DAILY MEAN
The average temperature for a day computed by averaging either the hourly readings or, more
commonly, the maximum and minimum temperatures.
DALTON'S LAW
States that the total pressure exerted
by a mixture of gases is equal to the sum of the partial pressures of
the gases. Formulated by John Dalton, an English physicist.
DATA BUOYS
Buoys placed throughout the Gulf of
Mexico and along the Atlantic and Pacific coasts of the United States
that relay information on air and water temperature,
wind speed, air
pressure, and wave conditions via radio signals.
DAWN
The first appearance of light in the eastern sky before sunrise. It marks the beginning of morning
twilight. The visual display is created by the scattering of light reaching
the upper atmosphere prior to the sun's rise to the observer's horizon.
Related term: daybreak
DAY
Considered a basic unit of time as defined by the earth's motion. It represents the time needed
for one complete revolution of the earth about its own axis. Also know
as a sidereal day, it is approximately equal to 23 hours, 56 minutes,
and 4.09 seconds. Related term: night
DEBRIS CLOUD
Considered a rotating cloudof debris or dust
that is on the ground or near the ground. The debris cloud appearing
beneath a thunderstorm will most likely confirm the presence of a tornado.
DEEPENING
Used in describing the history of a low pressure
system or an area of cyclonic circulation, it means a decrease in the
central pressure of the system. Although it usually describes the action
of a pressure system on a constant
pressure chart, it also means a surface low is increasing in cyclonic
circulation and acquiring more energy. The opposite of filling.
DEGREE
A measure of temperature difference representing a single division on a temperature scale. Related
terms: Celsius, Fahrentheit, and Kelvin
DEGREE DAY
A measure of the departure of the mean daily temperature from a given standard. That
is one degree day for each degree (Fahrenheit or Celsius) of departure
above or below the standard during one day. Related terms: cooling
degree dayand heating
degree day
DENSE FOG ADVISORY
Advisory issued when fog reduces visibility to 1/8 mile or less, creating possible
hazardous conditions.
DENSITY
The ratio of the mass of a substance to the volume it occupies. In oceanography, it is
equivalent to specific gravity and represents the ratio of the weight
of a given volume of sea water to that of an equal volume of distilled
water at 4.0 degrees C or 39.2 degrees F.
DENSITY ALTITUDE
The altitude at which a given
density is found in the standard atmosphere. Used in aviation,
it is computed from the station pressure at takeoff and the virtual
temperature at the particular altitude under consideration.
DEPRESSION
In meteorology, it is another name for an area of low pressure, a low, or trough. It
also applies to a stage of tropical
cyclonedevelopment and is known as a tropical depression to distinguish
it from other synoptic features.
DERECHO
A line of intense, widespread, and fast-moving thunderstorms that moves across a great
distance. They are characterized by damaging straight-line winds over
hundreds of miles. Spanish for straight.
DEW
Condensation in the form of small water drops that forms on grass and other small objects near
the ground when the temperature has fallen to the dew point, generally
during the nighttime hours.
DEW POINT
The temperature to which air must be cooled at a constant pressure to become saturated.
View Current Dew Point at MNSU
DIABLO WINDS
Dry winds in the Diablo mountain range
in central California that can exceed 60 miles per hour. Similar to
the Santa Ana winds, they develop as the wind flows from high pressure over Nevada to lower pressure
along the central California coast.
DIFFLUENCE
A rate at which wind flow spreads apart along an axis oriented normal to the flow in
question. The opposite of confluence.
DIFFRACTION
The result of light waves interfering with other after passing through a narrow
aperture, causing them to bend or spread.
DIRECTIONAL SHEAR
The shear created by a rapid change in wind
direction with height.
DISCONTINUITY
Comparatively large contrast in meteorological elements over a relatively small distance
or period of time. In oceanography, it is the abrupt change or jump of a variable at a line or surface.
DISTURBANCE
This has several applications. It can apply to a low or cyclone that is small
in size and influence. It can also apply to an area that is exhibiting
signs of cyclonic development. It may also apply to a stage of tropical
cyclonedevelopment and is known as a tropical disturbance to distinguish
it from other synoptic features.
DIURNAL
Pertaining to actions or events that occur during a twenty-four hour cycle or recurs every
twenty-four hours. Meteorological elements that are measured diurnally
include clouds, precipitation,
pressure, relative
humidity, temperature,
and wind.
DIVERGENCE
Wind movement that results in a horizontal net outflow of air from a particular
region. Divergence at lower levels is associated with a downward movement
of air from aloft. Contrast with convergence.
DOG DAYS
The name given to the very hot summer
weather that may persists for four to six weeks between mid-July through
early September in the United States. In western Europe, this period
may exist from the first week in July to mid-August and is often the
period of the greatest frequency of thunder. Named for Sirius,
the Dog Star, which lies in conjunction with the sun during this period,
it was once believed to intensify the sun's heat during the summer months.
DOLDRUMS
Located between 30 degrees North and 30 degrees South latitudes in the vicinity of the
equator, this area typically has calm or light and variable winds. Also
a nautical term for the equatorial trough. Related terms: Intertropical
Convergence Zone (ITCZ), Horse
Latitudes
DOPPLER RADAR
Weather radar
that measures direction and speed of a moving object, such as drops
of precipitation, by determining
whether atmospheric motion is horizontally toward or away from the radar.
Using the Doppler effect, it measures the velocity of particles. Named
for J. Christian Doppler, an Austrian physicist, who in 1842 explained
why the whistle of an approaching train had a higher pitch than the
same whistle when the train was going away. Related terms: NEXRAD:
Visit WALTER Radar Tutorial
DOWNBURST
A severe localized downdraft from a thunderstorm or shower. This outward burst of cool
or colder air creates damaging winds at or near the surface. Sometimes
the damage resembles tornadic damage. Related term: microburst
DOWNDRAFT
A sudden descent
of cool or cold air to the ground, usually with precipitation,
and associated with a thunderstorm or shower. Related term: updraft
DOWNPOUR
A heavy rain. Related
term: cloudburst
DOWNSLOPE EFFECT
The warming of an air flow as it descends a hill or mountain slope. Related term: upslope
DRAINAGE WIND
A katabatic
wind, it is caused by the cooling of air along the slopes of a mountain.
Related term: mountain breeze
DRIFTING SNOW
Snow
particles blown from the ground by the wind to a height of less than
six feet.
DRIFTS
Normally used when
referring to snow or sand particles are deposited behind obstacles or
irregularities of the surface or driven into piles by the wind.
DRIZZLE
Slowly falling precipitation
in the form of tiny water droplets with diameters less than 0.02 inches
or 0.5 millimeters. It falls from stratus
clouds and is often associated with low visibility
and fog. It is reported as "DZ" in an observation and on the METAR.
DROPSONDE
A radiosonde dropped with a parachute from an aircraft rather than lifted by a balloon
to measure the atmosphere below.
DROUGHT
Abnormal dry weather for a specific area that is sufficiently prolonged for the lack of water
to cause serious hydrological
imbalance.
DRY ADIABAT
The line on a Skew T-Log P chart that depicts
the lifting of dry air, or air that is unsaturated. As a parcel rises
adiabatically, its pressure decreases and its temperature falls due
to the expansion of the air parcel. When an air parcel is unsaturated
and rises, then the temperature decreases at a rate of 1 degrees C per 100 meters
(5.5 degrees F per 1,000 feet). Related term: moist
adiabat and adiabatic process
DRY BULB THERMOMETER
A thermometer
used to measure the ambient temperature. The temperature recorded is
considered identical to air temperature. One of the two thermometers
that make up a psychrometer.
DRY LINE
The boundary between the dry desert air mass of the
Southwest U.S. and the moist air mass from the Gulf of Mexico. It usually
lies north-south across the central and southern High Plains states
during spring and summer. The passage of a dry line results in a sharp
decrease in humidity, clearing
skies, and a wind shift from
southeasterly or south to southwesterly or west. Its presence influences
severe weather development in the Great Plains.
DRY SLOT
An area of dry, and usually cloud-free,
air that wraps into the southern and eastern sections of a synoptic
scale or mesoscale low pressure system. Best seen on a satellite picture,
such as a water vapor image.
DUSK
The period of waning light from the time of sunset to dark.
Related terms: twilight and dawn
DUST
Small particles of earth or other matter suspended in the air.
It is reported as "DU" in an observation and for wide spread dust on
the METAR.
DUST BOWL
The term given to the area of the Great Plains including Texas, Oklahoma, Kansas, Colorado,
and New Mexico that was most greatly affected during the Great Drought
of the 1930's. Related term: drought
DUST DEVIL
A small, rapidly rotating column of wind, made visible
by the dust, dirt or debris it picks up. It usually occurs in arid or
semi-arid areas and is most likely to develop on clear, dry, hot afternoons
in response to surface heating. Related term: whirlwind
DUSTSTORM
A severe weathercondition characterized by strong winds and dust-filled
air over a large area. Visibility
is reduced to between 5/8ths and 5/16ths statute mile. It is reported
as "DS" in an observation and on the METAR.
D-VALUE
The deviation of actual altitude
along a constant pressure surface from the standard atmosphere altitude
of that surface.
DYNAMICS
A branch of mechanics that deals with forces and their relations to patterns of motion. In
metorology, this relates especially
to windand water patterns.
