Subj : Weather Terms (D) Part 4
To   : All
From : Daryl Stout
Date : Mon Sep 05 2016 12:06 am

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 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� F per 1000 ft or 9.8� 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
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.

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 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 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.


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