A

Absolute Humidity - The mass of water vapor in a given volume of air. It represents the density of water vapor in the air.

Acid Rain - Cloud or rain droplets combine with gaseous pollutants, such as oxides of sulfur and nitrogen, to make falling rain or snow acidic.

Advection - Transport of an atmospheric property by the wind.

Advection Fog - Occurs when warm, moist air moves over a cold surface and the air cools to below its dew point.

Air mass Thunderstorm - A thunderstorm not associated with a front. Air mass thunderstorms typically are associated with warm, humid air in the summer months; they develop during the afternoon in response to afternoon heating by the sun, and dissipate rather quickly after sunset.

Air Mass - A large body of air that has similar horizontal temperature and moisture properties.

Air Pressure - The pressure exerted by the weight of air above a given point, usually expressed in millibars (mb) or inches of mercury (in. Hg).

Altitude - A measure of height of an object, usually with reference to the earth's surface, or sea level.

Altocumulus - A middle cloud, usually white or gray. Often occurs in layers of patches with wavy, rounded masses or rolls.

Anemometer - An instrument designed to measure wind speed.

Anticyclonic Rotation - Rotation in the opposite sense as the Earth's rotation, i.e., clockwise in the Northern Hemisphere as would be seen from above. The opposite of cyclonic rotation.

Atmosphere - A gaseous covering to a planet that is bound by gravity. Planets have very different atmospheres and each has very different properties.

B

Backing Winds - Winds that shift in a counterclockwise direction with time at a given location (e.g. from northerly to westerly). The opposite of veering winds.

Barometer - An instrument that measures atmospheric pressure.

Beach Erosion - The movement of beach materials by wave action, currents and tides, or wind.

Blizzard - A weather condition characterized by low temperatures and strong winds (greater than 35mph) bearing a great amount of either falling or blowing snow.

Blizzard Warning - Snow, strong winds and low temperatures will combine to produce a blinding snow, deep drifts and life threatening wind chill. This means temperatures below 20 degrees and winds above 35mph.

Blowing Snow - wind-driven snow that reduces visibility near the ground. Blowing snow can be falling snow or snow that has already accumulated but is picked up and blown by strong winds.

Bow Echo - A radar echo that is linear but bent outward in a bow shape. Damaging straight-line winds often occur near the "crest" or center of a bow echo. Areas of circulation also can develop at either end of a bow echo, which sometimes can lead to tornado formation - especially in the left (usually northern) end, where the circulation shows cyclonic rotation.

C

Ceiling - The height of the lowest layer of clouds when the weather reports describe the sky as broken or overcast.

Ceilometer - An instrument that automatically records cloud height.

Celsius Scale - A temperature scale where zero is assigned to the temperature where water freezes and 100 to the temperature where water boils (at sea level).

Cirrocumulus - A high cloud that appears as a white patch of cloud without shadows. It consists of very small elements in the form of grains or ripples.

Cirrostratus - A high cloud appearing as a whitish veil that may totally cover the sky. Often produces halo phenomena.

Cirrus - A high cloud composed of ice crystals in the form of thin, white, featherlike clouds in patches, filaments, or narrow bands.

Climate - The accumulation of daily and seasonal weather events over a long period of time.

Cloudburst - Any sudden and heavy rain shower.

Cold Advection - Transport of cold air into a region by horizontal winds.

Cold Front - A transition zone where a cold air mass advances and replaces a warm air mass.

Condensation - The process by which water vapor becomes a liquid.

Convection - Warm air rising or cold air sinking or both.

Convergence - Convergence in a horizontal wind field indicates that more air is entering a given area than is leaving at that level. To compensate for the resulting "excess," vertical motion may result: upward forcing if convergence is at low levels, or downward forcing (subsidence) if convergence is at high levels. Upward forcing from low-level convergence increases the potential for thunderstorm development.

Cumulus - A cloud in the form of individual, detached domes or towers that are usually dense and well defined.

Cyclogenesis - Development or intensification of a low-pressure center (cyclone).

D

Desert - Area which averages less than 10” of precipitation per year. Temperature ranges from very hot in the summer to below freezing at night during the winter.

Dew - Water that has condensed onto objects near the ground when their temperatures have fallen below the dew point of the surface air.

Dew Point - The temperature to which the air must be cooled to condense. For example, if the air temperature was 65 degrees and the dew point was 65 degrees the humidity would be 100% and the air would be totally saturated. The larger the spread of temperature and dew point, the drier the air. This spread is called the dew point depression.

Diurnal – Daily occurrence; related to actions which are completed in the course of a calendar day, and which typically recur every calendar day (e.g., diurnal temperature rises during the day, and diurnal falls at night).

Divergence – Spreading out of air molecules away from each other and a certain location. More air is leaving than entering the location. The opposite of convergence, divergence at upper levels of the atmosphere enhances upward motion, and hence the potential for thunderstorm.

Doppler Radar - A radar that determines the velocity of falling precipitation either toward or away from the radar unit.

Downburst - A severe localized downward gust of air that can be experienced beneath a severe thunderstorm.

Downslope winds- typically warm and dry, occur in many parts of the world where mountains stand in the path of strong air currents. Dry air decending in elevation warms to a higher temperature than air at the surface. In the European Alps they are known as the foehn. The foehn has other names in other places: zonda in Argentina, halny wiatr in Poland, koembang in Java, and Santa Ana in California. In the Rocky Mountains, where warm, dry downslope winds can melt a foot of snow in less than a hour, they are called the chinook--after Native Americans of the Pacific northwest, where the winds originate.

Drifting Snow - An uneven distribution of snowfall caused by strong surface winds.

Drought - A period of abnormally dry weather sufficiently long enough to cause serious effects on agriculture and other activities in the affected area.

Dry Line - A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains. It typically lies north-south across the central and southern high Plains states during the spring and early summer, where it separates moist air from the Gulf of Mexico (to the east) and dry desert air from the southwestern states (to the west).

Dust Devil - A small atmospheric vortex not associated with a thunderstorm, which is made visible by a rotating cloud of dust or debris (dust whirl). Dust devils form in response to surface heating during fair, hot weather; they are most frequent in arid or semi-arid regions.

E

El Nino - An extensive ocean warning that begins along the coast of Peru and Equador.

Evaporation Fog - Fog produced when sufficient water vapor is added to the air by evaporation.

Eye - A region in the center of a hurricane (tropical storm) where the winds are light and skies are clear to partly cloudy.

Eye Wall - A wall of dense thunderstorms that surrounds the eye of a hurricane.

F

Fahrenheit Scale - A temperature scale where 32 is assigned to the temperature where water freezes and 212 to the temperature where water boils (at sea level).

Flash Flood - A flood which is caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours.

Flood - The filling with water of a normally dry area of land caused by an increased water level in a stream, river or drainage ditch or by the ponding of rainwater.

Flood Warning - Flooding has been reported or is imminent. Take the necessary precaution if you are in a flood prone area.

Flood Watch - Flooding is possible within the watch area. Sometimes this is called a Flash Flood Watch or an Urban and Small Stream Advisory to indicate the possibility of rapidly rising water and flooding or high water on streets, underpasses and around storm drains.

Fog - A cloud with its base at the Earth's surface.

Freeze - A condition occurring over a widespread area when the surface air temperature remains below freezing for a sufficient time to damage certain agricultural crops.

Freezing Rain/Drizzle - Occurs when super cooled rain or drizzle freezes upon contact with surfaces such as the ground, trees, power lines, etc.

Front - The transition zone between two different air masses.

Frost - A covering of ice produced by water vapor freezing on exposed surfaces when the air temperature falls below the frost point.

Frostbite - The partial freezing of exposed parts of the body, causing injury to the skin and sometimes to deeper tissues.

Frost/Freeze Warning - Below freezing temperatures are expected during the growing season and may cause significant damage to plants and crops.

Fujita Scale (or F Scale) - A scale of wind damage from tornadoes in which wind speeds are inferred from an analysis of wind damage:

F0 (weak):	40- 72 mph, light damage
F1 (weak):	73-112 mph, moderate damage.
F2 (strong):	113-157 mph, considerable damage.
F3 (strong):	158-206 mph, severe damage.
F4 (violent):	207-260 mph, devastating damage.
F5 (violent):	261-318 mph, (rare) incredible damage.

Funnel Cloud - A tornado whose circulation has not reached the ground. Often appears as a rotating cone like cloud that extends downward from the base of a thunderstorm.

G

Gust-Front - The boundary between air flowing into a thunderstorm and the precipitation-cooled air flowing out of the storm. An arcus or shelf cloud may be seen above its surface position. There is a noticeable wind shift and temperature drop that occurs when the gust-front passes (similar to a cold front).

H

Hail - Balls or chunks of ice larger than 1/4 inch in diameter which are produced due to strong updrafts in thunderstorms.

Haze - Fine, dust, salt or pollution particles dispersed through a portion of the atmosphere. Individually these are not visible but cumulatively they will diminish visibility.

Heat Advisory - Issued when the "heat index" is expected to exceed 105 degrees during the day and 80 degrees during the night for at least two consecutive days.

Heat Index - The temperature your body feels when the air temperature is combined with the relative humidity. As humidity increases, the rate of evaporation decreases. Our bodies are cooled by the evaporation of our perspiration. Therefore, if evaporation is slowed, the cooling of our bodies is slowed and we feel warmer.

Heavy Snow - In general, snowfall is accumulating at either of the following rates:

1. 4 inches or more in 12 hours or less.
2. 6 inches or more in 24 hours or less.

Heavy Surf -Large waves breaking on or near the shore resulting from swells spawned by a distant storm or persistent winds.

Heavy Surf Advisory - A forecast of heavy (high) surf that may pose a threat to life or property.

High - An area of high pressure around which the wind blows clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere.

High Wind Advisory - An advisory that sustained surface winds exceeding 25mph over land are either predicted or occurring for an unspecified period of time.

High Wind Warning - A warning for sustained surface winds greater than 40 mph lasting more than an hour or winds over 58 mph over land that are either predicted or occurring for an unspecified period of time.

Humidity - A general term that refers to the air's water vapor content.

Hurricane - A severe tropical cyclone having winds in excess of 64 knots (74 mi/hr).

Hurricane Season - The time of the year when most tropical storms and hurricanes occur. In the Atlantic, Caribbean, and Gulf of Mexico, and Central Pacific, the season is from June through November. The season begins two weeks earlier in the Eastern Pacific.

Hurricane Warning - Hurricane conditions are expected within 24 hours. Complete all storm preparations and evacuate if directed by local officials.

Hurricane Watch - Hurricane conditions (heavy rain, tidal flooding, and winds above 75 mph) are possible within 36 hours. Prepare to take immediate action in case a warning is issued.

I

Inversion - An increase in air temperature with height.

Isobar - A line connecting points of equal pressure.

Isodrosotherm - A line connecting points of equal dew point temperature.

Isohyet - A line connecting points of equal precipitation amounts.

Isopleth - General term for a line connecting points of equal value of some quantity. Isobars and isotherms etc. all are examples of isopleths.

Isotach - A line connecting points of equal wind speed.

Isotherm - A line connecting points of equal temperature.

J

Jet Stream - Relatively strong winds concentrated in a narrow stream in the atmosphere, normally referring to horizontal, high-altitude winds. The position and orientation of jet streams vary from day to day. General weather patterns (hot/cold, wet/dry) are related closely to the position, strength and orientation of the jet stream (or jet streams). A jet stream at low levels is known as a low-level jet.

L

Lapse Rate - The rate of change of an atmospheric variable, usually temperature, with height. A steep lapse rate implies a rapid decrease in temperature with height (a sign of instability) and a steepening lapse rate implies that destabilization is occurring.

Lifted Index (or LI) - A common measure of atmospheric instability. The value is obtained by computing the temperature that air near the ground would have if it were lifted to some higher level (around 18,000 feet, usually) and comparing that temperature to the actual temperature at that level. Negative values indicate instability - the more negative, the more unstable the air.

Lightning - A visible electrical discharge produced by thunderstorms.

Local Winds - Winds that tend to blow over a relatively small area.

Low - A cyclonic storm that most often forms along a front in middle and high latitudes.

M

Mixed Cloud - A cloud containing both water drops and ice crystals.

Moisture Advection - Transport of moisture by horizontal winds.

N

National Weather Service - The government service responsible for issuing hazardous weather products such as warnings and advisories in order to protect the public. They also issue forecasts, statements and outlooks.

NCEP - National Centers for Environmental Prediction

NOAA - National Oceanographic and Atmospheric Administration.

NSSL - National Severe Storms Laboratory, in Norman OK.

O

Orographic - Related to, or caused by, physical geography (such as mountains or sloping terrain).

Overrunning - A weather pattern in which a relatively warm air mass is in motion above another air mass of greater density at the surface.

P

Partly Cloudy - Means the sky has some amount of clouds, but usually between 30 and 70% coverage. Often used at night.

Partly Sunny - Means the sky has some amount of clouds, but usually between 30 and 70% coverage. Often used for periods when the sun is out.

Polar- temperatures below 32°F most of the time, with little or no precipitation.

Precipitation - Any form of water particles - liquid or solid-that falls from the atmosphere and reaches the ground.

R

Rainbow - An arc of concentric colored bands that spans a section of the sky when rain is present and the sun is positioned at the observer's back.

Relative Humidity - The ratio of the amount of water vapor actually in the air compared to the amount of water vapor the air can hold at that particular temperature and pressure.

Ridge - An elongated area of relatively high atmospheric pressure; the opposite of trough.

S

Sea Breeze - A coastal local wind that blows from the ocean onto the land and often begins during some time during the day at many coastal locations. The leading edge of the breeze is termed a sea breeze front.

Severe Thunderstorm Warning - Issued when severe weather has been reported or is being indicated by Doppler radar. Warnings indicate imminent danger and the appropriate action should be taken. A warning is issued when a thunderstorm may produce wind gusts over 55mph. and/or ¾" or larger hail.

Severe Thunderstorm Watch - An outlined area where severe thunderstorms are more likely to occur within a certain time frame. Sometimes will be referred to as a "watch box". During a watch you should keep informed and watch the weather situation closely.

Sleet - A type of precipitation consisting of transparent pellets of ice 5 millimeters or less in diameter. Sleet is rainfall that freezes before it hits the ground.

Snow Squalls - An intermittent heavy snow shower that greatly reduces visibility.

Sounding - A plot of the vertical profile of temperature and dew point (and often winds) above a fixed location. Soundings are used extensively in severe weather forecasting, to determine instability, locate temperature inversions, measure the strength of the cap, obtain the convective temperature.

SPC - Storm Prediction Center, in Norman, OK

Squall Line - A solid or nearly solid line or band of active thunderstorms.

Storm Surge - A rise above the normal water level along a shore caused by strong onshore winds and/or reduced atmospheric pressure. The surge height is the difference of the observed water level minus the predicted tide. Hurricanes often produce a high storm surge.

Straight-line Winds - Generally, any wind that is not associated with rotation, used mainly to differentiate them from tornadic winds.

Stratocumulus - A low cloud, predominantly, stratiform with low, lumpy, rounded masses, often with blue sky between them.

Stratus - A low, gray cloud layer with a rather uniform base whose precipitation is most commonly drizzle.

Subtropical - area between 30° and 40° latitude with temperatures always above 32°F and precipitation most of the year.

Summer Solstice - Approximately June 22 in the Northern Hemisphere when the sun is highest in the sky and directly overhead at latitude 23 1/2° N, the Tropic of Cancer.

Sunspots - Relatively cooler areas on the sun's surfaces. They represent regions of extremely high magnetic field.

T

Temperate- areas with distinct summer and winter seasons and moderate precipitation with temperatures will fall below 32^o F in winter and above 72^o F in summer.

Temperature - Temperature is a measure of the heat content of a body (the atmosphere in the case of weather). The molecular motion of a substance creates energy, which can be measured in terms of the heat it generates. Air, water, and soil can all be measured for temperature.

Thermodynamics - In general, the relationships between heat and other properties (such as temperature, pressure, density, etc.) In forecast discussions, thermodynamics usually refers to the distribution of temperature and moisture (both vertical and horizontal) as related to the diagnosis of atmospheric instability.

Thunder - The sound due to rapidly expanding gases along the channel of a lightning discharge.

Thunderstorm - A local storm produced by cumulonimbus clouds. Always accompanied by lightning and thunder.

Tornado - An intense, rotating column of air that protrudes from a cumulonimbus cloud in the shape of a funnel or a rope whose circulation is present on the ground.

Tornado Warning - A tornado has been reported or is being indicated as possible by Doppler radar. Immediate action should be taken.

Tornado Watch - Same as a severe thunderstorm watch but tornadoes are also possible in the watch area.

Trade Winds - The winds that occupy most of the tropics and blow from the subtropical highs to the equatorial low.

Tropical – often between 20°N and 20°S, very hot and humid with temperatures always greater than 65°F and rain most of the year.

Tropical Depression - A tropical cyclone in which the maximum 1-minute sustained surface wind is 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 39-73 mph. Tropical storms pose a threat to life and/or property in coastal areas.

Tropical Storm Warning - Tropical storm force winds are occurring or are expected within 24 hours.

Tropical Storm Watch - Tropical storm force winds between 37 and 74 mph are possible in the next 36 hours.

Tropopause - The upper boundary of the troposphere, usually characterized by an abrupt change in lapse rate from positive (decreasing temperature with height) to neutral or negative (temperature constant or increasing with height).

Troposphere - The layer of the atmosphere from the earth's surface up to the tropopause, characterized by decreasing temperature with height, vertical wind motion, appreciable water vapor content, and sensible weather (clouds, rain, etc.).

Trough - An elongated area of relatively low atmospheric pressure, usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.

Typhoon - A hurricane that forms in the western Pacific ocean.

U

Updraft - A small-scale current of rising air.

Upper Level System - A general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere.

Upslope Flow - Air that flows toward higher terrain, and hence is forced to rise. The added lift often results in widespread low cloudiness and stratiform precipitation if the air is stable, or an increased chance of thunderstorm development if the air is unstable.

Upstream - Toward the source of the flow, or located in the area from which the flow is coming.

V

Veering Winds - Winds which shift in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft).

Virga - Streaks or wisps of precipitation falling from a cloud but evaporating before reaching the ground.

Visibility - The greatest distance an observer can see and identify prominent objects.

W

Wall Cloud - A localized, persistent, often abrupt lowering from the base of a parent cloud. Wall clouds can range from a fraction of a mile up to nearly five miles in diameter, and normally are found on the south or southwest (inflow) side of the thunderstorm. When seen from within several miles, many wall clouds exhibit rapid upward motion and cyclonic rotation.

Warm Advection - Transport of warm air into an area by horizontal winds.

Warm Cloud - Clouds that form at temperatures above freezing.

Warm Front - A front that moves in such a way that warm air replaces cold air.

Waterspout - A column of rotating wind over water that has characteristics of a dust devil and tornado.

Water Vapor - Water in a vapor (gaseous) form; also refered to as atomospheric moisture. Water vapor is one of the most important parts of the atmosphere.

Weather - The condition of the atmosphere at any particular time and place.

Wet Bulb Temperature - The air temperature drops after it rains. In the summertime a thunderstorm can break the oppressive heat of the day. As it precipitates, evaporation occurs and the air temperature drops. The wet bulb temperature is the temperature at which no more evaporation will occur, and thus no further decrease in the temperature. The air will continue to cool until the air can evaporate no more moisture. The temperature, when the cooling continues until the evaporation stops and the air becomes saturated, is the wet bulb temperature.

Wind Chill - The perceived cooling that a person feels due to loss of body heat as wind passes over exposed skin. The loss of heat is greater the faster the wind blows.

Wind Chill Temperature - An "apparent temperature" that results from the combination of low temperature and high winds. A wind chill temperature is thought to feel the same as an air temperature equal to the wind chill temperature but with no wind. The higher the wind, the lower the wind chill temperature.

Wind Speed/Direction - Wind speed is the measure motion of the air with respect to the surface of the earth covering a unit distance over a unit time. Wind direction is an indicator of the direction that the wind is coming from. For example, a northerly wind is coming from the north and blowing toward the south.

Wind Vane - An instrument used to indicate wind direction.

Winter Solstice - Approximately December 22 in the Northern Hemisphere when the sun is lowest in the sky and directly overhead at latitude 12 1/2° S, the Tropic of Capricorn.

Winter Storm Warning - Same as a watch, except conditions are expected to begin within 24 hours or have already begun.

Winter Storm Watch - Severe winter conditions, such as accumulations of heavy snow and/or ice of 4" or more possible within the next 36-48 hours.

Winter Weather Advisory - Winter weather conditions are expected to cause significant inconveniences and may be hazardous. This can often be called a snow advisory or a freezing rain advisory for the specific expected weather. A snow advisory is for less than 4" accumulations.

Z

Zonal Flow - Large-scale atmospheric flow in which the east-west component (i.e., latitudinal) is dominant. The accompanying meridional (north-south) component often is weaker than normal. Compare with meridional flow.