The point on any given observer's celestial sphere diametrically opposite of one's zenith.

A division of the University Corporation for Atmospheric Research, the Center plans, organizes, and conducts atmospheric and related research programs in collaboration with universities. For further information, contact NCAR, located in Boulder, Colorado.

As part of the National Weather Service, the centers provide timely, accurate, and continually improving worldwide forecast guidance products. Some of the centers include the Climate Prediction Center, the Storm Prediction Center, and the Tropical Prediction Center. For further information, contact the NCEP.

The agency that archives climatic data from the National Oceanic and Atmospheric Administration, as well as other climatological organizations. For further information, contact the NCDC, located in Asheville, North Carolina.

A branch of the Tropical Prediction Center, it is the office of the National Weather Service that is responsible for tracking and forecasting tropical cyclones over the North Atlantic, Caribbean, Gulf of Mexico, and the Eastern Pacific. For further information, contact the NHC, located in Miami, Florida.

The division of the National Weather Service that produces, processes, handles, and distributes meteorological and oceanographic information to users throughout the Northern Hemisphere, specifically U.S. governmental organizations. For further information, contact the NMC, located in Camp Spring, Maryland.

A branch of the U.S. Department of Commerce, it is the parent organization of the National Weather Service. It promotes global environmental stewardship, emphasizing atmospheric and marine resources. For further information, contact the NOAA, located in Silver Spring, Maryland.

The division of the National Weather Service that maintains a watch for thunderstorm activity on the lower forty-eight states. They are responsible for preparing thunderstorm outlooks and issuing tornado and severe thunderstorm watches. The Center is located in Kansas City, Missouri.

A branch of the National Oceanic and Atmospheric Administration, it provides accurate and timely forecasts and warnings of hazardous weather events, especially flash floods, hail, lightning, tornadoes, and other severe wind storms. For further information, contact the NSSL, headquartered in Norman, Oklahoma.

A primary branch of the National Oceanic and Atmospheric Administration, it is responsible for all aspects of observing and forecasting atmospheric conditions and their consequences, including severe weather and flood warnings.

A unit of length used in marine navigation that is equal to a minute of arc of a great circle on a sphere. One international nautical mile is equivalent to 1,852 meters or 1.151 statue miles. Refer to a sea mile.

The time after civil twilight, when the brighter stars used for celestial navigation have appeared and the horizon may still be seen. It ends when the center of the sun is 12 degrees below the horizon, and it is too difficult to perceive the horizon, preventing accurate sighting of stars. See twilight.

A tide of decreased range, which occurs about every two weeks when the moon is at one quarter or three-quarters full. Compare with a spring tide.

Occurs when the rotation of the atmosphere advects lower values of vorticity into an area. This is created by divergence aloft. Contrast with positive vorticity advection.

The local name for winds blowing downward from desert uplands through the Newhall Pass southward into the San Fernando Valley, north of Los Angeles.

The unit of force giving a mass of about one kilogram (2.205 pounds) an acceleration of about one meter (1 yard) per second per second.

Acronym for NEXt Generation Weather RADar. A network of advanced Doppler radars implemented in the United States between 1992 and 1996, it detects the location and intensity of precipitation out to a range of 143 miles from the radar site. NEXRAD Doppler radar is highly sensitive and can detect precipitation from very light rain and snow up to the strongest thunderstorms with accuracy and detail. Sometimes, however, the radar's extreme sensitivity will cause ground clutter and other non-precipitation echoes to be displayed in the vicinity of the radar site.

The period of the day between dusk and dawn.

This cloud exhibits a combination of rain or snow, and sometimes the base of the cloud cannot be seen because of the heaviness of precipitation. They are generally associated with fall and winter conditions, but can occur during any season.

A colorless, tasteless, ordorless gas that is the most abundant constituent of dry air. It comprises 78.09%.

Rarely seen clouds of tiny ice particles that form approximately 75 to 90 kilometers above the earth's surface. They have been seen only during twilight (dusk and dawn) during the summer months in the higher latitudes. They may appear bright against a dark night sky, with a blue-silver color or orange-red.

Thunderstorms which develop after sunset. They are often associated with the strengthening of the low level jet and are most common over the Plains states. They also occur over warm water and may be associated with the seaward extent of the overnight land breeze.

A cyclonic storm occurring off the east coast of North America. These winter weather events are notorious for producing heavy snow, rain, and tremendous waves that crash onto Atlantic beaches, often causing beach erosion and structural damage. Wind gusts associated with these storms can exceed hurricane force in intensity. A nor'easter gets its name from the continuously strong northeasterly winds blowing in from the ocean ahead of the storm and over the coastal areas.

The recognized standard value of a meteorological element as it has been averaged in a given location over a fixed number of years. Normals are concerned with the distribution of data within limits of common occurrence. The parameters may include temperatures (high, low, and deviation), pressure, precipitation (rain, snow, etc.), winds (speed and direction), thunderstorms, amount of clouds, percent relative humidity, etc.

A semi-permanent, subtropical area of high pressure in the North Pacific Ocean. It is strongest in the Northern Hemispheric summer and is displaced towards the equator during the winter when the Aleutian Low becomes more dominate. Comparable systems are the Azores High and the Bermuda High.

A short-term weather forecast for expected conditions in the next few hours.

The use of numerical models, such as the fundamental equations of hydrodynamics subjected to observed initial conditions, to forecast the weather. These models are run on high-speed computers at the National Meteorological Center.