D

Daily maximum temperature

Maximum temperature during a continuous time interval of 24 hours.

Daily minimum temperature

Minimum temperature during a continuous time interval of 24 hours.

Daily range temperature

Temperature range during a continuous time interval of 24 hours.

Dangerous semicircle / Navigable semicircle

Region of a tropical cyclone, situated to the right of its path in the Northern Hemisphere and to the left in the Southern Hemisphere, where the seas are higher and the winds more violent and then to carry shipping into the cyclone’s path. The other half of the cyclone is termed the “navigable semicircle”.

Dart leader

Leader stroke of a lightning discharge, other than the first in the series of a compound discharge, in which the ionized channel is established in a single jump.

Dayglow

Daytime form of airglow; though probably stronger than the corresponding night-time emission, it is less readily distinguished against the stronger daytime background radiation.

Day of snow lying

Day during which at least half of the ground at the station is covered by snow.

Debris

Remains of something that has been otherwise broken up or destroyed.

Deeping of a depression

Decrease with time of the central pressure of a depression.

Deforestation

Process of clearing forests, which can lead to erosion, floods, drought, loss of biodiversity, extinction of species and an increment in the carbon dioxide presence in the atmosphere.

Degree-day

Algebraic difference expressed in degrees, between the mean temperature of a particular day and a reference temperature. For a specific period (months, years), the number of degree-days is the algebraic sum of the degree-days for all days in the period.

Dendritic snow crystals

Ice crystals whose macroscopic form is characterized by intricate branching structures which are treelike in form and which possess hexagonal symmetry in ideal form.

Density of dry air

Ratio of the mass of dry air to the volume which it occupies.

Density of moist air

Ratio of the mass of moist air to the volume which it occupies.

Deposition

Process by which a substance changes from gaseous to solid phase. Process by which traces gases or particles are transferred from the atmosphere to the Earth’s surface. It can be wet or dry, depending on the phase that the material is in during the process.

Deposition nuclei

Nuclei on which deposition occurs.

Desert climate / Arid climate

A climatic type which is characterized by insufficient moisture to support appreciable plant life.

Desertification

Land degradation in arid, semi arid and dry sub-humid areas. The primary factor that causes desertification is human activities, usually arising from increased demands from growing populations.

Designated pollutant

Air pollutant identified as potentially harmful, deserving monitoring (controlled by government regulation) and study.

Detrainment

Outflow of air from a convective element, transferring some of its heat and moisture to the surrounding atmosphere.

Dew

Water condensed and deposited on objects near ground, whose temperatures have fallen enough, generally by nocturnal radiational cooling, but is still above freezing.

Dewbow

Bow, similar to a rainbow, often hyperbolic in shape, formed by the refraction and reflection of the Sun’s rays in dew drops.

Dew point

Temperature to which an air parcel must be cooled in order to achieve saturation, given a constant pressure and constant water vapor content.

Dew point temperature

Diffraction / Diffraction phenomenon

Interference pattern produced within the geometrical shadow thrown by an obstacle, due to the bending (by an amount which varies with wavelength) of light waves caused by the obstacle. Examples of diffraction phenomena are the aureole, Bishop’s ring, corona and glory.

Diffusion

Transport and spreading of matter or of a conservative property just by the random motions of molecules, which do not move in organized groups (molecular diffusion) or by eddy motions (turbulent diffusion). This last one is the prevalent process in the atmosphere.

Dispersion

Variation of the complex index of refraction with frequency, either classified as normal or anomalous; in fact, every material substance presents anomalous dispersion at some frequency. By its means, a beam of light can be spatially separated into its components, resulting in phenomena as rainbows and halos. Also designates the spreading of atmospheric constituents, such as air pollutants.

Dissipation

Diminish of atmospheric kinetic energy due to its transfer from larger to smaller scales of motion, ending in molecular motions.

Disturbance

Small sized low or cyclone, with relatively small effect. Area where weather elements show signs of development of a cyclonic circulation. Any deviation in flow or pressure that results in cloudiness and precipitation. Any individual circulatory system within the primary circulation of the atmosphere.

Diurnal

Actions that take place or are completed within 24 hours or every 24 hours. Most references are made to diurnal cycles, ranges or variations. One of the most consistent features of the study of the weather is the diurnal variability of meteorological elements.

Divergence

Expansion or spreading out of a vector field. The spreading out of a given air parcel, opposite to convergence.

Dobson Unit (DU)

Unit of measure of ozone amount in the atmosphere; one Dobson unit is the equivalent of 2.69 × 1016 molecules of ozone per square centimeter at standard temperature and pressure.

Doppler effect

A shift in the frequency of an electromagnetic or sound wave due to the relative movement of the source or the observer.

Doppler radar

Radar that detects and makes use of the Doppler effect to measure the radial velocity of a target moving towards or away from the radar, and to differentiate moving targets from fixed ones.

Douglas Scale

Scale which purpose is to estimate the sea’s roughness for navigation. The Douglas Scale consists of two codes, one for estimating the state of the sea and the other for describing sea swell, expressed in 10 degrees, from Degree 0 to Degree 9.

Downpour

Heavy rain.

Downslope/Downvalley wind

Wind directed down a slope, usually used to refer to winds produced by processes larger in scale than the slope. Downslope winds experience warming, drying, increasing stability and clearing of clouds, as this kind of flow produces subsidence. It is a component of the mountain-valley wind systems; same as katabatic wind.

Drizzle

Precipitation in very fine drops of water, with less of 0.5 mm of diameter, uniformly distributed and very close to one another, falling from a cloud.

Droplet

Small spherical particle of any liquid; in meteorology, commonly used to denote water droplets.

Drought

Period of abnormally dry weather, long enough to produce severe hydrological imbalance. Prolonged absence or serious deficiency of precipitation.

Dry adiabatic lapse rate

The rate of decrease of temperature with height of a parcel of dry air (unsaturated air parcel), lifted by a reversible adiabatic process. When applied to vertical motion, the rate is -9.8° C degrees per kilometer which equals to -1 °C/100m.

Dry spell

Period of abnormally dry weather. Usually refers to less severe conditions than a drought.

Dry tongue

Protrusion or intrusion of dry air into a region of higher moisture content.

Dust

Solid small and irregular particles, many of them microscopic in size, suspended in the atmosphere. It gives distant objects a grayish hue. When turbulence is not strong enough to keep it suspended, it falls back to the surface.

Dust devil

Sand devil, dervish, devil, satan, desert devil, and sand auger.

Dust wall

Leading edge or front of a duststorm or sandstorm, which appears as a huge high vertical wall, which moves rapidly.

Dust whirl / Dust devil or Sand whirl

Ensemble of dust, sand and debris raised from the ground in a usually short-lived small vortex or whirling column of varying height and small diameter. They rotate cyclonically as well as anticyclonically with an almost vertical axis. Usually, dust whirls appear over dry and dusty or shady areas.

Duststorm / Duster, Black blizzard

Ensemble of particles of dust violently lifted to great heights by a strong and turbulent wind over an extensive area. It is an unusual and frequently severe weather condition.

Dvorak´s Objective Technology (ODT)

Widely used system to objectively estimate the intensity of tropical cyclones based exclusively on visible and infrared satellite images.