Subj : Weather Terms (D) Part 1
To   : All
From : Daryl Stout
Date : Mon Sep 05 2016 12:06 am

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.

Data Point
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.

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 6ø 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 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.

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.

Deformation Zone
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).

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 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.

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
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.

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 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
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.


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