Glossary

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Acid rain - precipitation which has become more acidic (lowered pH) as it descends through the atmosphere by combining with airborne pollutants.

Albedo a radiation value which indicates the amount of incoming solar radiation that is reflected by a surface.

Air Mass - a body of air that extends hundreds or thousands of kilometers horizontally and is relatively uniform in temperature and moisture content.

Air-Mass Thunderstorm - a thunderstorm which forms from localized convection within an unstable air mass.

Annual Flood - the maximum discharge peak during a given water year (October 1 - September 30).

Annual Temperature Range a temperature value which indicates the difference between the average temperature of the warmest month and the average temperature of the coolest month.

Anomaly - something different, abnormal, peculiar, or not easily classified (e.g., temperature deviation from the normal)

Aquifer - A geologic formation, group of formations, or part of a formation that contains sufficient saturated permeable material to yield significant quantities of water to springs and wells.

Atmospheric Greenhouse Effect - the warming of the atmosphere by absorption and reemission of infrared radiation by molecules in the atmosphere.

Atmospheric Pressure - the pressure exerted on an object as a result of the weight of the column of air above the object. Also known as Barometric Pressure .

Barometer - an instrument for determining the pressure of the atmosphere.

Base Flood - the national standard for floodplain management is the base, or one percent chance flood. This flood has at least one chance in 100 of occurring in any given year. It is also called a 100 year flood.

Blizzard - severe weather condition characterized by low temperatures, winds of 32 mph or higher, and sufficient snow for visibility to be reduced to less than 500 ft.

Ceiling - The height above the earth's surface given to the lowest cloud layer or obscuring phenomena when the sky cover is reported as broken, overcast, or obscuration and not classified "thin" or "partial."

Climate - the statistical collection of weather conditions at a place over a period of years. Specifically, the meteorological conditions, including temperature, precipitation, and wind that characteristically prevail in a particular region.

Climatology - the science that deals with climates and their phenomena.

Cloud - a visible mass of minute water and/or ice particles in the atmosphere suspended above the earth's surface.

Cold-Air Funnel - a funnel cloud or (rarely) a small, relatively weak tornado that can develop from a small shower or thunderstorm when the air aloft is unusually cold (hence the name). They are much less violent than other types of tornadoes.

CG - a cloud-to -ground lightning flash.

Cold Front - an advancing edge of a cold air mass.

Condensation - the physical process by which a gas becomes a liquid; the opposite of evaporation.

Conduction - the transfer of energy by molecular motion from warmer to colder regions through a substance or between objects in direct contact, and without any net external motion.

Convection - in general, the transport and mixing of the properties of a fluid (e.g., heat, moisture, etc.) by means of mass motion within the fluid; in meteorology, atmospheric motions generally are divided into those in the horizontal, or advection, and those in the vertical, or convection; convection typically results from surface heating and the subsequent rising of warm air

Convectional Precipitation a type of precipitation that occurs when moist air, warmed by the surface of the ground rises, cools, and cannot hold the moisture which falls as precipitation.

Cyclone - an atmospheric circulation that rotates counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere, that usually has a diameter of 2000 to 3000 kilometers

Degree Days an indicator for the growing season for plants measured by the difference between the mean daily temperature and the plant growth temperature of 5.5 degrees C.

Dew - water condensed upon the surfaces of objects near the ground when temperatures of the surface air have fallen below the dew point due to cooling during the night but are still above freezing.

Dew Point (Dewpoint Temperature) - a measure of atmospheric moisture; the temperature to which air must be cooled for saturation to occur (given a constant pressure and constant water-vapor content).

Direct Flood Damage - the damage done to property, structures, goods, etc., by a flood as measured by the cost of replacement and repairs.

Direct Runoff - the runoff entering stream channels promptly after rainfall or snowmelt. Superposed on base runoff, it forms the bulk of the hydrograph of a flood.

Discharge - the rate at which water passes a given point. Discharge is expressed in a volume per time with units of L3/T. Discharge is often used interchangeably with streamflow.

Dispersion - the process of separating radiation into various wavelengths.

Diurnal - daily; related to actions which are completed during a single calendar day, and which typically recur every calendar day (e.g., diurnal temperature cycle of temperature increase and decrease).

Doppler Radar - a radar system that utilizes the Doppler effect for measuring the radial velocity of the wind (i.e., the motion toward or away from the radar).

Doppler Shift (or Doppler Effect) - the change in frequency with which energy from a given source reaches an observer when the source and the observer are in motion relative to each other.

Downburst - an intense localized downdraft which may be experienced beneath a thunderstorm, typically a severe thunderstorm; it results in an outward burst of damaging winds on or near the ground.

Downdraft - a relatively small-scale current of air with marked downward motion.

Downstream - in the same direction as a stream or other flow, or toward the direction in which the flow is moving.

Drainage Area (Watershed or Catchment Area) - an area having a common outlet for its surface runoff.

Drainage Basin - a part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water.

Drought - a period of abnormally dry weather sufficiently prolonged from the lack of precipitation to cause a serious hydrologic imbalance.

Dry Air - in meteorology, air that contains no water vapor.

Easterlies - any winds with components from the east.

El Niño - a warming of the ocean current along the coasts of Peru and Ecuador that is generally associated with dramatic changes in the weather patterns of the region; a major El Niño event generally occurs every 3 to 7 years and is associated with changes in the weather patterns worldwide.

Equinox - days of the year when the sun appears directly over the Equator. This corresponds to the first day of spring or autumn on the calendar. Daylength is exactly 12 hours everywhere in the world.

Evaporation - the process by which a liquid is transformed into a gas.

Evapotranspiration - water loss due to the combined effects of evaporation from soil surfaces and transpiration from growing plants.

Exosphere - the upper most layer of the earth's atmosphere; the only layer where atmospheric gases can escape into outer space.

FEMA acronym for F ederal E mergency M anagement A gency

Flash Flood - a local flood of great volume and short duration generally resulting from heavy rainfall in the immediate vicinity.

Flood - the condition that occurs when water rises and overflows the natural or artificial confines of a body of water on to normally dry land, or accumulates in low-lying areas.

Flux - the amount of flow per unit time (i.e. energy flux or radiation flux).

Fog - a cloud with its base in direct contract with the ground.

Freeze (noun) - a condition which occurs over a widespread area when the surface air temperature remains below freezing long enough to damage certain agricultural crops.

Frequency - the rate at which an event will reoccur; the reciprocal of the period. In radiation, the number of wave crests that pass by a point each second, described in hertz.

Front - the boundary or transition zone between two dissimilar air masses.

Frost Free Period a value which indicated the total number of days between the last expected frost in the spring and the first expected frost in the fall.

Funnel Cloud - a rotating cloud column or inverted cloud cone extending downward from a cloud base that is not in contact with the ground.

Global Radiation - the total direct solar radiation and diffuse sky radiation (e.g., that reflected or scattered) received by surface of the earth.

Greenhouse Gases - the gases that absorb terrestrial radiation and contribute to the greenhouse effect; the main greenhouse gasses are water vapor, methane, CO2 and ozone.

Greenwich Mean Time (GMT) - global time scale based on the local time observed on the Prime Meridian; also called Zulu (Z) time or Universal Coordinated Time (UTC).

Ground Water water that flows under the surface of the earth.

Growing Season an indicator for the number of days in the year with an average temperature above 5.5 degrees C.

Gust Front - a boundary between cold air from the thunderstorm downdraft and warm, humid surface air

Gustnado (or Gustinado) - [Slang] a small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm

Heat Index (HI) - a value that represents the temperature it actually feels at a given air temperature and relative humidity.

Humidity - some measure of the water vapor content of the air

Hydrologic Cycle - the description of the transport of water substance between the earth, the atmosphere, and the seas.

Hydrology - the scientific study of the waters of the earth, especially with relation to the effects of precipitation and evaporation upon the occurrence and character of water on or below the land surface.

Indian Summer - a period of abnormally warm or mild weather in late autumn or early winter.

Insolation - incoming solar radiation; sunshine.

Latent Heat - the heat released or absorbed by a substance during a phase change.

Lightning - any visible electrical discharge produced by thunderstorms.

Local Winds - winds which blow over a small area at a speed or in a direction generally different from those which are appropriate to the general pressure distribution of the region.

Major Flooding - a general term including extensive inundation and property damage. (Usually characterized by the evacuation of people and livestock and the closure of both primary and secondary roads).

Mean Annual Temperature - the average temperature for the entire year at any given location.

Mean Daily Temperature - the average of the highest and lowest temperatures during a 24-hour period.

Mesoclimate - the climate of a small area of the earth's surface which may differ from the general climate of the district.

Mesocyclone - a vertical column of (counterclockwise) rotating air within a severe thunderstorm which may be a precursor to a funnel or tornado; typically a mesocyclone is 2-6 miles in diameter. The circulation of a mesocyclone covers an area much larger than the tornado that may develop within it. Properly used, mesocyclone is a Doppler radar feature that meets specific criteria for magnitude, vertical depth, and duration.

Minor Flooding - a general term indicating minimal or no property damage but possibly some public inconvenience.

Moderate Flooding - t he inundation of secondary roads; transfer to higher elevation necessary to save property -- some evacuation may be required.

Moisture Advection - the transport of moisture by horizontal winds.

Moisture Convergence - a measure of the degree to which moist air is converging into a given area.

Multiple-Vortex (or Multi-Vortex) Tornado - a tornado in which two or more condensation funnels or debris clouds are present at the same time, often rotating about a common center or about each other.

NOAA - acronym for the N ational O ceanographic and A tmospheric A dministration.

Normal - a calculated average of climate data, such as temperature or precipitation, for a standard 30-year interval updated every 10 years. Current normals are calculated from 1971 2000 data; the next normal dataset will be derived from observations between 1981 and 2010 inclusive.

NWS - acronym for N ational W eather S ervice.

One Percent Chance Flood (One Hundred Year Flood) - flood magnitude that has one chance in 100 of being exceeded in any future 1-year period. The occurrence of floods is assumed to be random in time, or regularity of occurrence is implied. The exceeding of a 1-percent chance is no guarantee, therefore, that a similar size flood will not occur next week. The risk of experiencing a large flood within time periods longer than 1 year increases in a nonadditive fashion. For example, the risk of exceeding a 1-percent chance flood one or more times during a 30-year period is 25 percent and during a 70-year period is 50 percent.

Ozone - a triatomic form of oxygen that is nearly colorless gas of pungent odor; it is formed naturally in the upper atmosphere, generally between the heights of about 10 and 50 km and it results in the absorption of a large part of the sun's most intense incoming radiation.

Precipitation - any form of water particles, either liquid or solid, which falls from the atmosphere and reaches the ground.

Pressure - a force per unit area or a stress characterized by uniformity in all directions.

RADAR - acronym for RA dio D etection A nd R anging; a radio device or system for locating an object by means of ultrahigh-frequency radio waves reflected from the object and received, observed, and analyzed by the receiving part of the device in such a way that characteristics (as distance and direction) of the object may be determined.

Real-Time - refers to the rapid retrieval, processing and transmission of data.

Reflection - the process whereby radiation (or other waves) incident upon a surface is directed back into the medium through which it traveled.

Refraction - the process whereby radiation experiences a change in direction as a result of a change in density of the medium or media through which it travels.

Response Time - the amount of time in which it will take a watershed to react to a given rainfall event.

River Basin (watershed) - Drainage area of a river and its tributaries.

River Flooding - the rise of a river to an elevation such that the river overflows its natural banks causing or threatening damage.

Runoff - that part of precipitation that flows toward the streams on the surface of the ground or within the ground. Runoff is composed of baseflow and surface runoff.

Severe Thunderstorm - a thunderstorm with wind gusts of 50 knots (58 mph) or greater, hail at least three-quarters of an inch in diameter, and/or a tornado or funnel cloud.

Solstice - days of the year when the sun appears directly overhead the Tropic of Cancer (northern hemisphere) or the Tropic of Capricorn (southern hemisphere). This corresponds to the first day of summer (greatest daylength) or winter (least daylength) on the calendar.

Squall Line - any line or narrow band of active thunderstorms which is not directly along a frontal boundary.

Stage - the level of the water surface above a given datum at a given location.

Stationary Front - the boundary between two air masses neither of which is replacing the other.

Stream Gage - a site along a stream where the stage (water level) is read either by eye or measured with recording equipment.

Streamflow - water flowing in the stream channel. It is often used interchangeably with discharge.

Surface Runoff - the runoff that travels overland to the stream channel. Rain that falls on the stream channel is often lumped with this quantity.

Surface Water water that flows above the ground such as lakes, springs, streams, rivers, etc.

Temperature an element of the climate which measures the degree of heat or cold in the atmosphere.

Thunder - the sound that follows a flash of lightning and is caused by sudden expansion of the air in the path of the electrical discharge.

Thunderstorm - a local storm produced by a cumulonimbus cloud, always with lightning and thunder, and usually accompanied by strong gusts of wind, heavy rain, and sometimes hail.

Tornado - a violently rotating column of air protruding from a cumulonimbus cloud and in contact with the ground; a condensation funnel does not need to reach to the ground for a tornado to be present.

Tornado Warning - issued by the local National Weather Service office when a tornado or funnel cloud has been sighted visually or detected by radar; the location and direction of movement, if know, are given, and residents of the warning area should take immediate safety precautions.

Tornado Watch - issued by the Storm Prediction Center in Norman , OK as a precautionary alert when conditions are favorable for the development of tornadoes in the specific area.

Topography - the configuration of a surface including its relief and the position of its natural and man-made features. Topography is also described as the graphic representation of the surface features of a place or region on a map, indicating their relative positions and elevations. 

Transpiration - the process by which plants transfer water within to water vapor in the atmosphere.

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

USGS acronym for U nited S tates G eological S urvey.

Visibility - the greatest distance toward the horizon that prominent objects can be identified visually with the naked eye.

Warm Front - the advancing edge of a warm air mass.

Warning - a product issued by the local National Weather Service office when a particular weather hazard is either imminent or has been reported. A warning indicates the need to take action to protect life and property. The type of hazard is reflected in the type of warning (e.g., tornado warning, blizzard warning).

Watch - a National Weather Service product indicating that conditions are favorable for the occurrence of a particular hazard. A watch is a recommendation for planning, preparation, and increased awareness.

Weather - the state of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness.

Zulu (or Z) time : an older name for Universal Coordinated Time (or Greenwich Mean Time) . The current 24-hour clock time in Greenwich , England ; a system by which all meteorological observations can be identified worldwide without any reference to time zones.