Report Title: Major and Catastrophic Storms and Floods in Texas  
Report Guide
Glossary of Terms
Introductory Materials
Substantial flood peaks
Links to related web resouces
Assorted documents related to Texas storms
Dedication and Credits
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  Glossary of Hydrologic and Meteorologic Terms  

Cubic feet per second (ft3/s, cfs) - The rate of discharge representing a volume of 1 cubic foot passing a given point during 1 second and equivalent to 7.48 gallons per second or 448.8 gallons per minute.

Discharge - The volume of water that passes a given point within a given period of time.

Drainage area - The area, measured in a horizontal plane, enclosed by a topographic divide from which direct surface runoff from precipitation normally drains by gravity into the stream upstream from the specified point.

Drainage basin - The part of the surface of the Earth that is occupied by a drainage system with a common outlet for its surface runoff, consisting of a surface stream or a body of impounded water with all tributary surface streams and bodies of impounded water.

Feeder bands - In tropical systems, feeder bands are the spiral bands of showers and thunderstorms that follow the outer bands and precede the center of the storm. They are often the location of severe weather and can produce copious rains. Circulation around tropical systems can bring the feeder bands over the same areas repeatedly, contributing to high rainfall totals.

Gage height - The water-surface elevation referred to some arbitrary datum. The gage height added to the elevation of the datum of the gage represents the water-surface elevation. For example, the elevation of the datum of the gage might be 100.00 feet, which, when added to a gage height of 12.50 feet, represents a water-surface elevation of 112.50 feet.

Isohyetal - Line of equal precipitation.

Low-level jet - A relatively fast-moving (20 to 60 miles per hour) layer of air that forms 1,000 to 3,000 feet above the surface. The low-level jet is a summertime nocturnal event, forming above the nighttime inversion as the air near the surface cools bringing low-level stratus clouds (late night and morning low clouds) to south-central Texas. It is also associated with flow off the Gulf of Mexico during spring through fall, bringing moisture rapidly back into Texas and the Plains as high pressure systems move eastward behind departing cold fronts. This low-level jet provides much of the moisture needed for thunderstorm development in this area.

Precipitable water (PW) - The total atmospheric water vapor contained in a vertical column extending between any two specified levels, generally from the ground to the top of the upper-air sounding, expressed in terms of the height to which the water would stand if completely condensed and collected in a vessel of equal cross section as the column. Two inches is a very moist, tropical atmosphere capable of producing copious amounts of rain. In central Texas 2-inch PWs are routinely seen only in association with inland tropical activity.

Runoff - The part of the precipitation that appears in surface streams.

Runoff in inches - The depth to which the drainage area would be covered if all the runoff for a given period were uniformly distributed on it.

Steering winds - The flow exerting influence over the movement of a disturbance, such as a thunderstorm. Steering winds for thunderstorms typically extend from 10,000 to 20,000 feet.

Streamflow - The discharge that occurs in a natural channel.

Tropical/barotropic - Barotropic describes the condition of the atmosphere when lines of constant temperature are parallel to lines of constant pressure through the depth of the atmosphere. True barotropic conditions are rarely achieved but come closest in tropical weather systems. Wind shear, the change of wind speed and (or) direction with height, is weak in barotropic systems making them conducive to the production of heavy rain.

Tropopause - The boundary between the troposphere and the stratosphere. "Weather," as we know it, occurs within the troposphere, that part of the atmosphere closest to the Earth's surface (6.2 to 12.4 miles deep). Temperature decreases with height within the troposphere. The tropopause is marked by an abrupt change of lapse rate (the change of temperature with height).

Upper-level diffluence - Diffluence is the rate at which adjacent flow is diverging along an axis normal to the flow at the point in question. Upper-level diffluence (between 15,000 and 30,000 feet), a spreading out of the air flow, places lower-speed winds in the region critical to thunderstorm movement. This slow movement leads to higher rainfall totals as storms remain over a location longer and contributes to storms developing and moving continually over the same areas.

Watershed - The divide separating one drainage basin from another. However, over the years, the term has evolved to represent the drainage basin.