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DALR

(abbr) Dry Adiabatic Lapse Rate. The rate of cooling (for ascending air), or warming (for descending air) when air parcels are displaced by whatever mechanism. Usually taken to be 10degC/1km (or 3degC/1000ft).

dam

Dekametres (i.e. 10's of metres) - often used on upper air charts: thus a 500 hPa height quoted as 540 dam is equivalent to 5400 metres. (NB: although 'DM' may still be seen on some model output, this is regarded as a non-standard abbreviation. 'dm' should definitely not be used, as it is the abbreviation for decimetres, i.e. tenths of a metre)

Daughter cell

As the precipitation downdraught associated with a marked Cumulonimbus event meets the ground, it will spread out in all directions. Where this cold outflow current meets the low level inflow (relative to the cloud motion) 'head-on', then this is a point of maximum convergence, leading to forced lifting of the air at that point, and provided the air is unstable enough, and convection is not otherwise inhibited (e.g. widescale descent), then a new convective cloud event will be initiated - a daughter cell.

Decoupling

Even given the strongest pressure gradients, surface-based friction will slow airflow in the lowest 800 m or so of the atmosphere, leading to the familiar cross-isobaric flow (from high to low pressure). With strong 'free-air' gradients (Vgr) (roughly above 25 knots or 12 m/s), surface winds will bear some relationship to Vgr; however, below these (approximate) levels, come nightfall under clear skies, surface cooling will lead to stabilisation of the lowest layers and the atmosphere finds it increasingly difficult to transfer momentum from the 'free-air' levels to the near-surface. The surface wind may drop away completely as the surface-based inversion develops (often within the course of half-an-hour), allowing mist, fog or surface frost to form (other factors being in place): this process has come to be known as 'decoupling' of the boundary-layer air from the flow inferred by the isobaric flow. Once the flow is decoupled, then the surface cools even more efficiently, thus reinforcing the nocturnal inversion. (See "Stable and unstable air masses" for discussion of stability etc., and "Why does the wind blow?" for matters concerning surface wind-flow.)

DENEB

(used in METAR reports) - fog dispersal operations are in progress (probably obsolete now so included for historical purposes).

Deterministic forecast

A forecast that says rain will occur at such-and-such a place within a given time band, i.e. a 'yes/no' forecast, is an example of deterministic forecasting. (See also probability forecasting.)

Dew Point

(strictly dew-point temperature) The temperature (of an air sample that contains water vapour), to which that sample must be cooled (Pressure and humidity content being held constant) to achieve saturation with respect to a water surface. It can be measured indirectly using a wet & dry hygrometer (ordinary dry bulb thermometer, and another/adjacent thermometer with its bulb covered in a damp muslin - hygrometric tables or calculator then being used to calculate the dew point, relative humidity, vapour pressure); also by a 'dew-cell' type of instrument that measures relative humidity, from which the dew point can be calculated, or it can be measured directly by a dew-point hygrometer. The screen/surface dew-point temperature is used in air mass analysis, and also in the calculation of night-minimum and fog-point temperatures, as well as being used in the estimation of convective condensation levels, human-comfort indices, probability of snow at the surface etc. Dew point values above the surface (from radio-sonde ascents) are used to define cloudy or potentially cloudy layers etc., in the upper air (see also Frost point).

Dew-point depression (DPD)

The numerical difference between the temperature of a sample and its dew-point. The greater the difference, the lower the relative humidity. Values (deg. C) of less than 3 would be considered to indicate 'high' relative humidity; those of 7 or greater would indicate 'low' relative humidity. (See "What is the dew point depression?")

Diffluent

(see Confluent)

Discontinuity

Where a steep gradient (i.e. sharp change over a small horizontal distance) occurs in a meteorological variable (i.e. temperature, humidity, wind direction etc.), there is said to exist a discontinuity in that variable.

Diurnal cycle

Changes which take place over the course of a 24hr period. The most obvious cycle is the rise and fall of surface temperature.

Divergence

When air flows in such a way that the area occupied by a particular 'group' of air particles grows ('spreads apart'), the pattern is said to be divergent. Divergence in the atmosphere is also (along with convergence/q.v.) associated with vertical motion, and hence development (or weakening) of weather systems, depending upon the level where the divergence is dominant in a particular atmospheric column. For example, divergent flow aloft is coupled to, and may be the primary cause of, upward motion, leading to widespread cloud formation/cyclogenesis etc. ( see also diffluent.)

DM

(obsolete abbreviation for dekametre - see entry for dam)

Downward penetration of Snow

Falling snow modifies the temperature structure of the atmospheric boundary layer as both melting & evaporation takes place. (See this question in the FAQ). Even if snow does not initially penetrate to the surface (after having fallen out of the parent cloud), if the Wet Bulb Freezing Level (q.v.) is low enough, the intensity of the precipitation is more than just 'light' and the mean wind strength in the melting layer is not too strong, then the snow level can descend considerably below initial conditions. The depth of this 'downward penetration of snow' as it is called, increases as the intensity of rain increases, and/or the wind speed decreases. It will be immediately apparent that the prospect for error in snow forecasting due to these variables in 'marginal' situations will be large!

DPD

(abbr) (see Dew-point depression)

DR

Used in METAR reports - low drifting (snow, sand etc.), not appreciably affecting the visibility, e.g. DRSN.

Drought

Partial & absolute droughts are terms that are no longer used in official summaries; they were introduced in 1887 by G.J. Symons in 'British Rainfall', (with the term 'dry spell' added in 1919) but ceased to be used circa 1960. Drought hydrology is a complex field of study and as statistical & data-processing techniques have become sophisticated, there was no longer an official requirement for the use of these rather crude definitions. It is however useful to know what the definitions were, and even today, for 'hobby-use', defining periods of drought using these standards can be an interesting exercise. They are detailed elsewhere in this Glossary. (See Absolute drought; partial drought; dry spell).

Droughts - types of

Droughts as defined above are essentially meteorological; in other words, they are defined in terms of the amount of rain (or rather lack of rain) that occurs. Hydrological drought episodes by contrast take account of the wider 'cycle' of water use, with focus on the imbalance between precipitation input (rain, snow etc.) against water availability via aquifer storage, reservoir levels, land-surface run-off / riverflow etc. Finally, agricultural droughts are usually defined in terms of the soil moisture deficit (SMD) across a growing season - the degree of irrigation (natural or artificial) needed to bring a particular soil type back to 'optimal' production - balancing the outgoing moisture due to evaporation & transpiration.

Dry Intrusion

(or 'dry slot') -- A narrow region, virtually cloud-free which separates a baroclinic leaf (q.v.), and the adjacent frontal cloud. This region is the result of abruptly descending upper tropospheric/lower stratospheric air into a rapidly developing and potentially damaging low pressure system -- hence the low humidity content/absence of cloud. Water vapour imagery (see "What are the various types of Satellite imagery available?") in particular is used to diagnose this feature, and the rate of 'darkening' of the dry slot gives a clue to the rate of development of the whole storm complex.

Dry spell

A period of at least 15 consecutive days, to none of which is credited 0.04 inches / 1.0 mm of precipitation. (See also drought, absolute drought, partial drought).

DS

Duststorm (used in METAR/TAF reports etc.); visibility generally < 1km due to dust raised by strong winds over a large area.

DU

Dust (widespread, in suspension) (used in METAR/TAF reports etc.) Visibility is 5000 m or less.

DWD

Deutscher Wetterdienst (German Weather Service), based at Offenbach. Visit their web site at: http://www.dwd.de/

Dynamic precipitation

For precipitation production, other conditions being satisfied (i.e. enough humidity, required temperature structure, sufficient depth of cloud), there must be a supply of upward motion through the cloud producing the rain, snow or whatever. In the case of dynamic precipitation, the primary agent for providing upward motion is broad-scale ascent due to, for example, short-wave troughs in the prevailing upper flow, jetstream developmental areas, mass convergence or strong warm advection. [ Computer models in operational use deduce dynamic precipitation by testing for super-saturation of a layer taking into account the total water content (all phases) in a layer: the excess found is precipitated out. The type (dynamic or convective) giving the greatest amount is (usually) that seen on output charts. ](See also Convective and Orographic precipitation).

DZ

Drizzle (as in METAR/TAF reports).