Acid Rain
Rain containing relatively high concentrations of acid-forming chemicals that have been released into the atmosphere and combined with water vapor; harmful to the environment.
Complex Terrain
Typically used to refer to mountainous terrain. In general usage, it may also refer to coastal regions and heterogeneous landscapes.
Coronal Rain
(Abbrev. CRN) In solar-terrestrial terms, material condensing in the corona and appearing to rain down into the chromosphere as observed at the solar limb above strong sunspots.
Critical Rainfall Probability
(Abbrev. CRP) - In hydrologic terms, the Probability that the actual precipitation during a rainfall event has exceeded or will exceed the flash flood guidance value.
Curtain Drain
In hydrologic terms, a drain constructed at the upper end of the area to be drained, to intercept surface or ground water flowing toward the protected area from higher ground, and carry it away from the area. Also called an Intercepting Drain.
Drainage Area
In hydrologic terms, an area having a common outlet for its surface runoff (also see Watershed and Catchment Area).
Drainage Basin
In hydrologic terms, a part of the surface of the earth that is occupied by a drainage system, which consists of a surface stream or a body of impounded surface water together with all tributary surface streams and bodies of impounded surface water.
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".
Entrainment Zone
A shallow region at the top of a convective boundary layer where fluid is entrained into the growing boundary layer from the overlying fluid by the collapse of rising convective plumes or bubbles.
Excess Rain
In hydrologic terms, effective rainfall in excess of infiltration capacity.
Excessive Rainfall Outlook (ERO)
A graphical product in which the Weather Prediction Center (WPC) forecasts the probability that rainfall will exceed flash flood guidance (FFG) within 40 kilometers (25 miles) of a point.
Freezing Rain
Rain that falls as a liquid but freezes into glaze upon contact with the ground.
Freezing Rain Advisory
Issued when freezing rain or freezing drizzle is forecast but a significant accumulation is not expected. However, even small amounts of freezing rain or freezing drizzle may cause significant travel problems.
French Drain
In hydrologic terms, an underground passageway for water through the interstices among stones placed loosely in a trench
HDRAIN
An Hourly Digital Rainfall Product of the WSR-88D.
Height Above the Nearest Drainage (HAND)
A relative elevation methodology used in inundation mapping, which determines the height of every point on a land surface above the nearest stream reach to which it drains.
Net Rainfall
In hydrologic terms, the portion of rainfall which reaches a stream channel or the concentration point as direct surface flow.
Rain
Precipitation that falls to earth in drops more than 0.5 mm in diameter.
Rain Foot
Slang for a horizontal bulging near the surface in a precipitation shaft, forming a foot-shaped prominence. It is a visual indication of a wet microburst.
Rain Forest
A forest which grows in a region of heavy annual precipitation. There are two major types, tropical and temperate.
Rain Gauge
An instrument for measuring the quantity of rain that has fallen.
Rain Induced Fog
When warm rain falls through cooler air, water evaporates from the warm rain. It subsequently condenses in the cool air forming fog. Such fog can be quite dense. It generally will persist as long as the rain continues. Since temperature rises little during the day, there is little diurnal variation in rain induced fog. Improvement in visibility cannot be expected until the rain stops or moves out of the affected area.
Rain Shadow
An area of reduced precipitation on the lee side of a mountain barrier caused by warming of air and dissipation of cloudiness as air descends the barrier.
Rain Shield
In a hurricane, a solid or nearly solid area of rain that typically becomes heavier as one approaches the eye. The outer edge is well defined and its distance from the eye varies greatly from storm to storm. The wind, both sustained and peak gusts, keeps increasing as much as one moves through the rain shield toward the storm's eye.
Rain-free Base
A dark, horizontal cloud base with no visible precipitation beneath it. It typically marks the location of the thunderstorm updraft. Tornadoes may develop from wall clouds attached to the rain-free base, or from the rain-free base itself - especially when the rain-free base is on the south or southwest side of the main precipitation area. Note that the rain-free base may not actually be rain free; hail or large rain drops may be falling. For this reason, updraft base is more accurate.
Rainbow
A luminous arc featuring all colors of the visible light spectrum (red, orange, yellow, green, blue, indigo, and violet). It is created by refraction, total reflection, and the dispersion of light. It is visible when the sun is shining through air containing water spray or raindrops, which occurs during or immediately after a rain shower. The bow is always observed in the opposite side of the sky from the sun.
Rainfall
The amount of precipitation of any type, primarily liquid. It is usually the amount that is measured by a rain gauge. Refer to rain for rates of intensity and the quantitative precipitation for forecasting.
Rainfall Estimates
A series of NEXRAD products that employ a Z-R relationship to produce accumulations of surface rainfall from observed reflectivity.
Snow Grains
Precipitation consisting of white, opaque ice particles usually less than 1 mm in diameter.
Subtle Heavy Rainfall Signature
This heavy rain signature is often difficult to detect on satellite. These warm top thunderstorms are often embedded in a synoptic-scale cyclonic circulation. Normally, they occur when the 500 mb cyclonic circulation is quasi-stationary or moves slowly to the east or northeast (about 2 degrees per 12 hours). The average surface temperature is 68ºF with northeasterly winds. The average precipitable water (P) value is equal to or greater than 1.34 inches and the winds veer with height, but they are relatively light. The heavy rain often occurs north and east of the vorticity maximum across the lower portion of the comma head about 2 to 3 degrees north or northeast of the 850 mb low.
Terrain Forced Flow
An airflow that is modified or channeled as it passes over or around mountains or through gaps in a mountain barrier.
Three-Hour Rainfall Rate
This WSR-88D Radar product displays precipitation total (in inches) of the current and past two clock hours as a graphical image. It displays hourly precipitation total (in inches) as a graphical image (polar format with resolution 1.1 nm by 1 degree). It is updated once an hour. It is used to:
1) Assess rainfall intensities and amounts over a longer viewing interval; and
2) Possibly adjust flash flood guidance values since the product corresponds to the timing of Flash Flood Guidance values.
Tipping-Bucket Rain Gage
A precipitation gage where collected water is funneled into a two compartment bucket; 0.01, 0.1 mm, or some other designed quantity of rain will fill one compartment and overbalance the bucket so that it tips, emptying into a reservoir and moving the second compartment into place beneath the funnel. As the bucket is tipped, it actuates an electric circuit.
Training
Repeated areas of rain, typically associated with thunderstorms, that move over the same region in a relatively short period of time and are capable of producing excessive rainfall totals. Train(ing) echoes can frequently be a source of flash flooding.

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