Weather Glossary


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D Dust
D Region In solar-terrestrial terms, a daytime layer of the earth's ionosphere approximately 50 to 90 km in altitude.
Daily Climatological Report As the name indicates, this climatological product is issued daily by each National Weather Service office. Most of the climatological data in this report are presented in a tabular form; however, some narrative statements may also be used in the product. The report is organized so that similar items are grouped together (i.e., temperature, precipitation, wind, sunrise and sunset times, etc.).
Daily Flood Peak In hydrologic terms, the maximum mean daily discharge occuring in a stream during a given flood event.
DALR Dry Adiabatic Lapse Rate
Dam In hydrologic terms, any artificial barrier which impounds or diverts water. The dam is generally hydrologically significant if it is:
1. 25 feet or more in height from the natural bed of the stream and has a storage of at least 15 acre-feet.
2. Or has an impounding capacity of 50 acre-feet or more and is at least six feet above the natural bed of the stream.
Dam Failure In hydrologic terms, catastrophic event characterized by the sudden, rapid, and uncontrolled release of impounded water.
DAMBRK In hydrologic terms, the Dam Break Forecasting Model.
DAPM In hydrologic terms, the Data Acquisition Program Manager.
Dark Surge on Disk (DSD) In solar-terrestrial terms, dark gaseous ejections visible in H-alpha.
Dart Leader A faint, negatively charged channel that travels more or less directly and continuously from cloud to ground.
DATACOL In hydrologic terms, the Software System that supports RFC gateway functions.
DATANET In hydrologic terms, it is the hydrologic Data Network Analysis Software.
Dawn Same as Civil Dawn; the time of morning at which the sun is 6 degrees below the horizon. At this time, there is enough light for objects to be distiguishable and that outdoor activities can commence.
Day Length Duration of the period from sunrise to sunset.
dBZ Nondimensional "unit" of radar reflectivity which represents a logarithmic power ratio (in decibels, or dB) with respect to radar reflectivity factor, Z.
DCP (Data Collection Platform) In hydrologic terms, an electronic device that connects to a river or rainfall gage that records data from the gage and at pre-determined times transmits that data through a satellite to a remote computer.
DDS Data Distribution System.
Dead Storage In hydrologic terms, the volume in a reservoir below the lowest controllable level.
Debris Cloud A rotating "cloud" of dust or debris, near or on the ground, often appearing beneath a condensation funnel and surrounding the base of a tornado. This term is similar to dust whirl, although the latter typically refers to a circulation which contains dust but not necessarily any debris. A dust plume, on the other hand, does not rotate. Note that a debris cloud appearing beneath a thunderstorm will confirm the presence of a tornado, even in the absence of a condensation funnel.
Decadal Occurring over a 10-year period, such as an oscillation whose period is roughly 10 years ("Pacific Decadal Oscillation").
Declination The latitude that the sun is directly over at a given time. The declination is ~23?Ǭ?N at the summer solstice, ~23?Ǭ?S at the winter solstice, and 0?Ǭ? (over the equator) at the spring and autumn equinoxes.
Deep Percolation Loss In hydrologic terms, water that percolates downward through the soil beyond the reach of plant roots.
Deep Seepage In hydrologic terms, infiltration which reaches the water table.
Deep well In hydrologic terms, a well whose pumping head is too great to permit use of a suction pump.
Deepening A decrease in the central pressure of a surface low pressure system. The storm is intensifying.
Deformed Ice In hydrologic terms, a general term for ice which has been squeezed together and forced upwards and downwards in places. Subdivisions are rated ice, ridge ice, hummocked ice, and other similar deformations.
Degradation In hydrologic terms, the geologic process by means of which various parts of the surface of the earth are worn down and carried away and their general level lowered, by the action of wind and water.
Degree Day A measure that gauges the amount of heating or cooling needed for a building using 65 degrees as a baseline. Electrical, natural gas, power, and heating, and air conditioning industries utilize heating and cooling degree information to calculate their needs. For more specific definitions and how to calculate degree days, see the definitions for Heating Degree Days and Cooling Degree Days.
Delta In hydrologic terms, an alluvial deposit, often in the shape of the Greek letter "delta", which is formed where a stream drops its debris load on entering a body of quieter water.
Delta T A simple representation of the mean lapse rate within a layer of the atmosphere, obtained by calculating the difference between observed temperatures at the bottom and top of the layer. Delta Ts often are computed operationally over the layer between pressure levels of 700 mb and 500 mb, in order to evaluate the amount of instability in mid-levels of the atmosphere. Generally, values greater than about 18 indicate sufficient instability for severe thunderstorm development.
Dendrites In hydrologic terms, thin branch-like growth of ice on the water surface.
Dendritic In hydrologic terms, the form of the drainage pattern of a stream and it's tributaries when it follows a treelike shape, with the main trunk, branches, and twigs corresponding to the main stream, tributaries, and subtributaries, respectively, of the stream.
Dense Fog Advisory Issued when fog reduces visibility to 1/8 mile or less over a widespread area.
Density Current In hydrologic terms, a flow of water maintained by gravity through a large body of water, such as a reservoir or lake, and retaining its unmixed identity because of a difference in density.
Density of Snow In hydrologic terms, the ratio, expressed as a percentage, of the volume which a given quantity of snow would occupy if it were reduced to water, to the volume of the snow. When a snow sampler is used, it is the ratio expressed as percentage of the scale reading on the sampler to the length of the snow core or sample.
Depletion Curve In hydrologic terms, the part of the hydrograph extending from the point of termination of the Recession Curve to the subsequent rise or alternation of inflow due to additional water becomming available for stream flow.
Depression A region of low atmospheric pressure that is usually accompanied by low clouds and precipitation. The term is also sometimes used as a reference to a Tropical Depression.
Depression Storage In hydrologic terms, the volume of water contained in natural depressions in the land surface, such as puddles.
Depth of Runoff In hydrologic terms, the total runoff from a drainage basin, divided by its area. For convenience in comparing runoff with precipitation, the term is usually expressed in inches of depth during a given period of time over the drainage area or acre-feet per square mile.
Derecho (Pronounced day-RAY-cho), a widespread and usually fast-moving windstorm associated with convection. Derechos include any family of downburst clusters produced by an extratropical MCS, and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles across.
Derived Products Processed base data on the Doppler radar.
Desertification A tendency toward more prominent desert conditions in a region.
Design Criteria In hydrologic terms, the hypothetical flood used in the sizing of the dam and the associated structures to prevent dam failure by overtopping, especially for the spillway and outlet works.
Detention Basins In hydrologic terms, structures which are built upstream from a populated area so that precipitation flows do not flood and cause the loss of life or property. They are normally dry, but are designed to detain surface water temporarily during, and immediately after a runoff event. Their primary function is to attenuate the storm flows by releasing flows at a lower flow rate. There are no gates or valves allowed on the outlet so that water can never be stored on a long-term basis. Typical detention times in such a basin would be on the order of 24 to 72 hours although some are as long as 5 to 10 days.
Detention Storage In hydrologic terms, the volume of water, other than depression storage, existing on the land surface as flowing water which has not yet reached the channel.
Detritus In hydrologic terms,
(1) the heavier mineral debris moved by natural watercourses, usually in bed-load form.
(2) the sand, grit, and other coarse material removed by differential sedimentation in a relatively short period of detention.
Dew Moisture that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature.
Dew Point (Abbrev. DWPT) - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant). A higher dew point indicates more moisture present in the air. It is sometimes referred to as Dew Point Temperature, and sometimes written as one word (Dewpoint).
Dew Point Depression The difference in degrees between the air temperature and the dew point.
Dew Point Front A narrow zone (mesoscale feature) of extremely sharp moisture gradient and little temperature gradient. It separates moist air from dry air. Severe weather can be associated with this front. It is also known as a "dryline" or "dry front".
DEWP On a buoy report, the dewpoint temperature taken at the same height as the air temperature measurement.
DFUS Diffuse
Diabatic A process which occurs with the addition or loss of heat. The opposite of adiabatic. Meteorological examples include air parcels warming due to the absorption of infrared radiation or release of latent heat.
Diablo Wind Similar to Santa Ana winds in southern California. These winds occur below canyons in the East Bay hills (Diablo range) and in extreme cases can exceed 60 mph. They develop due to high pressure over Nevada and lower pressure along the central California coast.
Diagnostic Model A computer model used to calculate air pollution concentrations. A diagnostic model produces a wind field over an area by interpolating from actual wind observations.
Diamond Dust A fall of non-branched (snow crystals are branched) ice crystals in the form of needles, columns, or plates.
Differential Motion Cloud motion that appears to differ relative to other nearby cloud elements, e.g. clouds moving from left to right relative to other clouds in the foreground or background. Cloud rotation is one example of differential motion, but not all differential motion indicates rotation. For example, horizontal wind shear along a gust front may result in differential cloud motion without the presence of rotation.
Differential Rotation In solar-terrestrial terms, the change in solar rotation rate with latitude. Low latitudes rotate at a faster angular rate (approx. 14 degrees per day) than do high latitudes (approx. 12 degrees per day).
Diffuse Ice In hydrologic terms, poorly defined ice edge limiting an area of dispersed ice; usually on the leeward side of an area of floating ice.
Difluence (or diffluence) - A pattern of wind flow in which air moves outward (in a "fan-out" pattern) away from a central axis that is oriented parallel to the general direction of the flow. It is the opposite of confluence.

Difluence in an upper level wind field is considered a favorable condition for severe thunderstorm development (if other parameters are also favorable). But difluence is not the same as divergence. In a difluent flow, winds normally decelerate as they move through the region of difluence, resulting in speed convergence which offsets the apparent diverging effect of the difluent flow.
DIR 1. Direction

2. On a buoy report, the ten-minute average wind direction measurements in degrees clockwise from true North.
Direct Flood Damage In hydrologic terms, the damage done to property, structures, goods, etc., by a flood as measured by the cost of replacement and repairs.
Direct Hit A close approach of a tropical cyclone to a particular location. For locations on the left-hand side of a tropical cyclone's track (looking in the direction of motion), a direct hit occurs when the cyclone passes to within a distance equal to the cyclone's radius of maximum wind. For locations on the right-hand side of the track, a direct hit occurs when the cyclone passes to within a distance equal to twice the radius of maximum wind. Compare indirect hit, strike.
Direct Runoff In hydrologic terms, the runoff entering stream channels promptly after rainfall or snowmelt. Superposed on base runoff, it forms the bulk of the hydrograph of a flood.
Direct Solar Radiation The component of solar radiation received by the earth's surface only from the direction of the sun's disk (i.e. it has not been reflected, refracted or scattered).
Directional Shear The component of wind shear which is due to a change in wind direction with height, e.g., southeasterly winds at the surface and southwesterly winds aloft. A veering wind with height in the lower part of the atmosphere is a type of directional shear often considered important for tornado development.
Disappearing Solar Filament (DSF) In solar-terrestrial terms, the sudden (timescale of minutes to hours) disappearance of a solar filament (prominence).
Discharge In hydrologic terms, 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.
Discharge Curve In hydrologic terms, a curve that expresses the relation between the discharge of a stream or open conduit at a given location and the stage or elevation of the liquid surface at or near that location. Also called Rating Curve and Discharge Rating Curve.
Discharge Table In hydrologic terms,

1. A table showing the relation between two mutually dependant quantities or variable over a given range of magnitude.

2. A table showing the relation between the gage height and the discharge of a stream or conduit at a given gaging station. Also called a Rating Table.
Disdrometer Equipment that measures and records the size distribution of raindrops.
Disk The visible surface of the sun (or any heavenly body) projected against the sky.
Dispersion The process of separating radiation into various wavelengths.
Distribution (Hydro)Graph In hydrologic terms, a unit hydrograph of direct runoff modified to show the proportions of the volume of runoff that occur during successive equal units of time.
Diurnal Daily; 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).
Diurnal Cycles Variations in meteorological parameters such as temperature and relative humidity over the course of a day which result from the rotation of the Earth about its axis and the resultant change in incoming and outgoing radiation.
Diurnal Temperature Range The temperature difference between the minimum at night (low) and the maximum during the day (high).
Divergence The expansion or spreading out of a vector field; usually said of horizontal winds. It is the opposite of convergence. Divergence at upper levels of the atmosphere enhances upward motion, and hence the potential for thunderstorm development (if other factors also are favorable).
Diversion In hydrologic terms, the taking of water from a stream or other body of water into a canal, pipe, or other conduit.
Divide In hydrologic terms, the high ground that forms the boundary of a watershed. A divide is also called a ridge.
Dividing Streamline In the blocked flow region upwind of a mountain barrier, the streamline that separates the blocked flow region near the ground from the streamlines above which go over the barrier.
Dividing Streamline Height The height above ground of the dividing streamline, as measured far upwind of a mountain barrier. See dividing streamline.
DMNT Dominant
DMSH Diminish
DNR Department of Natural Resources
DNS Dense
DNSTRM Downstream
Dobson Unit Unit used to measure the abundance of ozone in the atmosphere. One Dobson unit is the equivalent of 2.69/ x 1016 molecules of ozone/cm2.
DOC Department of Commerce
DOH Development and Operations Hydrologist
Doldrums The regions on either side of the equator where air pressure is low and winds are light.
Domain In air pollution modeling, the geographical area over which a simulation is performed.
Domestic Consumption In hydrologic terms, the quantity, or quantity per capita, of water consumed in a municipality or district for domestic uses or purposes during a given period, generally one day. It is usually taken to include all uses included within the term Municipal Use of Water and quantity wasted, lost, or otherwise unaccounted for.
Domestic Use of water In hydrologic terms, the use of water primarily for household purposes, the watering of livestock, the irrigation of gardens, lawns, shrubbery, etc., surrounding a house or domicile.
Doppler Radar Radar that can measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar antenna).
Down-Valley Wind A thermally driven wind directed down a valley's axis, usually occurring during nighttime; part of the along-valley wind system.
Downburst A strong downdraft current of air from a cumulonimbus cloud, often associated with intense thunderstorms. Downdrafts may produce damaging winds at the surface.
Downdraft (Abbrev. DWNDFT) - A small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm. A downburst is the result of a strong downdraft.
Downslope Flow A thermally driven wind directed down a mountain slope and usually occurring at night; part of the along-slope wind system.
Downstream In the same direction as a stream or other flow, or toward the direction in which the flow is moving.
Downstream Slope In hydrologic terms, the slope or face of the dam away from the reservoir water. This slope requires some kind of protection (e.g.; grass) from the erosive effects of rain and surface flow
Downwash A deflection of air downward relative to an object that causes the deflection.
Downwelling Radiation The component of radiation directed toward the earth's surface from the sun or the atmosphere, opposite of upwelling radiation.
DP 1. Deep

2. Dew Point
DPD On a buoy report, dominant wave period (seconds) is the period with the maximum wave energy.
DPNG Deepening
DPTH Depth
Drainage Area In hydrologic terms, an area having a common outlet for its surface runoff (also see Watershed and Catchment Area).
Drainage Basin In hydrologic terms, 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.
Drainage Density In hydrologic terms, the relative density of natural drainage channels in a given area. It is usually expressed in terms of miles of natural drainage or stream channel per square mile of area, and obtained by dividing the total length of stream channels in the area in miles by the area in square miles.
Drainage Divide In hydrologic terms, the boundary line, along a topographic ridge or along a subsurface formation, separating two adjacent drainage basins.
Drainer A valley or basin from which air drains continuously during nighttime rather than becoming trapped or pooled.
Drains (Relief Wells) In hydrologic terms, a vertical well or borehole, usually downstream of impervious cores, grout curtains or cutoffs, designed to collect and direct seepage through or under a dam to reduce uplift pressure under or within a dam. A line of such wells forms a "drainage curtain".
Drawdown In hydrologic terms, the lowering of the surface elevation of a body of water, the water surface of a well, the water table, or the piezometric surface adjacent to the well, resulting from the withdrawl of water therefrom.
DRCTN Direction
Dredging In hydrologic terms, the scooping, or suction of underwater material from a harbor, or waterway. Dredging is one form of channel modification. It is often too expensive to be practical because the dredged material must be disposed of somewhere and the stream will usually fill back up with sediment in a few years. Dredging is usually undertaken only on large rivers to maintain a navigation channel.
DRFT Drift
Drifting Ice In hydrologic terms, pieces of floating ice moving under the action of wind and/ or currents.
Drifting Snow Drifting snow is an uneven distribution of snowfall/snow depth caused by strong surface winds. Drifting snow may occur during or after a snowfall. Drifting snow is usually associated with blowing snow.
Drizzle Precipitation consisting of numerous minute droplets of water less than 0.5 mm (500 micrometers) in diameter.
Drop-size Distribution The distribution of rain drops or cloud droplets of specified sizes.
Drought Drought is a deficiency of moisture that results in adverse impacts on people, animals, or vegetation over a sizeable area. NOAA together with its partners provides short- and long-term Drought Assessments.
Drought Assessments At the end of each month, CPC issues a long-term seasonal drought assessment. On Thursdays of each week, the CPC together with NOAA National Climatic Data Center, the United States Department of Agriculture, and the National Drought Mitigation Center in Lincoln, Nebraska, issues a weekly drought assessment called the United States Drought Monitor. These assessments review national drought conditions and indicate potential impacts for various economic sectors, such as agriculture and forestry.
Drought Index In hydrologic terms, computed value which is related to some of the cumulative effects of a prolonged and abnormal moisture deficiency. (An index of hydrological drought corresponding to levels below the mean in streams, lakes, and reservoirs.)
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). The dry line typically advances eastward during the afternoon and retreats westward at night. However, a strong storm system can sweep the dry line eastward into the Mississippi Valley, or even further east, regardless of the time of day. A typical dry line passage results in a sharp drop in humidity (hence the name), clearing skies, and a wind shift from south or southeasterly to west or southwesterly. (Blowing dust and rising temperatures also may follow, especially if the dry line passes during the daytime. These changes occur in reverse order when the dry line retreats westward. Severe and sometimes tornadic thunderstorms often develop along a dry line or in the moist air just to the east of it, especially when it begins moving eastward.
Dry Punch [Slang], a surge of drier air; normally a synoptic-scale or mesoscale process. A dry punch at the surface results in a dry line bulge. A dry punch aloft above an area of moist air at low levels often increases the potential for severe weather.
Dry Adiabat A line of constant potential temperature on a thermodynamic chart.
Dry Adiabatic Lapse Rate The rate at which the temperature of a parcel of dry air decreases as the parcel is lifted in the atmosphere. The dry adiabatic lapse rate (abbreviated DALR) is 5.5?Ǭ?F per 1000 ft or 9.8?Ǭ?C per km.
Dry Crack In hydrologic terms, a crack visible at the surface but not going right through the ice cover, and therefore it is dry.
Dry Floodproofing In hydrologic terms, a dry floodproofed building is sealed against floodwaters. All areas below the flood protection level are made watertight. Walls are coated with waterproofing compounds or plastic sheeting. Openings like doors windows, sewer lines and vents are closed, whether permanently, with removable shields, or with sandbags. The flood protection level should be no more than 2 or 3 feet above the top of the foundation because the buildings walls and floors cannot withstand the pressure of deeper water.
Dry Line Storm Any thunderstorm that develops on or near a dry line.
Dry Line Bulge A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels. Severe weather potential is increased near and ahead of a dry line bulge.
Dry Microburst A microburst with little or no precipitation reaching the ground; most common in semi-arid regions. They may or may not produce lightning. Dry microbursts may develop in an otherwise fair-weather pattern; visible signs may include a cumulus cloud or small Cb with a high base and high-level virga, or perhaps only an orphan anvil from a dying rain shower. At the ground, the only visible sign might be a dust plume or a ring of blowing dust beneath a local area of virga.
Dry Slot A zone of dry (and relatively cloud-free) air which wraps east- or northeastward into the southern and eastern parts of a synoptic scale or mesoscale low pressure system. A dry slot generally is seen best on satellite photographs.
Dry Thunderstorm Generally a high-based thunderstorm when lightning is observed, but little if any precipitation reaches the ground. Most of the rain produced by the thunderstorm evaporates into relatively dry air beneath the storm cell. May also be referred to as "dry lightning".
Dry Weather Flow In hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel. This is also referred to as baseflow, or ground water flow.
Dry-adiabatic 1. An adiabatic process in a hypothetical atmosphere in which no moisture is present. 2. An adiabatic process in which no condensation of its water vapor occurs and no liquid water is present.
DSA Special Tropical Disturbance Statement
DSIPT Dissipate
Dst Index A geomagnetic index describing variations in the equatorial ringcurrent
DTRT Deteriorate
Duration Curve In hydrologic terms, a cumulative frequency curve that shows the percent of time during which specified units of items (e.g. discharge, head, power,etc.) were equaled or exceeded in a given period. It is the integral of the frequency diagram.
Duration of Ice Cover In hydrologic terms, The time from freeze-up to break-up of an ice cover.
Duration of Sunshine The amount of time sunlight was detected at a given point.
DURG During
DURN Duration
Dusk Same as Civil Dusk; the time at which the sun is 6 degrees below the horizon in the evening. At this time objects are distinguishable but there is no longer enough light to perform any outdoor activities.
Dust Devil A small, rapidly rotating wind that is made visible by the dust, dirt or debris it picks up. Also called a whirlwind, it develops best on clear, dry, hot afternoons
Dust Plume A non-rotating "cloud" of dust raised by straight-line winds. Often seen in a microburst or behind a gust front.
Dust Storm A severe weather condition characterized by strong winds and dust-filled air over an extensive area.
Dust Whirl A rotating column of air rendered visible by dust.
DVLP Develop
DVV Downward Vertical Velocity (sinking air)
DWNDFT Downdraft - A small-scale column of air that rapidly sinks toward the ground, usually accompanied by precipitation as in a shower or thunderstorm. A downburst is the result of a strong downdraft.
DWNSLP Downslope
DWPT Dew Point - A measure of atmospheric moisture. It is the temperature to which air must be cooled in order to reach saturation (assuming air pressure and moisture content are constant).
Dynamic Ice In hydrologic terms, pressure due to a moving ice cover or drifting ice. Pressure occuring at movement of first contact termed Ice Impact Pressure
Dynamic Lifting The forced uplifting of air from various atmospheric processes