height, or cools at a low rate). Under stable conditions, convection (and thunderstorms) are inhibited, winds are generally on the light side, and pollution is easily trapped in the lower layers of the atmosphere (near the ground).

STATIONARY FRONT - A surface boundary between air masses (such as a cold or warm front) that has stalled and has little or no motion.

STRATUS - Low clouds that have a stratified form, containing liquid water droplets.

STREAMLINE - A line on a weather map showing the path a parcel of air would follow as it moves through the atmosphere.

SUBTROPICAL JET STREAM - A stream of air from the west, at about 14 km (9.3 miles) height and about 30 degrees north latitude. This jet stream is weaker than the polar jet stream and usually separates equatorial air masses from cooler air to the north.

SUPERCELL - The largest thunderstorms that approach a steady-state internal structure for a few hours and are able to produce large hail and tornados. The NWS defines supercells as thunderstorms that have a persistent, rotating updraft as part of their structure - these rotating updrafts have the potential to spawn deadly tornados.

SUPERCOOLED - Liquid water droplets between 0 and -40 degrees Celsius that would freeze immediately if particles were present to start the solidifying process.

SYNOPTIC - Deals with the gathering of meteorological data over a large area (such as the United States) at a specified instant in time, for the purpose of projecting the data into the future (to give weather forecasts).

SYNOPTIC CHART - A weather map produced by plotting synoptic meteorological data (such as winds, temperature or pressure).

THERMAL LOW - An area of low atmospheric pressure produced from continued surface heating as in the Desert Southwest. As the air is heated, it expands, becomes less dense and thus the pressure becomes lower.

THERMAL UPDRAFT - A rising air current in a large thunderstorm that is buoyant because of its warm temperature compared to surrounding air. "Thermals" are not always associated with thunderstorms; they are useful to soaring pilots and hang gliders, as the rising air currents are responsible for keeping the planes/gliders aloft.

THUNDER - The audible sound waves produced as a lightning discharge heats, and expands, a conducting column of air.

TORNADO - The most violent vortex storm on earth. Most commonly, tornados originate within a thunderstorm, although other processes can produce weaker tornados. Tornado strength can be classified on the Fujita scale of intensity which numbers from 0 to 5. F5 tornados are the most destructive known, with winds over 250 possible. F0 tornados are the weakest, though they can still produce significant damage.

TROPICAL DEPRESSION - A stormy region in the tropics more intense than a tropical disturbance but less intense than a tropical storm. Wind speeds in a tropical depression are less than 39 mph.

TROPICAL DISTURBANCE - A region of cloudy, relatively disorganized, weather in the tropics These sometimes develop into a tropical storm (and later into a hurricane).

TROPICAL STORM - A vortex storm that occurs over the tropical ocean. In a more specific sense, it is a stage in hurricane development where the intensity is slightly less than a hurricane. Sustained wind speeds in a tropical storm are 39 mph or greater.

TROUGH - A region of cyclonic (counter-clockwise) curvature of winds in the atmosphere, located around a low-pressure area. Troughs are generally associated with cloudy, cool, unsettled, and wet weather.

TURBULENCE - Very unorganized atmospheric motion, including gusts and lulls in the wind. It is responsible for random shaking motion experienced in an airplane during flight, and in extreme cases can damage or destroy an aircraft.

UPPER AIR SUPPORT - Strength provided to surface low-pressure systems by horizontal divergence in the upper atmosphere, or to surface high-pressure areas by horizontal convergence in the upper air.

URBAN HEAT ISLAND - The increased temperatures in urban areas compared with surrounding rural areas. This is due to the significant amount of brick, concrete, asphalt, etc, in a city which absorbs significant amounts of heat during the day and releases it gradually at night, keeping the urban areas warmer than the surrounding rural areas.

VEERING - A clockwise shift in wind direction, such as when the winds shift from the south to the west. Winds will veer during a cold frontal passage, as the southerly or southeast flow ahead of the front quickly shifts to the west or northwest following the front.

VORTICITY - The amount of rotation, or spin, of the air - also defined as circulation per unit area. Cyclonic vorticity (counter-clockwise spin) is essential for the creation of upper air divergence, rising air, and the development of surface low pressure systems.

WARM FRONT - The leading edge of a warm air mass. Warm fronts are usually associated with temperature rises, cloudy and wet weather. Clouds in a warm front are generally layered, and less convection (thunderstorm activity) is seen along warm fronts, as compared to cold fronts.

WARNING - A warning issued by the National Weather Service means the hazardous weather is IMMINENT or OCCURRING. Warnings are issued for severe thunderstorms, tornadoes, flash floods, blizzards, hurricanes, and other phenomena.

WATCH - A watch issued by the National Weather Service means that conditions are favorable for the formation of the phenomena mentioned in the watch, such as flash floods or severe thunderstorms.

WIND CHILL - The air temperature we FEEL that is corrected for the influence of wind speed. In MOST cases, the wind chill temperature will be lower than the actual temperature, but in certain cases of high temperature and humidity, wind will actually make the perceived temperature hotter than the actual temperature! Above a certain wind speed, additional wind will not lower the wind chill value.

WIND SHEAR - A difference in wind speed OR direction between two wind currents in the atmosphere. Strong vertical wind shear (where the wind changes speed or direction significantly from the surface to some point in the middle or upper atmosphere) is essential for the formation of tornadoes, assuming that the atmosphere is sufficiently unstable for the generation of strong thunderstorms.