Subj : Weather Terms (D) Part 4 To : All From : Daryl Stout Date : Mon Sep 05 2016 12:06 am Drainage Density In hydrologic terms, the relative density of natural drainage channels in a given area. It is usually expressed in terms of miles of natural drainage or stream channel per square mile of area, and obtained by dividing the total length of stream channels in the area in miles by the area in square miles. Drainage Divide In hydrologic terms, the boundary line, along a topographic ridge or along a subsurface formation, separating two adjacent drainage basins. Drainer A valley or basin from which air drains continuously during nighttime rather than becoming trapped or pooled. Drains (Relief Wells) In hydrologic terms, a vertical well or borehole, usually downstream of impervious cores, grout curtains or cutoffs, designed to collect and direct seepage through or under a dam to reduce uplift pressure under or within a dam. A line of such wells forms a "drainage curtain". Drawdown In hydrologic terms, the lowering of the surface elevation of a body of water, the water surface of a well, the water table, or the piezometric surface adjacent to the well, resulting from the withdrawl of water therefrom. DRCTN Direction Dredging In hydrologic terms, the scooping, or suction of underwater material from a harbor, or waterway. Dredging is one form of channel modification. It is often too expensive to be practical because the dredged material must be disposed of somewhere and the stream will usually fill back up with sediment in a few years. Dredging is usually undertaken only on large rivers to maintain a navigation channel. DRFT Drift Drifting Ice In hydrologic terms, pieces of floating ice moving under the action of wind and/ or currents. Drifting Snow Drifting snow is an uneven distribution of snowfall/snow depth caused by strong surface winds. Drifting snow may occur during or after a snowfall. Drifting snow is usually associated with blowing snow. Drizzle Precipitation consisting of numerous minute droplets of water less than 0.5 mm (500 micrometers) in diameter. Drop-size Distribution The distribution of rain drops or cloud droplets of specified sizes. Drought Drought is a deficiency of moisture that results in adverse impacts on people, animals, or vegetation over a sizeable area. NOAA together with its partners provides short- and long-term Drought Assessments. Drought Assessments At the end of each month, CPC issues a long-term seasonal drought assessment. On Thursdays of each week, the CPC together with NOAA National Climatic Data Center, the United States Department of Agriculture, and the National Drought Mitigation Center in Lincoln, Nebraska, issues a weekly drought assessment called the United States Drought Monitor. These assessments review national drought conditions and indicate potential impacts for various economic sectors, such as agriculture and forestry. Drought Index In hydrologic terms, computed value which is related to some of the cumulative effects of a prolonged and abnormal moisture deficiency. (An index of hydrological drought corresponding to levels below the mean in streams, lakes, and reservoirs.) Dry Adiabat A line of constant potential temperature on a thermodynamic chart. Dry Adiabatic Lapse Rate The rate at which the temperature of a parcel of dry air decreases as the parcel is lifted in the atmosphere. The dry adiabatic lapse rate (abbreviated DALR) is 5.5ø F per 1000 ft or 9.8ø C per km. Dry Crack In hydrologic terms, a crack visible at the surface but not going right through the ice cover, and therefore it is dry. Dry Floodproofing In hydrologic terms, a dry floodproofed building is sealed against floodwaters. All areas below the flood protection level are made watertight. Walls are coated with waterproofing compounds or plastic sheeting. Openings like doors windows, sewer lines and vents are closed, whether permanently, with removable shields, or with sandbags. The flood protection level should be no more than 2 or 3 feet above the top of the foundation because the buildings walls and floors cannot withstand the pressure of deeper water. Dry Line A boundary separating moist and dry air masses, and an important factor in severe weather frequency in the Great Plains. 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. Dry Line Bulge A bulge in the dry line, representing the area where dry air is advancing most strongly at lower levels. Severe weather potential is increased near and ahead of a dry line bulge. Dry Line Storm Any thunderstorm that develops on or near a dry line. Dry Microburst A microburst with little or no precipitation reaching the ground; most common in semi-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. Dry Punch [Slang], a surge of drier air; normally a synoptic-scale or mesoscale process. A dry punch at the surface results in a dry line bulge. A dry punch aloft above an area of moist air at low levels often increases the potential for severe weather. Dry Slot A zone of dry (and relatively cloud-free) air which wraps east- or northeastward into the southern and eastern parts of a synoptic scale or mesoscale low pressure system. A dry slot generally is seen best on satellite photographs. Dry Thunderstorm Generally a high-based thunderstorm when lightning is observed, but little if any precipitation reaches the ground. Most of the rain produced by the thunderstorm evaporates into relatively dry air beneath the storm cell. May also be referred to as "dry lightning". Dry Weather Flow In hydrologic terms, streamflow which results from precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel. This is also referred to as baseflow, or ground water flow. Dry-adiabatic 1. An adiabatic process in a hypothetical atmosphere in which no moisture is present. 2. An adiabatic process in which no condensation of its water vapor occurs and no liquid water is present. DSA Special Tropical Disturbance Statement DSIPT Dissipate Dst Index A geomagnetic index describing variations in the equatorial ringcurrent. DTRT Deteriorate Duration Curve In hydrologic terms, a cumulative frequency curve that shows the percent of time during which specified units of items (e.g. discharge, head, power,etc.) were equaled or exceeded in a given period. It is the integral of the frequency diagram. Duration of Ice Cover In hydrologic terms, The time from freeze-up to break-up of an ice cover. Posted by VPost v1.7.081019 --- Virtual Advanced Ver 2 for DOS * Origin: The Thunderbolt BBS (801:1/7) þ Synchronet þ The Curmudeon's Place .