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
Runoff - The part of the precipitation that appears in surface
Runoff in inches - The depth to which the drainage area would
be covered if all the runoff for a given period were uniformly distributed
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