National Weather Service United States Department of Commerce

Weather Glossary: D's

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
The maximum mean daily discharge occurring in a stream during a given flood event.
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, or
  2. 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
Catastrophic event characterized by the sudden, rapid, and uncontrolled release of impounded water.
Day-Second Feet
Often abbreviated as DSF. See Second-Day Feet.
A logarithmic expression for power, referenced to 1 milliwatt. dBm = 10 log (power / 1 mW). See decibel.
A logarithmic expression for reflectivity factor, referenced to (1 mm6 / 1 m3). dBZ = 10 log (z / 1 mm6 m3). See decibel.
DCP (Data Collection Platform)
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.
Data Distribution System.
Dead Fuel Moisture
Dead fuel moisture responds solely to ambient environmental conditions and is critical in determining fire potential. Dead fuel moisture is classed by time lag. Dead fuels in NFDRS have four timbale classes:
  • 1-hr: Fine flashy fuels, less than 1/4" (6 mm) diameter. Responds quickly to weather changes. Computed from observation time temperature, humidity and cloudiness.
  • 10-hr: 1/4" to 1" (6 mm to 25 mm) diameters. Computed from observation time temperature, humidity, and cloudiness, or may be a standard set of "10-Hr Fuel Sticks" that are weighed as part of the fire weather observation.
  • 100-hr: 1" to 3" (25 mm to 76 mm) diameter. Computed from 24-hour average boundary condition composed of day length, hours of rain, and daily temperature/humidity ranges.
  • 1000-hr: 3" to 6" (76 mm to 152 mm) diameter. Computed from a 7-day average boundary condition composed of day length, hours of rain, and daily temperature/humidity ranges.
Dead Storage
The volume in a reservoir below the lowest controllable level.
Process of correcting for aliases in the velocity measurement. Also known as unfolding.
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.
Decibel (dB)
This is a logarithmic expression comparing the energy that the radar emits (Z1) to the energy that radar receives back from a radar target (Z2). It is expressed mathematically as
Z (dBZ) = 10 log (Z1/Z2) The solution to this equation lets the radar operator know the strength of a target. The value of Z is a function of the amount of radar beam energy that is back scattered by a target and detected as a signal (or echo). Higher values of Z (and dBZ) thus indicate more energy being back scattered by a target.

The amount of back scattered energy generally is related to precipitation intensity, such that higher values of dBZ that are detected from precipitation areas generally indicate higher precipitation rates. However, other factors can affect reflectivity, such as width of the radar beam, precipitation type, drop size, or the presence of ground clutter or AP.

WSR-88D radars can detect reflectivities as low as -32 dBZ near the radar site, but significant (measurable) precipitation generally is indicated by reflectivities of around 15 dBZ or more. Values of 50 dBZ or more normally are associated with heavy thunderstorms, perhaps with hail, but as with most other quantities, there are no reliable threshold values to confirm the presence of hail or severe weather in a given situation. See VIP for threshold dBZ values associated with each VIP level.
The tendency for the surface wind to become much lighter than wind above it at night when the surface temperature cools.
Deep Percolation Loss
Water that percolates downward through the soil beyond the reach of plant roots.
Deep Seepage
Infiltration which reaches the water table.
Deep Well
A well whose pumping head is too great to permit use of a suction pump.
Deformed Ice
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.
Degree Day

It gauges the amount of heating or cooling needed for a building using 65°F (18°C) as a baseline. To compute heating/cooling degree-days, take the average temperature for a day and subtract the reference temperature of 65°F (18°C). If the difference is positive, it is called a Cooling Degree Days.

If the difference is negative, it is called a Heating Degree Days. The magnitude of the difference is the number of days.

For example, if your average temperature is 50°F (10°C) for a day in September, the difference of the average temperature for that day and the reference temperature of 65°F (18°C) would yield a minus 15°F (minus 8°C). Therefore, you know that you are going to have Heating Degree Days that day.

Since the magnitude of the difference is 15°F (8°C), you know that you are going to have 15 Heating Degree Days (8°C). Electrical, natural gas, power, and heating, and air conditioning industries utilize heating and cooling degree information to calculate their needs.

An acronym for Delaware/Maryland/Virginia.
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
  1. 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.
  2. A way of determining whether lake effect snow showers are likely to develop over the Great Lakes. Typically, this is done by taking the absolute value of the difference in temperature between the 850 mb temperature and the lake water temperature. Typically, 13°C or more is needed for lake effect snow showers to develop over The Great Lakes. However, an 8°C difference can help enhance system snow.
Thin branch-like growth of ice on the water surface.
The form of the drainage pattern of a stream and its 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
A fog in which the visibility is <¼ mile (0.4 kilometer).
Dense Fog Advisory
This product is issued by the National Weather Service when widespread fog reduces visibility to less than or equal to ¼ mile (0.4 kilometer).
Density Current
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
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.
Depth of Runoff
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.
Depletion Curve
That 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 becoming available for stream flow.
The change from water vapor (a gas) directly to ice (a solid) without going through the liquid water phase. It is the opposite of Sublimation.
Depression Storage
The volume of water contained in natural depressions in the land surface, such as puddles.
(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 mesoscale convective weather system (MCS), and can produce damaging straight-line winds over areas hundreds of miles long and more than 100 miles (160 kilometers) across. There are 2 types of derecho-producing convective systems: serial and progressive.
Derived Products
Processed base data on the Doppler radar.
Design Criteria
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
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
The volume of water, other than depression storage, existing on the land surface as flowing water which has not yet reached the channel.
  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.
Developing Gale
Used in the National Weather Service High Seas Forecast. It refers to an extratropical low or an area in which gale force winds of 34 knots (39 mph / 63 km/h) to 47 knots (54 mph / 87 km/h) are expected by a certain time period.
Developing Storm
Used in the National Weather Service High Seas Forecast. It refers to an extratropical low or an area in which storm force winds of 48 knots (55 mph / 89 km/h) or greater are expected by a certain time period.
Water droplets that form upon surfaces on or near the ground when air is cooled toward its dew point.
Dew Point (Dew-Point Temperature)
A measure of atmospheric moisture. The temperature to which air must be cooled, at constant pressure and moisture content, in order for saturation to occur. The higher the dew point, the greater amount of water vapor in the air mass.
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.
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.
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 diluent flow, winds normally decelerate as they move through the region of difluence, resulting in speed convergence which offsets the apparent diverging effect of the diluent flow.
The bending of light around objects, such as clouds and fog droplets, producing fringes of light and dark colored bands.
Diffuse Ice
Poorly defined ice edge limiting an area of dispersed ice; usually on the leeward side of an area of floating ice.
Dipole Pattern
The systematic polarity pattern or spatial distribution of concentrated + and - strike points of lightning flashes on the ground. The + and - centers may be a couple of hundred miles apart.
Dipolar Structure
The dominate accumulations of + and - charges in a thunderstorm cell (+ charge in the upper part of the cloud and - charge in the bottom part of the cloud).
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.
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.
Dirty Ridge
Most of the time, upper-level ridges bring fairly clear weather as the storms are steered around the ridge. Sometimes, however, strong storms undercut the ridge and create precipitation. Ridges that experience this undercutting by storms are known as dirty ridges because of the unusual occurrence of precipitation.
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
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
1. A table showing the relation between two mutually dependent 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 gauging station. Also called a Rating Table.
A disruption of the atmosphere that usually refers to a low-pressure area, cool air and inclement weather.
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).
A measure of 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).
The taking of water from a stream or other body of water into a canal, pipe, or other conduit.
The high ground that forms the boundary of a watershed. A divide is also called a ridge.
Development and Operations Hydrologist.
Domestic SIGMET (SIGnificant METeorlogical Information)
This NWS aviation product advises of weather potentially hazardous to all aircraft other than convective activity.
  • In the conterminous U.S., SIGMETS covered are:
    • Severe icing,
    • Severe or extreme turbulence,
    • Dust storms and sandstorms lowering visibilities to less than three miles (five kilometers), and/or
    • Volcanic Ash
  • In Alaska and Hawaii, SIGMETS are also issued for the following events:
    • Tornadoes,
    • Lines of thunderstorms,
    • Embedded thunderstorms, and/or
    • Hail greater than or equal to 1" (25.4 mm) in diameter.
For the lower 48 states and adjacent coastal waters, Convective SIGMETS are issued hourly for Thunderstorm-related aviation hazards. These SIGMET items are considered to be widespread because they must be affecting or be forecast to affect an area of at least 3,000 square miles (4,800 square kilometers) at any one time. However, if the total area to be affect during the forecast period is very large, it could be that only a small portion of this total area would be affected at any one time.
Doppler Dilemma
The interdependence of maximum unambiguous range and maximum unambiguous velocity on the pulse repetition frequency (PRF). As PRF is increased (decreased), the range of unambiguous velocities increases (decreases), but the unambiguous range decrease (increases). Vmax * Rmax = c * l / 8, where Vmax is maximum unambiguous velocity, Rmax is maximum unambiguous range, c is speed of light, and l is wavelength.
Doppler Radar

A new Weather Surveillance Radar (WSR-88D) system developed in 1988. About 120 systems were installed at Weather Forecast Offices. An additional 24 systems were installed at Department of Defense (Air Force Bases) sites.

This powerful and sensitive Doppler system generates many useful products for meteorologists, among them: standard reflectivity echoes, wind velocity or atmospheric air motion pictures, and areal 1-hour, 3-hour, or storm-total precipitation images. This radar can also measure radial velocity, the instantaneous component of motion parallel to the radar beam (i.e., toward or away from the radar antenna).

Doppler Shift
The change in observed frequency of wave energy due to the relative motion of the observer and wave source. For example, as a train approaches your location, you hear a higher pitch sound. After the train has passed your location, you will hear a lower pitch sound. The Doppler radar uses this change in frequency to determine the velocity and direction of the wind.

A strong down draft, initiated by a thunderstorm, that induces an outburst of damaging straight-line winds on or near the ground. Downburst winds can produce damage similar to a strong tornado. The damage from aloft often looks like a star with debris spreading out from the center in straight lines.

Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder. Downbursts come in the following to 2 categories: microburst and macroburst.

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/Upslope Flow

Air that descends down a mountain chain or over sloping terrain (pressurized air moving from high pressure to low pressure), resulting in subsequent drying, and in some cases, dramatic warming of air that can quickly melt a snow cover. Local names for downslope winds or "foehn winds" in the western United States are Chinook Winds, East Winds, North Winds and Mono Winds.

Usually associated with little or no clouds. On the other hand, the upslope flow is representative of air being lifted by rising terrain and is normally associated with extensive clouds and/or precipitation.

In the same direction as a stream or other flow, or toward the direction in which the flow is moving.
Drainage Area
An area having a common outlet for its surface runoff (also see Watershed and Catchment Area).
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. See basin.
Drainage Density
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
The boundary line, along a topographic ridge or along a subsurface formation, separating two adjacent drainage basins.
Drains (Relief Wells)
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".
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 withdrawal of water therefrom.
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.
Drifting Ice
Pieces of floating ice moving under the action of wind and/ or currents.
Drifting Snow
It 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 (DZ)
Drizzle Intensity Visibility
Light >½ statute mile (>0.8 kilometer)
Moderate ¼ to ½ mile (0.4 to 0.8 kilometer)
Heavy <¼ mile (<0.4 kilometer)

Fairly uniform precipitation composed exclusively of fine drops with diameters less than 0.02" (0.5 mm) which are very close together. Drizzle appears to float while following air currents, although unlike fog droplets, it falls to the ground.

The intensity of drizzle is based solely on visibility. The table (right) shows drizzle intensity versus visibility.

A period of abnormally dry weather sufficiently prolonged from the lack of precipitation to cause a serious hydrologic imbalance.
Drought Index
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 Adiabat
A line of constant potential temperature on a thermodynamic chart.
Dry Adiabatic Rate
The rate of change of temperature in rising or descending unsaturated air parcel. The rate of adiabatic cooling or warming is 5.5°F per 1,000 feet (10°C per 1,000 meters).
Dry Crack
Crack visible at the surface but not going right through the ice cover, and therefore it is dry.
Dry Floodproofing
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 (61 to 91 cm) above the top of the foundation because the buildings walls and floors cannot withstand the pressure of deeper water.
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; see dry punch).

The dry line's eastward movement at the surface is often not as fast as it is aloft. Dry air at about 1,000 feet (305 meters) can overrun the moist air, creating what is called a capping inversion. Hot air over warm air acts like a lid that keeps heated air from rising. On some days this lid is very thick preventing the blossoming of storms.

But on other days, the lid holds the unstable air down for only so long before it breaks through the inversion and clouds explode into thunderstorms. Storms may grow to 50,000 feet (15,200 meters) or higher in minutes as moist air rockets skyward at 100 mph (160 km/h), creating large hail, torrential rain, deadly lightning and strong winds. And with low-level winds changing speed and direction with height, from south to southwest then to west, storms are likely to rotate, which can lead to powerful tornadoes.

The thunderstorms most likely to produce tornadoes are called supercells and are often a product of the severe weather trigger known as the dry line. When the dry line retreats westward at night, the dew points will rise rapidly and the wind will shift from the west or southwest to the south or southeast. See Low Precipitation storm. It is also known as a dewpoint front or "dry front".
Dryline Bulge
A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels (i.e., a surface dry punch). Severe weather potential is increased near and ahead of a feature.
Dryline Punch
A surge of drier air; normally a synoptic-scale or mesoscale process. A dry punch at the surface results in a dry line bulge.
Dryline Storm
Generally, any thunderstorm that develops on or near a dry line. The term often is used synonymously with a Low Precipitation (LP) thunderstorm, since the latter almost always occurs near the dry line.
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. Compare with wet microburst.
Dry Punch
Slang for a surge of drier air; normally a synoptic-scale or mesoscale process. A dry punch at the surface results in a dryline bulge. A dry punch aloft above an area of moist air at low levels often increases the potential for severe weather.
Dry Slot
A zone of dry (and relatively cloud-free) air which wraps east or northeast 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 slot should not be confused with clear slot, which is a storm-scale phenomenon.
Dry Weather Flow
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.
Dual Doppler
The use of two Doppler radars to measure two different radial velocities; with some math, these two radial wind components can be synthesized to a spatial distribution of fully 2-D (horizontal) winds.
The phenomenon by which the radar signal propagates along the boundary of two dissimilar air masses. The radar ranges with ducted propagation are greatly extended; holes can also appear in the coverage. Ducting occurs when the upper air is exceptionally warm and dry in comparison with the air at the surface. When this occurs, the radar is able to detect targets at abnormally long ranges and it can cause holes in the radar coverage area. Ducting occurs when dN/dh <= - 157 N-units/km.
Duration Curve
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
The time from freeze-up to break-up of an ice cover.
Dust Devil

A small, vigorous whirlwind, usually of short duration, rendered visible by dust, sand, and debris picked up from the ground. They range from 10 feet (3 meters) to greater than 100 feet (30 meters) in diameter, and can extend up to 1,000 feet (300 meters) above the ground. They form are caused by intense surface heating.

This heating causes the air to rapidly rise and thus, a mini low-pressure system is formed. They are usually found in desert or dry climatic regions where dust and dirt can be easily lifted. Only rarely do they cause any damage. Wind speeds can reach 30 to 60 mph (48 to 97 km/h).

Dust Plume
A non-rotating "cloud" of dust raised by straight-line winds. Often seen in a microburst or behind a gust front. If rotation is observed, then the term dust whirl or debris cloud should be used.
Dust Storm (DS)
Severe weather conditions marked by strong winds and dust filled air over an extensive area. Visibility is reduced to ½ mile (0.8 kilometer) or less.
Dust Whirl
A rotating column of air rendered visible by dust. Similar to debris cloud; see also dust devil, gustnado, tornado.
Dynamic Ice
Pressure due to a moving ice cover or drifting ice. Pressure occurring at movement of first contact termed Ice Impact Pressure.
Dynamic Range
The ratio, usually expressed in decibels, of the maximum to the minimum signal that a system can handle. Used to describe the limits of receivers.
Generally, any forces that produce motion or affect change. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.