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Mauna Kea Weather Center Meteorology Glossary/Dictionary

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  • day - 1. A basic time increment defined by the earth's motion; specifically, a complete revolution of the earth about its own axis. The sidereal day is defined as the time required for the earth to make one complete revolution in an absolute coordinate system, that is, with respect to the stars. The day in common use is the mean solar day, derived, by means of the equation of time, from the apparent solar day, which is determined directly from the apparent relative motion of the sun and earth. The civil day is a modification of the mean solar day, which renders it practical as a time measure for ordinary purposes. See also year, lunar day. 2. The period from midnight to midnight, local civil time; that is, a civil day. 3. The period of daylight, as opposed to that of darkness. 4. See pendulum day. 5. See observational day.

  • decay - In ocean wave studies, the loss of energy from wind-generated ocean waves after they have ceased to be acted on by the wind. This process is accompanied by an increase in wave length and a decrease in wave height.

  • deep easterlies - Same as equatorial easterlies.

  • deep trades - Same as equatorial easterlies.

  • deepening - A decrease in the central pressure of a pressure system as depicted on a constant-height chart, or an analogous decrease in height on a constant-pressure chart; the opposite of filling. The term is usually applied to low pressure rather than to high pressure, although technically it is acceptable in either sense. The deepening of a low is commonly accompanied by the intensification of its cyclonic circulation, and the term is frequently used to imply the process of cyclogenesis. Deepening can be quantitatively expressed in at least two ways: either 1) as the time rate of central-pressure decreases; or 2) as that component of the pressure tendency at any fixed point that is attributable neither to the motion of the pressure system relative to that point nor to the diurnal influence of atmospheric tides. Compare cyclogenesis.

  • degradation - The lessening of a quality of data or images because of any optical, electronic, or mechanical distortions (noise) in the data collection or image forming systems.

  • degree - 1. A unit of temperature. See absolute temperature scale, Celsius temperature scale, centigrade temperature scale, Fahrenheit temperature scale, Kelvin temperature scale, Reaumur temperature scale. 2. A unit of angular distance; 1/360 part of a circle. See also radian.

  • delay - Applied to any time-varying physical quantity, usually periodic, its displacement in time relative to a similar reference quantity. For example, the temperature in soil depends on depth and time. If the surface temperature varies sinusoidally, so does the temperature at any depth, but it is out of phase (is delayed) relative to the surface temperature. There is a phase shift or phase delay or phase lag of soil temperature relative to surface temperature. Although these three terms are most often applied to waves, nothing inherent in the concept of a delay restricts it to waves. Compare lag.

  • density - 1. The ratio of the mass of any substance to the volume occupied by it (usually expressed in kilograms per cubic meter, but any other unit system may be used); the reciprocal of specific volume. In a continuous medium the density is defined by a limiting process and is a point function. 2. The ratio of any quantity to the volume or area it occupies; for example, flux density, power density, ion density, electron density, drainage density.

  • departure - Same as deviation.

  • dependence - See statistical independence.

  • depression - 1. In general, a point or limited area of locally lower elevation in a particular surface. 2. In meteorology, an area of low pressure; a low or trough. This is usually applied to a certain stage in the development of a tropical cyclone, to migratory lows and troughs, and to upper-level lows and troughs that are only weakly developed. This use of the term is most common in the European literature. See V-shaped depression. 3. Same as depression angle. 4. See wet-bulb depression, dewpoint spread.

  • detrainment - The transfer of air from an organized air current to the surrounding atmosphere; the opposite of entrainment.

  • development - The process of intensification of an atmospheric disturbance, most commonly applied to cyclones and anticyclones. From the dynamical viewpoint, development usually implies the generation of vorticity in the atmosphere due to the action of the baroclinic structure of the flow or to the addition of energy to the disturbance, as distinct from the barotropic redistribution of vorticity.

  • deviation - (Also called departure.) In statistics, the difference between two numbers. It is commonly applied to the difference of a variable from its mean, or to the difference of an observed value from a theoretical value. See standard deviation, geostrophic departure.

  • dew - Water condensed onto grass and other objects near the ground, the temperatures of which have fallen below the dewpoint of the surface air due to radiational cooling during the night, but are still above freezing; hoarfrost may form if the dewpoint is below freezing (see frost point). If the temperature falls below freezing after dew has formed, the frozen dew is known as white dew. The conditions favorable to dew formation are 1) a radiating surface, well insulated from the heat supply of the soil, on which vapor may condense; 2) a clear, still atmosphere with low specific humidity in all but the surface layers, to permit sufficient effective terrestrial radiation to cool the surface; and 3) high relative humidity in the surface air layers, or an adjacent source of moisture such as a lake. Dew plays an important role in the propagation of certain plant pathogens, such as late potato blight, which require dew-covered leaves from certain stages of sporulation. Dew is responsible for the optical effect known as the heiligenschein.

  • diffluence - The rate at which adjacent flow diverges along an axis oriented normal to the flow at the point in question; the opposite of confluence. The diffluence may be measured by [∂]vn/[∂]n or V[∂]y[&psgr;]/[∂]n, where V is the speed of the wind, the n axis is oriented 90° clockwise from the direction of the wind vector, vn is the wind component in the n direction, and y[&psgr;] is the wind direction measured in degrees clockwise from a reference direction.

  • dip - Same as inclination.

  • display - 1. In radar, a presentation of the reflectivity, mean Doppler velocity, or other properties of the received signals in a form to facilitate analysis or interpretation. Common displays are the PPI, RHI, and THI. 2. Same as indicator.

  • dissipation - (Also called viscous dissipation.) In thermodynamics, the conversion of kinetic energy into internal energy by work done against the viscous stresses. Sometimes the rate of conversion per unit volume is meant. If the Navier-Stokes equations of viscous flow are employed, Rayleigh's mathematical expression for the rate of viscous (or frictional) dissipation per unit volume is where m[&mgr;] is the dynamic viscosity. The Navier-Stokes assumptions thus satisfy the primary requirement of the second law of thermodynamics that the rate of dissipation be positive and the process irreversible. In a turbulent fluid, which the atmosphere usually is, dissipation is the end result of the turbulent scale process, by which kinetic energy is transferred from its originating, or outer, scale to the dissipation scales by nonlinear dynamical interactions. Most dissipation occurs at scales near the Kolmogorov microscale l[&lgr;]d, given by l[&lgr;]d = (n[&ngr;]3/)1/4, where n[&ngr;] is the kinematic viscosity and is the rate of energy dissipation per unit mass. See also stress tensor, energy equation.

  • disturbance - In general, any agitation or disruption of a steady state. In meteorology, this has several rather loose applications: 1) used for any low or cyclone, but usually one that is relatively small in size and effect; 2) applied to an area where weather, wind, pressure, etc., show signs of the development of cyclonic circulation (see tropical cyclone); 3) used for any deviation in flow or pressure that is associated with a disturbed state of the weather, that is, cloudiness and precipitation; and 4) applied to any individual circulatory system within the primary circulation of the atmosphere. See wave disturbance, instability; compare perturbation.

  • diurnal - Daily, especially pertaining to actions that are completed within 24 hours and that recur every 24 hours; thus, most reference is made to diurnal cycles, variations, ranges, maxima, etc. The diurnal variability of nearly all of the meteorological elements is one of the most striking and consistent features of the study of weather. The diurnal variations of important elements at the earth's surface can be summarized as follows: 1) temperature maximum occurs after local noon and minimum near sunrise; 2) relative humidity and fog are the reverse of temperature; 3) wind generally increases and veers by day and decreases and backs by night (see heliotropic wind, land and sea breeze, mountain and valley wind); 4) cloudiness and precipitation over a land surface increase by day and decrease at night; over water the reverse is true, but to a lesser extent; 5) evaporation is markedly greater by day; 6) condensation is much greater at night; 7) atmospheric pressure varies diurnally or semidiurnally according to the effects of atmospheric tides.

  • diurnal heating - In the ocean, heating from solar radiation (primarily shortwave) that can lead to transient thermoclines and the afternoon effect.

  • divergence - The expansion or spreading out of a vector field; also, a precise measure thereof. In mathematical discussion, divergence is taken to include convergence, that is, negative divergence. The mean divergence of a field F within a volume is equal to the net penetration of the vectors F through the surface bounding the volume (see divergence theorem). The divergence is invariant with respect to coordinate transformations and may be written div F or F, where is the del operator. In Cartesian coordinates, if F has components Fx, Fy, Fz, the divergence is Expansions in other coordinate systems may be found in any text on vector analysis. In hydrodynamics, if the vector field is unspecified, the divergence usually refers to the divergence of the velocity field (see also mass divergence). In meteorology, because of the predominance of horizontal motions, the divergence usually refers to the two-dimensional horizontal divergence of the velocity field where u and v are the x and y components of the velocity, respectively. This divergence is denoted by any of the following symbols: divH V, H V, div2 V, 2 V, divp V, p V, where the last two quantities involve derivatives in the isobaric surface. The order of magnitude of the horizontal divergence in meteorological motions is of considerable dynamic importance: The geostrophic wind has divergence of the order of 10-6s-1; the wind field associated with migratory cyclonic systems, 10-5s-1; motions of smaller scale (such as gravity waves, frontal waves, and cumulus convection) have characteristic divergence one or two orders of magnitude greater. See balance equation, deformation; compare diffluence, curl, vorticity.

  • domain - An area of human activity presumed to contain expertise and knowledge suitable for the basis of an expert or knowledge-based system. Expert system domains should be kept as narrow as possible without becoming trivial. For example, prediction of severe storms rather than general weather prediction, or analysis of cloud type rather than the whole of satellite meteorology.

  • drag - (Or resistance.) The frictional impedance offered by air to the motion of bodies passing through it. More precisely, the component of aerodynamic force parallel to the direction of mean flow. At very low speeds, most of the drag exerted by the air on a body moving through it is due to viscous drag (or skin friction) acting through a fairly thin boundary layer. In the case of spheres, the low-speed air resistance is given by Stokes's law. For higher speeds, so-called form drag or pressure drag arises as a result of separation of the laminar boundary layer creating a wake region of chaotic flow in which the pressure is reduced. In general, for large Reynolds numbers, form drag is far more significant than viscous drag. The velocity dependence of air resistance changes more or less continuously from linear dependence in the viscous range to velocity-squared dependence at high speeds. The latter dependence is given by Rayleigh's formula, drag = CDr[&rgr;]L2U2, where r[&rgr;] is the density of the medium, L a characteristic linear dimension of the body, and U the speed of the body relative to the fluid. The drag coefficient CD is proportional to an experimentally determined power of the Reynolds number. At speeds approaching that of sound, an additional source of drag arises out of the formation of shock waves.

  • drain - 1. To provide channels, such as open ditches or drain tile, so that excess water can be removed by surface or by internal flow. 2. To lose water from the soil by percolation. (Glossary of Terms in Soil Science, Agriculture Department of Canada, Publication 1459, revised 1976)

  • drift - 1. The effect of the velocity of fluid flow upon the velocity (relative to a fixed external point) of an object moving within the fluid; the vector difference between the velocity of the object relative to the fluid and its velocity relative to the fixed reference. In air navigation, drift is often couched in terms of angular difference between heading and course, and thus can be produced only by a crosswind; when the wind velocity is parallel to the heading of the aircraft (direct headwind or tailwind), the drift is considered to be zero. The calculation of the drift (leeway) effects upon an ocean-going vessel is complicated by having to consider the combined effects of two fluids in motion. 2. "In geology, materials in transport by ice; deposits made by glacial ice on land, in the sea, and in bodies of meltwater." (Glossary of Arctic and Subarctic Terms, 1955.) 3. The speed of an ocean current. In publications for the mariner, drifts are usually given in miles per day or in knots. 4. See snowdrift. 5. The horizontal track of an object, for example, clouds, caused by the wind or fluid motion. Also refers to a shift in the calibration of a satellite sensor or change in the orbital track of a satellite.

  • drizzle - (Sometimes popularly called mist.) Very small, numerous, and uniformly distributed water drops that may appear to float while following air currents. Unlike fog droplets, drizzle falls to the ground. It usually falls from low stratus clouds and is frequently accompanied by low visibility and fog. In weather observations, drizzle is classified as 1) light, the rate of fall being from a trace to 0.3 mm (0.01 in.) per hour; 2) moderate, the rate of fall being from 0.3 to 0.5 mm (0.01-0.02 in.) per hour, and 3) heavy, the rate of fall being greater than 0.5 mm (0.02 in.) per hour. When precipitation equals or exceeds 1 mm (0.04 in.) per hour, all or part of the precipitation is usually rain. However, true drizzle falling as heavily as 1.3 mm (0.05 in.) per hour has been observed. By convention, drizzle drops are taken to be less than 0.5 mm (0.02 in.) in diameter. Larger drops are considered raindrops. Compare mist.

  • drop - Liquid particle, with shape maintained as a balance between surface tension and air drag when falling at terminal velocity under gravity in the atmosphere; drops less than 1 mm (0.04 in.) are approximately spherical. The shape may also be influenced by ambient electric field or thunderstorm strength.

  • dry air - 1. In atmospheric thermodynamics and chemistry, air that is assumed to contain no water vapor. Compare moist air. 2. Generally, air with low relative humidity.

  • dry convection - A vertical exchange of air without precipitation at the ground.

  • dry spell - A period of precipitation below a specified amount. The specific period and amount of precipitation vary depending on the particular activity under discussion.

  • duct - Applied to the atmosphere and ocean, any region with vertically varying properties such that waves of any kind (e.g., electromagnetic and acoustic) launched in certain directions are guided by or trapped within the region rather than propagating radially from their source. For a duct to exist, attenuation must be negligible over distances comparable to the characteristic linear dimensions of the duct.

  • duration - In connection with wind waves, the duration is the length of time that winds generating surface waves have been present. This, in addition to the fetch (which is the distance over which these winds act) and wind direction, determines the amplitude and direction of the wind-generated waves.

  • dust - Solid materials suspended in the atmosphere in the form of small irregular particles, many of which are microscopic in size. It imparts a tannish or grayish hue to distant objects. The sun's disk is pale or colorless or has a yellowish tinge at all periods of the day. Dust cannot be a stable component of the atmosphere because it must eventually fall back to the earth's surface when winds and turbulence become too weak to bear it aloft. Dust is due to many natural and artificial sources, for example, volcanic eruptions, salt spray from the seas, blowing solid particles, plant pollen, bacteria, and smoke and ashes from forest fires and industrial combustion processes. It was once thought that dust particles were a main source of condensation nuclei; this is no longer regarded as probable as most dusts are not sufficiently hygroscopic. Compare smoke, haze; see duststorm, dust devil.

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