D
The terms and definitions found in the glossary relate to the science of meteorology.
For descriptions and definitions of words used in the Bureau's forecasts and warnings, please refer to Weather Words.
- DALR
- The Dry Adiabatic Lapse Rate (DALR) is the rate at which the temperature of unsaturated air changes as a parcel ascends or decends through the atmosphere.
The DALR is approximately 9.8 degree Celsius per 1 km. - Deep-layer mean wind
- Average wind over a deep layer of the atmosphere, giving a representative steering wind. In this case, deep is defined as spanning from the lower atmosphere all the way to the upper air (5-10 kilometres high or up to a pressure level of at least 500hPa)
- 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 the time period and area you have already chosen.
Click here for more information. - Delta-T
- Delta T is used by the agricultural industry. It is an important indicator for acceptable spraying conditions. It is indicative of evaporation rate and droplet lifetime. Delta T is calculated by subtracting the wet bulb temperature from the dry bulb temperature. Click here for more information.
- Desert
- There are many different definitions which describe a desert. In general, arid areas (or deserts) are areas with low precipitation. For example, the climate maps on our website use a condition of median annual rainfall less than 350 mm to describe an arid area.
- Dew
- Droplets of water deposited when air cools and the water vapor in it condenses.
- Dew-point temperature
- This is a measure of the moisture content of the air and is the temperature to which air must be cooled in order for dew to form. The dew-point is generally derived theoretically from dry and wet-bulb temperatures, with a correction for the site's elevation.
If the dry-bulb temperature is the same as the dew-point, the air is said to be saturated and the relative humidity is 100%. - Diurnal variation
- The changes of value, for example, of a meteorological element within the course of a (solar) day. More especially, it denotes the systematic changes that occur during the average day.
- Downburst
- Violent and damaging downdraught striking the surface of the Earth violently. Associated with a severe thunderstorm.
- Drizzle
- Fairly uniform precipitation (rain) composed exclusively of very small water droplets (less than 0.5 mm in diameter) very close to one another.
- Drought
- Prolonged absence or marked deficiency of precipitation (rain).
- Dry-bulb temperature
- This is the shade temperature (degrees Celsius) registered by a mercury-in-glass thermometer exposed in a white louvered box or meteorological screen which is raised on legs one metre above the ground.
- Dry
- (As used by forecasters) 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.
- DST - Daylight Saving Time
- Also known as summer time, Daylight Saving Time occurs when the time on local clocks is advanced forward by one hour at the beginning of the defined period of DST, and returned back by one hour at the end of DST. The exact dates between which DST is to apply can be obtained from the relevant Australian State governments. Within Australia DST across the three time zones is generally denoted by: EDT - Australian Eastern Daylight Time; CDT - Australian Central Daylight Time; and WDT - Australian Western Daylight Time.
- Dust storm
- A storm which carries large amounts of dust into the atmosphere. Ensemble of particles of dust or sand energetically lifted to great heights by a strong and turbulent wind.