Introduction
The Hype
The Impacts
Interactive Timeline
La Niña
Prediction Methods
The Preparation
About the Site
Archives
Search
Learn Anything?
Forum
______________________
Glossary
A-M | N-Z
N
NATIONAL CENTER FOR ATMOSPHERIC RESEARCH (NCAR)
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.
NATIONAL CENTERS FOR ENVIRONMENTAL PREDICTION (NCEP)
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 Aviation Weather Center, the Climate Prediction Center, the Storm Prediction Center,
and the Tropical Prediction Center. Formerly known as NMC. For further information,
contact the NCEP, with central offices located in Silver Spring, Maryland.
NATIONAL CLIMATIC DATA CENTER (NCDC)
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.
NATIONAL HURRICANE CENTER (NHC)
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.
NATIONAL METEOROLOGICAL CENTER (NMC)
Now incorporated into the National Centers for Environmental Prediction, it was the
division of the National Weather Service that produced, processed, handled, and
distributed meteorological and oceanographic information to users throughout the Northern
Hemisphere, specifically U.S. governmental organizations.
NATIONAL OCEANIC AND ATMOSPHERIC ADMINISTRATION (NOAA)
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.
NATIONAL SEVERE STORMS FORECAST CENTER (NSSFC)
As of October 1995, the responsibilities of this Center were divided into two branches,
the Storm Prediction Center and the Aviation Weather Center.
NATIONAL SEVERE STORMS LABORATORY (NSSL)
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.
NATIONAL WEATHER SERVICE (NWS)
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. For further information,
contact the NWS.
O
OCCLUDED FRONT
Also known as an occlusion, it is a complex front formed when a cold front overtakes a
warm front. It develops when three thermally different air masses conflict. The type of
frontal boundary they create depends on the manner in which they meet. See cold front and
warm front.
OCEAN
The intercommunicating body of salt water occupying the depressions of the earth's
surface, or one of its major primary subdivisions, bounded by the continents, or the
equator, and other imaginary lines. A sea is subdivision of an ocean.
OCEANOGRAPHY
The study of the ocean, embracing and integrating all knowledge pertaining to the ocean's
physical boundaries, the chemistry and physics of sea water, and marine biology.
OVERCAST
The amount of sky cover for a cloud layer that is 8/8ths, based on the summation layer
amount for that layer.
P
PRECIPITATION
Any and all forms of water, liquid or solid, that falls from clouds and reaches the
ground. This includes drizzle, freezing drizzle, freezing rain, hail, ice crystals, ice
pellets, rain, snow, snow pellets, and snow grains. The amount of fall is usually
expressed in inches of liquid water depth of the substance that has fallen at a given
point over a specified time period.
PRESSURE
The force per unit area exerted by the weight of the atmosphere above a point on or above
the earth's surface. Also known as atmospheric pressure or barometric pressure.
R
RADAR
Acronym for RAdio Detection And Ranging. An electronic instrument used to detect distant
objects and measure their range by how they scatter or reflect radio energy. Precipitation
and clouds are detected by measuring the strength of the electromagnetic signal reflected
back. Doppler radar and NEXRAD are examples.
RAIN
Precipitation in the form of liquid water droplets greater than 0.5 mm. If widely
scattered, the drop size may be smaller. It is reported as "R" in an observation
and on the METAR. The intensity of rain is based on rate of fall. "Very light"
(R--) means that the scattered drops do not completely wet a surface. "Light"
(R-) means it is greater than a trace and up to 0.10 inch an hour. "Moderate"
(R) means the rate of fall is between 0.11 to 0.30 inch per hour. "Heavy" (R+)
means over 0.30 inch per hour.
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.
ROSSBY WAVES
The movement of ridges and troughs in the upper wind patterns, primarily the jet stream,
circling the earth. Named for Carl-Gustaf Rossby, a U.S. Weather Bureau (NWS) employee,
who first theorized about the existence of the jet stream in 1939.
S
SATELLITE
Any object that orbits a celestial body, such as a moon. However, the term is often used
in reference to the manufactured objects that orbit the earth, either in a geostationary
or a polar manner. Some of the information that is gathered by weather satellites, such as
GOES9, includes upper air temperatures and humidity, recording the temperatures of cloud
tops, land, and ocean, monitoring the movement of clouds to determine upper level wind
speeds, tracing the movement of water vapor, monitoring the sun and solar activity, and
relaying data from weather instruments around the world.
SEA SURFACE TEMPERATURE (SST)
The temperature of the water's surface. It is measured using buoy and ship data, infrared
satellite imagery, and coastal observations.
SKY
The vault-like apparent surface against which all aerial objects are seen from the earth.
SKY COVER
The amount of the celestial dome that is hidden by clouds and/or obscurations.
STORM
An individual low pressure disturbance, complete with winds, clouds,and precipitation.
Examples include thunderstorms, tornadoes, or even tropical cyclones. The name is
associated with destructive or unpleasant weather. Storm-scale refers to disturbances the
size of individual thunderstorms.
STRATIFORM
Clouds composed of water droplets that exhibit no or have very little vertical
development. The density of the droplets often blocks sunlight, casting shadows on the
earth's surface. Bases of these clouds are generally no more than 6,000 feet above the
ground. They are classified as low clouds, and include all varieties of stratus and
stratocumulus. The opposite in type are the vertical development of cumuliform clouds.
STRATOCUMULUS
A low cloud composed of layers or patches of cloud elements. It can form from cumulus
clouds becoming more stratiformed and often appears as regularly arranged elements that
may be tessellated, rounded, or roll-shaped with relatively flat tops and bases. It is
light or dark gray in color, depending on the size of the water droplets and the amount of
sunlight that is passing through them.
STRATUS
One of the three basic cloud forms (the others are cirrus and cumulus). It is also one of
the two low cloud types. It is a sheetlike cloud that does not exhibit individual
elements, and is, perhaps, the most common of all low clouds. Thick and gray, it is seen
in low, uniform layers and rarely extends higher than 5,000 feet above the earth's
surface. A veil of stratus may give the sky a hazy appearance. Fog may form from a stratus
cloud that touches the ground. Although it can produce drizzle or snow, it rarely produces
heavy precipitation. Clouds producing heavy precipitation may exist above a layer of
stratus.
U
UPWELLING
The process by which water rises from a lower to a higher depth, usually as a result of
divergence and offshore currents. It influences climate by bringing colder, more
nutrient-rich water to the surface. A vital factor of the El Nino event.
V
VISIBILITY
A measure of the opacity of the atmosphere, and therefore, the greatest distance one can
see prominent objects with normal eyesight. The National Weather Service has various terms
for visibility. Surface visibility is the prevailing visibility determined from the usual
point of observation. Prevailing visibility is considered representative of visibility
conditions at the station. Sector visibility is the visibility in a specified direction
that represents at least a 45 degree arc of the horizon circle. Tower visibility is the
prevailing visibility determined from the airport traffic control tower (ATCT) at stations
that also report surface visibility.
W
WALKER CIRCULATION
The large-scale vertical overturning of air in the equatorial and tropical portions of the
world. It always refers to the east-west, longitudinal or zonal overturning of air. There
may be as many as 4 or 5 Walker circulations present at any given time across the global
tropics, but usually there are three, typically observed over Southeast Asia, Africa and
Central/South America. Sinking branches of Walker circulations are typically observed over
the tropical central Atlantic, Pacific and Indian Oceans. Latitudinal positions of Walker
circulations shift seasonally and are nearly always centered in the summer hemisphere. In
winter Walker circulations are located between 5N and 15S. In summer they are located
between 8N and 15N.
WARM FRONT
The leading edge of an advancing warm air mass that is replacing a retreating relatively
colder air mass. Generally, with the passage of a warm front, the temperature and humidity
increase, the pressure rises, and although the wind shifts (usually from the southwest to
the northwest in the Northern Hemisphere), it is not as pronounced as with a cold frontal
passage. Precipitation, in the form of rain, snow, or drizzle, is generally found ahead of
the surface front, as well as convective showers and thunderstorms. Fog is common in the
cold air ahead of the front. Although clearing usually occurs after passage, some
conditions may produced fog in the warm air. See occluded front and cold front.
WEATHER
The state of the atmosphere at a specific time and with respect to its effect on life and
human activities. It is the short term variations of the atmosphere, as opposed to the
long term, or climatic, changes. It is often referred to in terms of brightness,
cloudiness, humidity, precipitation, temperature, visibility, and wind.
WESTERLIES
Usually applied to the broad patterns of persistent winds with a westerly component. It is
the dominant persistent atmospheric motion, centered over the midlatitudes of each
hemisphere. Near the earth's surface, the westerlies extend from approximately 35 to 65
degrees latitude, while in the upper levels they extend further polarward and equatorward.
WIND
Air that flows in relation to the earth's surface, generally horizontally. There are four
areas of wind that are measured: direction, speed, character (gusts and squalls), and
shifts. Surface winds are measured by wind vanes and anemometers, while upper level winds
are detected through pilot balloons, rawin, or aircraft reports.