D

Day – sunrise to sunset.

Day of the week (eg. Monday) – midnight to midnight.

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.

Dense fog – a fog in which the visibility is less than one-quarter mile.

Depression – a region of low atmospheric pressure that is usually accompanied by low clouds and precipitation.

Dew point – the temperature to which the air must be cooled for water vapor to condense and form fog or clouds.

Diamond dust – a fall of non-branched (snow crystals are branched) ice crystals in the form of needles, columns, or plates.

Difluence – 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.

Disturbance – a disruption of the atmosphere that usually refers to a low pressure area, cool air and inclement weather.

Downburst – a strong downdraft resulting in an outward burst of damaging winds on or near the ground. Downburst winds can produce damage similar to a strong tornado.

Downdraft – a column of generally cool air that rapidly sinks to the ground, usually accompanied by precipitation as in a shower or thunderstorm.

Downslope wind – air that descends an elevated plain and consequently warms and dries. Occurs when prevailing wind direction is from the same direction as the elevated terrain and often produces fair weather conditions.

Draft – a draft is a small gusty air current that moves upward or downward abruptly; hence the terms updraft and downdraft.

Drifting snow – uneven distribution of snowfall caused by strong surface winds. Drifting snow does not reduce visibility. In Canada, it is defined as snow blown to a height of less than two meters.

Drizzle – small, slowly falling water droplets, with diameters between .2 and .5 millimeters. In Australia, it is defined as fairly uniform precipitation (rain) composed exclusively of very small water droplets (less than 0.5 mm in diameter) very close to one another.

Drought – abnormally dry weather in a region over an extended period sufficient to cause a serious hydrological (water cycle) imbalance in the affected area. This can cause such problems as crop damage and water-supply shortage. In Australia, it is defined as the prolonged absence or marked deficiency of precipitation (rain).

Dry – free from rain. Normally used when preceding weather has also been relatively dry and dry weather is expected to continue for a day or so.

Dust storm – an area where high surface winds have picked up loose dust, reducing visibility to less than one-half mile. In Australia, it is described as a storm which carries large amounts of dust into the atmosphere.