From Daryl Stout@57:57/10 to All on Fri Oct 16 00:06:14 2020
This weather glossary contains information on more than 2000 terms,
phrases and abbreviations used by the National Weather Service (NWS)...
the government agency who makes weather forecasts, and issues weather advisories, watches, and warnings, for the United States, and its
Many of these terms and abbreviations are used by NWS forecasters to communicate between each other and have been in use for many years and
before many NWS products were directly available to the public. It is the purpose of this glossary to aid you in better understanding NWS products.
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.
Dry Adiabatic Lapse Rate
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.
In hydrologic terms, catastrophic event characterized by the sudden,
rapid, and uncontrolled release of impounded water.
In hydrologic terms, the Dam Break Forecasting Model.
In hydrologic terms, the Data Acquisition Program Manager.
Dark Surge on Disk (DSD)
In solar-terrestrial terms, dark gaseous ejections visible in H-alpha.
A faint, negatively charged channel that travels more or less directly
and continuously from cloud to ground.
In the context of hydrologic observations, a location on a river/stream
for which observed data is input to RFC or WFO hydrologic forecast
procedures, or included in public hydrologic products. Flood forecasts
and warnings are not issued for data points.
In hydrologic terms, the Software System that supports RFC gateway
In hydrologic terms, it is the hydrologic Data Network Analysis Software.
Same as Civil Dawn; the time of morning at which the sun is 6ø below
the horizon. At this time, there is enough light for objects to
be distiguishable and that outdoor activities can commence.
Duration of the period from sunrise to sunset.
Nondimensional "unit" of radar reflectivity which represents a
logarithmic power ratio (in decibels, or dB) with respect to radar
reflectivity factor, Z.
(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.
Data Distribution System.
In hydrologic terms, the volume in a reservoir below the lowest
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
Occurring over a 10-year period, such as an oscillation whose period is
roughly 10 years ("Pacific Decadal Oscillation").
The latitude that the sun is directly over at a given time. The
declination is approximately 23ø N at the summer solstice, approximately
23ø S at the winter solstice, and 0ø (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.
In hydrologic terms, infiltration which reaches the water table.
In hydrologic terms, a well whose pumping head is too great to permit
use of a suction pump.
A decrease in the central pressure of a surface low pressure system. The
storm is intensifying.
The change in shape of a fluid mass by variations in wind, specifically
by stretching and/or shearing. Deformation is a primary factor in
frontogenesis (evolution of fronts) and frontolysis (decay of fronts).
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.
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.
A measure that gauges the amount of heating or cooling needed for a
building using 65ø 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.
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.
Change in temperature.
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) The difference in temperature between the surface of a lake and 850mb, typically used to determine lake effect snow potential.
In hydrologic terms, thin branch-like growth of ice on the water surface.
In hydrologic terms, 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 Advisory
Issued when fog reduces visibility to 1/8 mile or less over a widespread
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.
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
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.
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.
(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.
Processed base data on the Doppler radar.
A tendency toward more prominent desert conditions in a region.
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.
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.
In hydrologic terms, the volume of water, other than depression storage, existing on the land surface as flowing water which has not yet reached
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.
In the high seas and offshore forecasts, a headline used in the warnings section to indicate that gale/storm force winds are not now occurring but
are expected before the end of the forecast period.
Moisture that has condensed on objects near the ground, whose temperatures
have fallen below the dewpoint temperature.
(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".
On a buoy report, the dewpoint temperature taken at the same height as
the air temperature measurement.
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.
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.
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.
A fall of non-branched (snow crystals are branched) ice crystals in the
form of needles, columns, or plates.
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.
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).
In hydrologic terms, poorly defined ice edge limiting an area of
dispersed ice; usually on the leeward side of an area of floating ice.
(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.
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.
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.
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).
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).
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.
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.
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
Equipment that measures and records the size distribution of raindrops.
The visible surface of the sun (or any heavenly body) projected against
The process of separating radiation into various wavelengths.
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.
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).
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).
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).
In hydrologic terms, the taking of water from a stream or other body of
water into a canal, pipe, or other conduit.
In hydrologic terms, the high ground that forms the boundary of a
watershed. A divide is also called a ridge.
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.
Department of Natural Resources
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.
Department of Commerce
Development and Operations Hydrologist
The regions on either side of the equator where air pressure is low and
winds are light.
In air pollution modeling, the geographical area over which a simulation
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.
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).
A thermally driven wind directed down a valley's axis, usually occurring
during nighttime; part of the along-valley wind system.
A strong downdraft current of air from a cumulonimbus cloud, often
associated with intense thunderstorms. Downdrafts may produce damaging
winds at the surface.
(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.
A thermally driven wind directed down a mountain slope and usually
occurring at night; part of the along-slope wind system.
In the same direction as a stream or other flow, or toward the direction
in which the flow is moving.
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.
A deflection of air downward relative to an object that causes the
The component of radiation directed toward the earth's surface from
the sun or the atmosphere, opposite of upwelling radiation.
2. Dew Point
On a buoy report, dominant wave period (seconds) is the period with the
maximum wave energy.
In hydrologic terms, an area having a common outlet for its surface
runoff (also see Watershed and Catchment Area).
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.
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.
In hydrologic terms, the boundary line, along a topographic ridge or
along a subsurface formation, separating two adjacent drainage basins.
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".
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
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.
In hydrologic terms, pieces of floating ice moving under the action of
wind and/ or currents.
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.
Precipitation consisting of numerous minute droplets of water less than
0.5 mm (500 micrometers) in diameter.
The distribution of rain drops or cloud droplets of specified sizes.
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.
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.
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.)
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 5.5ø F per 1000 ft or 9.8ø C per km.
In hydrologic terms, a crack visible at the surface but not going right
through the ice cover, and therefore it is dry.
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.
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 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 Line Storm
Any thunderstorm that develops on or near a dry line.
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.
[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.
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.
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
1. An adiabatic process in a hypothetical atmosphere in which no moisture
2. An adiabatic process in which no condensation of its water vapor occurs
and no liquid water is present.
Special Tropical Disturbance Statement
A geomagnetic index describing variations in the equatorial ringcurrent.
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.
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.
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.
A non-rotating "cloud" of dust raised by straight-line winds. Often
seen in a microburst or behind a gust front.
A severe weather condition characterized by strong winds and dust-filled
air over an extensive area.
A rotating column of air rendered visible by dust.
Downward Vertical Velocity (sinking air)
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.
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).
In hydrologic terms, pressure due to a moving ice cover or drifting ice. Pressure occuring at movement of first contact termed Ice Impact
The forced uplifting of air from various atmospheric processes, such as
weather fronts, and cyclones.
Dynamic Wave Routing Model (DWOPER)
A computerized hydraulic routing program whose algorithms incorporate
the complete one-dimensional equations of unsteady flow.
Generally, any forces that produce motion or effect change. In
operational meteorology, dynamics usually refer specifically to those
forces that produce vertical motion in the atmosphere.