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Weather Glossary - D

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Daily maximum temperature

In Australia the maximum temperature recorded for a given day represents the highest temperature recorded in the 24 hours between 9am on that day and 9am the following day. This temperature is usually reached during the afternoon of the given day.

Daily minimum temperature

In Australia the minimum temperature recorded for a given day represents the lowest temperature recorded in the 24 hours between 9am the previous day and 9am that day. This temperature is usually reached during the early hours of the given day.

Daily rainfall

In Australia the rainfall recorded for a given day represents the total recorded in the 24 hours between 9am the previous day and 9am that day.

DALR

Dry Adiabatic Lapse Rate. The rate at which the temperature of dry (unsaturated) air changes as it is raised or lowered adiabatically through the atmosphere. The DALR is approximately -9.8K/km (temperature reduces as the air is raised). See also SALR.

dBZ

The colours shown on the weather radar images represent the different echo intensities (reflectivity) measured in dBZ (decibels of Z) during each elevation scan. "Reflectivity" is the amount of transmitted power returned to the radar receiver. Reflectivity (designated by the letter Z) covers a wide range of signals (from very weak to very strong). So, a more convenient number for calculations and comparison, a decibel (or logarithmic) scale (dBZ), is used.

The dBZ values increase as the strength of the signal returned to the radar increases. Each weather radar image you see includes a colour scale. The scale represents dBZ values of the energy reflected back to the radar from precipitation and other airborne material (dBZ values from 5 to 75).

The scale of dBZ values is also related to the intensity of rainfall. Typically, light rain is occurring when the dBZ value reaches 20. The higher the dBZ, the stronger the rainrate. Depending on the type of weather occurring and the area of the country, forecasters use a set of rainrates which are associated to the dBZ values.

Hail is a good reflector of energy and will return very high dBZ values. Since hail can cause the rainfall estimates to be higher than what is actually occurring, users need to be wary of converting these high dBZ values into rainfall rates. Typically, any value over 60 dBZ has a high correlation to hail within a storm.

See also VIP.

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 devil, 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.

Deciles

Used to give an element a ranking. For example, a decile rainfall map will show whether the rainfall is above average, average or below average for a chosen time period and area.

Deepening

Used to describe a decrease in the central pressure of a low pressure system.

Delta T

Delta T is an important indicator for acceptable spraying conditions in the agricultural industry. It is indicative of evaporation rate and droplet lifetime. Delta T is calculated by subtracting the wet bulb temperature from the dry bulb temperature.

Density

The ratio of the mass of a substance to the volume occupied by it. Generally given in units of kg/m. The density of air is around 1 kg/m, water around 1000 kg/m. The density of both air and water vary with temperature. Water has a higher density at 4°C than at 0°C and it is for this reason that ice floats on water.

Depression

In meteorology it is another name for a low pressure system, an area of low pressure or a trough.

Desert

A region of little or no rainfall. See also arid.

Dew

The droplets of water deposited on a surface when the water vapour in the surrounding air condenses.

Dew point

(or dew point temperature) 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 remain constant).

Diamond Dust

Very fine, small, unbranched ice crystals. Diamond dust is so fine that it hangs in the air and is only visible as they glitter in the sun. Diamond dust can also produce a range of halo effects around the sun and moon. Diamond dust forms at temperatures below -30C and is most common over the high antarctic plateau and parts of Canada and Siberia.

Also known as frost mist, frost in the air and snow mist.

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. Opposite to confluence.

Difluence in an upper level wind field is considered a favourable condition for severe thunderstorm development (if other parameters are also favourable). But difluence is not the same as divergence. In a difluent flow, winds decelerate as they move through the region of difluence, resulting in speed convergence which offsets the apparent diverging effect of the flow.

Directional shear

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.

Distribution of showers and precipitation

  • Few: Indicating timing not an area.
  • Isolated: Showers which are well separated in space during a given period.
  • Local: Restricted to relatively small areas.
  • Patchy: Occurring irregularly over an area.
  • Scattered: Irregularly distributed over an area. Showers which while not widespread, can occur anywhere in an area. Implies a slightly greater incidence than isolated.
  • Widespread: Occurring extensively throughout an area.

Disturbance

This term has several applications. Normally describes a low or trough that is small in size and influence or is exhibiting signs of development.

Diurnal

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).

Divergence

The expansion or spreading out of a vector field; usually said of horizontal winds. The opposite of convergence. Divergence at upper levels of the atmosphere enhances upward motion, and hence the potential for thunderstorm, rain or cloud development (if other factors also are favourable).

Doldrums

The region near the equator characterised by low pressure and light, shifting winds. See also equatorial trough.

Doppler radar

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).

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. Although usually associated with thunderstorms, downbursts can occur with showers too weak to produce thunder. See dry and wet microburst.

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.

Downstream

In the same direction as a stream or flow, or toward the direction in which the flow is moving.

Drizzle

Fairly uniform precipitation (rain) composed exclusively of very small water droplets (less than 0.5mm in diameter) very close to one another.

Drought

A prolonged absence or marked deficiency of precipitation.

Dry

Free from rain. Normally used when the preceding weather has been relatively dry, and dry weather is expected to continue for at least a day or so.

A dry climate is one in which annual precipitation is exceeded by potential evaporation and transpirtion.

Dry adiabat

A line of constant potential temperature on a thermodynamic chart. See sounding.

Dry bulb temperature

The shade temperature (degrees Celsius) registered by a mercury-in-glass thermometer exposed in a white louvered box called a "Stevenson Screen" which is raised on legs one metre above the ground.

Dry line

A boundary separating a moist and dry air mass. Shown on a synoptic chart as a trough. Dry lines are common across inland QLD during the warmer months of the year where convection will occur on the eastern side. An important factor in severe weather frequency in the Great Plains (US). 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. See LP storm.

Dry line bulge

A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels (i.e., a surface dry punch). Severe weather potential is increased near and ahead of a dry line bulge.

Dry line storm

Generally, any thunderstorm that develops on or near a dry line. The term is often used synonymously with low-precipitation thunderstorms, since the latter usually occur near the dry line.

Dry microburst

A microburst with little or no precipitation reaching the ground; most common in semi-arid or 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. Compare with wet microburst.

Dry Season

The months from April through to October where tropical areas of Australia are dominated by dry southeasterly winds (trade winds). Scrub fires are common in the latter part of the dry season due to vegetation drying out.

Dry slot

A zone of dry (and relatively cloud-free) air which wraps west- or south-westwards into the southern and eastern parts of a synoptic scale or mesoscale low pressure system. A dry slot is seen best on satellite photographs. A dry slot should not be confused with clear slot, which is a storm-scale phenomenon.

Duration of precipitation

  • Brief: Short duration
  • Intermittent:Precipitation which ceases at times
  • Occasional: Precipitation which while not frequent, is recurrent.
  • Continuos: Precipitation which does not cease, or ceases only briefly.
  • Periods of rain: rain is expected to fall most of the time, but there will be breaks.

Dust devil

A small atmospheric vortex not associated with a thunderstorm, which is made visible by a rotating cloud of dust or debris (dust whirl). Dust devils form in response to surface heating during fair, hot weather; they are most frequent in arid or semi-arid regions. Other names for dust devils include the 'willy-willy' and the 'cock-eyed bob'.

Dust plume

A non-rotating 'cloud' of dust raised by straight-line winds. Often seen in a microburst or behind a gust front.

If rotation is observed, then the term dust whirl or debris cloud should be used.

Dust storm

A storm which carries a large amount of dust into the atmosphere. Particles of dust and or sand are energetically lifted to great heights by strong and turbulent winds.

Dust whirl

A rotating column of air rendered visible by dust. Similar to debris cloud; see also dust devil, gustnado, tornado.

Dynamics

Generally, any forces that produce motion or affect change in the atmosphere. In operational meteorology, dynamics usually refer specifically to those forces that produce vertical motion in the atmosphere.

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Longreach breaks November heat records with 11-day stretch of temperatures above 40 degrees

Audience submitted image 19:04 EDT Longreach in central-west Queensland has broken November heatwave records with an 11-day stretch of 40 degrees Celsius-plus temperatures, the weather bureau says.

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