The following is a list of definitions of commonly used Weather Terms. For a more comprehensive list check out www.weather.com/glossary.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
ABSOLUTE HUMIDITY
A type of humidity that considers the mass of water vapor present per unit
volume of space. Also considered as the density of the water vapor. It
is usually expressed in grams per cubic meter.
AIR
This is considered the mixture of gases that make up the earth's atmosphere.
The principal gases that compose dry air are Nitrogen (N2) at 78.09%, Oxygen
(O2) at 20.946%, Argon (A) at 0.93%, and Carbon Dioxide (CO2) at 0.033%.
One of the most important constituents of air and most important gases
in meteorology is water vapor (H2O).
AIR MASS
An extensive body of air throughout which the horizontal temperature and moisture
characteristics are similar.
AIR POLLUTION
The soiling of the atmosphere by contaminants to the point that may cause injury
to health, property, plant, or animal life, or prevent the use and enjoyment
of the outdoors.
ALTIMETER
An instrument used to determine the altitude of an object with respect to a
fixed level. The type normally used by meteorologists measures the altitude
with respect to sea level pressure.
ALTITUDE
In meteorology, the measure of a height of an airborne object in respect to
a constant pressure surface or above mean sea level.
ANEMOMETER
An instrument which measures wind speed.
ANTARCTIC
Of or relating to the area around the geographic South Pole, from 90 degrees
South to the Antarctic Circle at approximately 66 1/2 degrees South latitude,
including the continent of Antarctica. Along the Antarctic Circle, the
sun does not set on the day of the summer solstice (approximately December
21st) and does not rise on the day of the winter solstice (approximately
June 21st).
ANTARCTIC
OCEAN
Although not officially recognized as a separate ocean body, it is commonly
applied to those portions of the Atlantic, Pacific, and Indian Oceans that
reach the Antarctic continent on their southern extremes.
ARCTIC
Of or relating to the area around the geographic North Pole, from 90 degrees
North to the Arctic Circle at approximately 66 1/2 degrees North latitude.
ARID
A term used for an extemely dry climate. The degree to which a climate lacks
effective, life-promoting moisture. It is considered the opposite of humid
when speaking of climates.
AURORA
It is created by the radiant energy emission from the sun and its interaction
with the earth's upper atmosphere over the middle and high latitudes. It
is seen as a bright display of constantly changing light near the magnetic
poles of each hemisphere. In the Northern Hemisphere, it is known as the
aurora borealis or Northern Lights, and in the Southern Hemisphere, this
phenomena is called the aurora australis.
AUTUMN
The season of the year which occurs as the sun approaches the winter solstice,
and characterized by decreasing temperatures in the mid-latitudes. Customarily,
this refers to the months of September, October, and November in the North
Hemisphere and the months of March, April, and May in the Southern Hemisphere.
Astronomically, this is the period between the autumnal equinox and the
winter solstice.
BALL LIGHTNING
A relatively rare form of lightning consisting of a luminous ball, often reddish
in color, which moves rapidly along solid objects or remains floating in
mid-air. Also known as globe lightning.
BAROGRAPH
An instrument that continuously records a barometer's reading of atmospheric
pressure. For an example, see aneroid barometer.
BAROMETER
An instrument used to measure atmospheric pressure. Two examples are the aneroid
barometer and the mercurial barometer.
BAROMETRIC
PRESSURE
The pressure exerted by the atmosphere at a given point. Its measurement can
be expressed in several ways. One is in millibars. Another is in inches or
millimeters of mercury (Hg). Also known as atmospheric pressure.
BEAUFORT WIND
SCALE
A system of estimating and reporting wind speeds. It is based on the Beaufort
Force or Number, which is composed of the wind speed, a descriptive term, and
the visible effects upon land objects and/or sea surfaces. The scale was devised
by Sir Francis Beaufort (1777-1857), hydrographer to the British Royal Navy.
BLACK ICE
Thin, new ice on fresh or salt water that appears dark in color because of
its transparency. Also refers to thin, transparent ice on road surfaces.
BLIZZARD
A severe weather condition characterized by low temperatures, winds 35 mph
or greater, and sufficient falling and/or blowing snow in the air to frequently
reduce visibility to1/4 mile or less for a duration of at least 3 hours.
A severe blizzard is characterized by temperatures near or below 10 degrees
Fahrenheit, winds exceeding 45 mph, and visibility reduced by snow to near
zero.
CELSIUS TEMPERATURE
SCALE
A temperature scale where water at sea level has a freezing point
of 0 degrees C (Celsius) and a boiling point of +100 degrees
C. More commonly used in areas that observe the metric system
of measurement. Created by Anders Celsius in 1742. Same as Centigrade.
In 1948, the Ninth General Conference on Weights and Measures
replaced "degree centigrade" with "degree Celsius."
CHINOOK
A type of foehn wind. Refers to the warm downslope wind in the Rocky Mountains
that may occur after an intense cold spell when the temperature could rise
by 20 to 40 degrees Fahrenheit in a matter of minutes. Also known as the
Snow Eater.
CLEAR ICE
A glossy, clear, or translucent ice formed by the relatively slow freezing
of large supercooled in water droplets. The droplets spread out over an
object, such as an aircraft wing's leading edge, prior to complete freezing
and forms a sheet of clear ice. Often synonymous with glaze.
CLIMATE
The historical record and description of average daily and in seasonal weather
events that help describe a region. Statistics are generally drawn over
several decades. The word is derived from the Greek klima, meaning inclination,
and reflects the importance early scholars attributed to the sun's influence.
CLOUD
A visible collection of minute particle matter, such as water droplets and/or
ice crystals, in the free air. A cloud forms in the atmosphere as a result
of condensation of water vapor. Condensation nuclei, such as in smoke or
dust particles, form a surface upon which water vapor can condense.
COLD FRONT
The leading edge of an advancing cold air mass that is underrunning and displacing
the warmer air in its path. Generally, with the passage of a cold front,
the temperature and humidity decrease, the pressure rises, and the wind
shifts (usually from the southwest to the northwest in the Northern Hemisphere).
Precipitation is generally at and/or behind the front, and with a fast-moving
system, a squall line may develop ahead of the front. See occluded front
and warm front.
CONDENSATION
The process by which water vapor undergoes a change in state from a gas to
a liquid. It is the opposite physical process of evaporation.
CRYSTALIZATION
The process of a substance going directly from a vapor form (water vapor) to
a solid (ice) at the same temperature, without going through the liquid
phase (water). The opposite of sublimation.
CURRENT
A horizontal movement of water, such as the Gulf Stream off the east coast
of North America, or air, such as the jet stream.
CYCLONE
An area of closed pressure circulation with rotating and converging winds,
the center of which is a relative pressure minimum. The circulation is
counterclockwise in the Northern Hemisphere and clockwise in the Southern
Hemisphere. Also called a low pressure system and the term used for a tropical
cyclone in the Indian Ocean. Other phenomena with cyclonic flow may be
referred to by this term, such as dust devils, tornadoes, and tropical
and extratropical systems. The opposite of an anticyclone or a high pressure
system.
DAWN
The first appearance of light in the eastern sky before sunrise. It marks the
beginning of morning twilight. The visual display is created by the scattering
of light reaching the upper atmosphere prior to the sun's rise to the observer's
horizon. Also known as daybreak.
DAY
Considered a basic unit of time as defined by the earth's motion. It represents
the time needed for one complete revolution of the earth about its own
axis. Also know as a sidereal day, it is approximately equal to 23 hours,
56 minutes, and 4.09 seconds. See night.
DEGREE
A measure of temperature difference representing a single division on a temperature
scale. See Celsius, Fahrentheit, and Kelvin scales.
DENSITY ALTITUDE
Density altitude is a measure used primarily by pilots, high-performance engine
mechanics and long-range shooters. The density altitude is a measure of
the density of the air, given in units of distance. It is a function of
temperature, relative humidity and air pressure.
DEW
Condensation in the form of small water drops that forms on grass and other
small objects near the ground when the temperature has fallen to the dew
point, generally during the nighttime hours.
DEW POINT
The dew point is the temperature at which dew would form assuming all other
conditions remained the same. The dew point is a function of the air temperature
and humidy. The dew point temperature can never be higher than the air
temperature. If the dew point temp and air temp are the same, than the
humidity must be 100%.
Okay, that's fine, but what does it really mean? The dew point is a very good
measure of comfort. If the dew point is high, the temperature and humidity
must also be high, and you are probably sweating profusely even while standing
still. If the dew point is low, then either the temperature or humidity or
both are very low, and you are feeling quite comfortable. This is a better
guage for comfort than temperature or humidity alone. It could be quite warm
but very dry (low dew point) and you would feel comfortable. It could also
be very humid but cool or cold (low dew point) and you would feel comfortable.
The temperature to which air must be cooled at a constant pressure to become
saturated.
DOG DAYS
The name given to the very hot summer weather that may persists for four to
six weeks between mid-July through early September in the United States.
In western Europe, this period may exist from the first week in July to
mid- August and is often the period of the greatest frequency of thunder.
Named for Sirius, the Dog Star, which lies in conjunction with the sun
during this period, it was once believed to intensify the sun's heat during
the summer months.
DOLDRUMS
A nautical term for the equatorial region of light winds between the trade
winds of the two hemispheres.
DROUGHT
Abnormal dry weather for a specific area that is sufficiently prolonged for
the lack of water to cause serious hydrological imbalance.
DRY BULB THERMOMETER
A thermometer used to measure the ambient temperature. The temperature recorded
is considered identical to air temperaure. One of the two therometers that
make up a psychrometer.
DUSK
The period of waning light from the time of sunset to dark. See twilight and
dawn.
EARTHQUAKE
A sudden, transient motion or trembling of the earth's crust, resulting from
the waves in the earth caused by faulting of the rocks or by volcanic activity.
ECLIPSE
The obscuring of one celestial body by another. See lunar eclipse or solar
eclipse.
EL NINO
The cyclical warming of East Pacific Ocean sea water temperatures off the western
coast of South America that can result in significant changes in weather
patterns in the United States and elsewhere. This occurs when warm equatorial
waters move in and displace the colder waters of the Humbolt Current, cutting
off the upwelling process.
EQUINOX
The point at which the ecliptic intersects the celestial equator. Days and
nights are most nearly equal in duration. In the Northern Hemisphere, the
vernal equinox falls on or about March 20 and the autumnal equinox on or
about September 22.
EVAPORATION
The physical process by which a liquid, such as water, is tranformed into a
gaseous state, such as water vapor. It is the opposite physical process
of condensation.
EYE
The center of a tropical storm or hurricane, characterized by a roughly circular
area of light winds and rain-free skies. An eye will usually develop when
the maximum sustained wind speeds exceed 78 mph. It can range in size from
as small as 5 miles to up to 60 miles, but the average size is 20 miles.
In general, when the eye begins to shrink in size, the storm is intensifying.
FAHRENHEIT
TEMPERATURE SCALE
A temperature scale where water at sea level has a freezing point of +32 degrees
F (Fahrenheit) and a boiling point of +212 degrees F. More commonly used in
areas that observe the English system of measurement. Created in 1714 by Gabriel
Daniel Fahrenheit (1696-1736), a German physicist, who also invented the alcohol
and mercury thermometers.
FLASH FLOOD
A flood that rises and falls quite rapidly with little or no advance warning,
usually as the result of intense rainfall over a relatively small area.
Flash floods can be caused by situations such as a sudden excessive rainfall,
the failure of a dam, or the thaw of an ice jam.
FLOOD
High water flow or an overflow of rivers or streams from their natural or artificial
banks, inundating adjacent low lying areas.
FLOOD PLAIN
Level land that may be submerged by flood waters.
FOG
A visible aggregate of minute water droplets suspended in the
atmosphere at or near the surface of the earth, reducing
horizontal visibility to less than 5/8 statute miles. It
is created when the temperature and the dew point of the
air have become the same, or nearly the same, and sufficient
condensation nuclei are present. It is reported as "FG" in an observation
and on the METAR.
FORECAST
A statement of expected future occurrences. Weather forecasting includes the
use of objective models based on certain atmospheric parameters, along
with the skill and experience of a meteorologist. Also called a prediction.
FREEZING POINT/FREEZE
The process of changing a liquid to a solid. The temperature at which a liquid
solidifies under any given set of conditions. Pure water under atmospheric
pressure freezes at 0 degrees Celsius or 32 degrees Fahrenheit. It is the
opposite of fusion. In oceanography, the freezing point of water is depressed
with increasing salinity.
FROST
The covering of ice crystals that forms by direct sublimation on exposed surfaces
whose temperature is below freezing.
GALE
On the Beaufort Wind Scale, a wind with speeds from 28 to 55 knots (32 to 63
miles per hour). For marine interests, it can be categorized as a moderate
gale (28 to 33 knots), a fresh gale (34 to 40 knots), a strong gale (41
to 47 knots), or a whole gale (48 to 55 knots). In 1964, the World Meteorological
Organization defined the categories as near gale (28 to 33 knots), gale
(34 to 40 knots), strong gale (41 to 47 knots), and storm (48 to 55 knots).
GREENHOUSE
EFFECT
The overall warming of the earth's lower atmosphere primarily due to carbon
dioxide and water vapor which permit the sun's rays to heat the earth, but
then restrict some heat-energy from escaping back into space.
HAIL
Precipitation that originates in convective clouds, such as
cumulonimbus, in the form of balls or irregular pieces
of ice, which comes in different shapes and sizes. Hail
is considered to have a diameter of 5 millimeter or more;
smaller bits of ice are classified as ice pellets, snow
pellets, or graupel. Individual lumps are called hailstones.
It is reported as "GR" in
an observation and on the METAR. Small hail and/or snow pellets is reported
as "GS" in an observation and on the METAR.
HEAT
A form of energy transferred between two systems by virtue of a difference
in temperature. The first law of thermodynamics demonstrated that the heat
absorbed by a system may be used by the system to do work or to raise its
internal energy.
HEAT EXHAUSTION
The effect of excessive heat, particularly when combined with high humidity,
on a human being. Signs of heat exhaustion include a general weakness,
heavy sweating and clammy skin, dizziness and/or fainting, and muscle cramps.
HEAT INDEX
The combination of air temperature and humidity that gives a description of
how the temperature feels. This is not the actual air temperature. For
an example, check out the heat index chart.
HEAT LIGHTNING
Lightning that appears as a glowing flash on the horizon. It is actually lightning
occurring in distant thunderstorms, just over the horizon and too far away
for thunder to be heard.
HEAT STROKE
Introduced to the body by overexposure to high temperatures, particularly when
accompanied by high humidity. The signs of heat stroke include when an
individual's body temperature is greater than 105 degrees Fahrenheit, the
skin is hot and dry, there is a rapid and irregular pulse, perspiration
has stopped, and one has lost consciousness. Seek imsfimageste medical
aid. May be called a sun-stroke when caused by direct exposure to the sun.
HEAT WAVE
A period of abnormally and uncomfortably hot weather. It could last from several
days to several weeks. The Weather Channel uses the following criteria
for a heat wave: a minimum of ten states must have 90 degree plus temperatures
and the temperatures must be at least five degrees above normal in parts
of that area for at least two days or more.
HORSE LATITIDES
Located between 30 degrees North and South in the vicinity of the equator,
this area typically has calm or light and variable winds. Another name
for the equatorial trough, the Intertropical Convergence Zone (ITCZ), or
the doldrums.
HUMIDITY
The amount of water vapor in the air. It is often confused with relative humidity
or dew point. Types of humidity include absolute humidity, relative humidity,and
specific humidity.
HURRICANE
The name for a tropical cyclone with sustained winds of 74 miles per hour (65
knots) or greater in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico,
and in the eastern North Pacific Ocean. This same tropical cyclone is known
as a typhoon in the western Pacific and a cyclone in the Indian Ocean.
HYDROMETEOR
Any any form of atmospheric water vapor, including those blown by the wind
off the earth's surface. Liquid or solid water formation that is suspended
in the air includes clouds, fog, ice fog, and mist. Drizzle and rain are
example of liquid precipitation, while freezing drizzle and freezing rain
are examples of freezing precipitation. Solid or frozen precipitation includes
ice pellets, hail, snow, snow pellets, snow grains, and ice crystals. Water
vapor that evaporates before reaching the ground is virga. Examples of
liquid or solid water particles that are lifted off the earth's surface
by the wind includes drifting and blowing snow and blowing spray. Dew,
frost, rime, and glaze are examples of liquid or solid water deposits on
exposed objects.
HYGROMETER
An instrument that measures the water vapor content of the atmosphere. See
the psychrometer as an example.
HYPOTHERMIA
Occurs when the core temperature of one's body falls below normal. It is the
failure of the body to maintain adequate production of heat under conditions
of extreme cold.
ICE The solid form of water. It can be found in the atmosphere in the form of ice crystals, snow, ice pellets, and hail, for example.
ICE CRYSTALS
Precipitation in the form of slowly falling, singular or unbranched
ice needles, columns, or plates. They make up cirriform clouds,
frost, and ice fog. Also, they produce optical phenomena
such as halos, coronas, and sun pillars. May be called "diamond dust". It is reported as "IC" in
an observation and on the METAR.
ICE JAM
An accumulation of broken river ice caught in a narrow channel, frequently
producing local flooding. Primarily occurs during a thaw in the late winter
or early spring.
ICE STORM
A severe weather condition characterized by falling freezing precipitation.
Such a storm forms a glaze on objects, creating hazardous travel conditions
and utility problems.
ICICLE
Ice that forms in the shape of a narrow cone hanging point down. It usually
forms when liquid water from a sheltered or heated source comes in contact
with below-freezing air and freezes more or less rapidly as it flows.
ICING
The forming or depositing of ice on an object. See glaze.
INCHES OF
MERCURY (Hg)
The name comes from the use of mercurial barometers which equate the height
of a column of mercury with air pressure. One inch of mercury is equivalent
to 33.86 millibars or 25.40 millimeters. See barometric pressure. First divised
in 1644 by Evangelista Torricelli (1608-1647), an Italian physicist and mathematician,
to explain the fundamental principles of hydromechanics.
INDIAN SUMMER
A period of abnormally warm weather in mid to late autumn with clear skies
and cool nights. A first frost normally precedes this warm spell.
JET STREAK
A region of accelerated wind speed along the axis of a jet stream.
JET STREAM
A narrow band of strong winds usually found at elevations from 20000 to 50000
feet.
KNOT
A nautical unit of speed equal to the velocity at which one nautical mile is
traveled in one hour. Used primarily by marine interests and in weather
observations. A knot is equivalent to 1.151 statute miles per hour or 1.852
kilometers per hour.
LIGHTNING
A sudden and visible discharge of electricity produced in response to the build
up of electrical potential between cloud and ground, between clouds, within
a single cloud, or between a cloud and surrounding air. For an example,
see ball lightning.
LUNAR ECLIPSE
An eclipse of the moon occurs when the earth is in a direct line between the
sun and the moon. The moon does not have any light of its own, instead,
it reflects the sun's light. During a lunar eclipse, the moon is in the
earth's shadow. It will often look dim and sometimes copper or orange in
color.
MERCURIAL
BAROMETER
An instrument used for measuring the change in atmospheric pressure. It uses
a long glass tube, open at one end and closed at the other. After first filling
the open end with mercury, it is then temporarily sealed and placed into a
cistern of mercury. A nearly perfect vacuum is established at the closed end
after the mercury descends. The height of the column of mercury in the tube
is a measurement of air pressure. As atmospheric pressure increases, the mercury
is forced from the cistern up the tube; when the atmospheric pressure decreases,
the mercury flows back into the cistern. Measurement is taken in inches of
mercury. Although mercurial barometers are very accurate, practicality has
led observers to use aneroid barometers. First used by Evangelista Torricelli
(1608-1647), an Italian physicist and mathematician, to explain the fundamental
principles of hydromechanics.
METEOROLOGY/METEOROLOGIST
The science and study of the atmosphere and atmospheric phenomena. Various
areas of meteorology include agricultural, applied, astrometerology, aviation,
dynamic, hydrometeorology, operational, and synoptic, to name a few. A
scientist who studies the atmosphere and atmospheric phenomena.
MIDDLE LATITUDES
The latitude belt roughly between 35 and 65 degrees North and South. Also referred
to as the temperate region.
MIST
A collection of microscopic water droplets suspended in the atmosphere. It
does not reduce visibility as much as fog and is often confused with drizzle.
MOISTURE
Refers to the water vapor content in the atmosphere, or the total water, liquid,
solid or vapor, in a given volume of air.
MONSOON
The seasonal shift of winds created by the great annual temperature variation
that occurs over large land areas in contrast with associated ocean surfaces.
The monsoon is associated primarily with the moisture and copious rains
that arrive with the southwest flow across southern India. The name is
derived from the word mausim, Arabic for season. This pattern is most evident
on the southern and eastern sides of Asia, although it does occur elsewhere,
such as in the southwestern United States.
MUD SLIDE
Fast moving soil, rocks and water that flow down mountain slopes and canyons
during a heavy a downpour of rain.
MUGGY
A subjective term for warm and excessively humid weather.
NOCTILUCENT
CLOUDS
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.
NOR'EASTER
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.
OBSERVATION
In meteorology, the evaluation of one or more meteorological elements, such
as temperature, pressure, or wind, that describe the state of the atmosphere,
either at the earth's surface or aloft. An observer is one who records
the evaluations of the meteorological elements.
OVERCAST
The amount of sky cover for a cloud layer that is 8/8ths, based on the summation
layer amount for that layer.
OZONE (O3)
A nearly colorless gas and a form of oxygen (O2). It is composed of an oxygen
molecule made up of three oxygen atoms instead of two.
OZONE LAYER
An atmospheric layer that contains a high proportion of oxygen that exists
as ozone. It acts as a filtering mechanism against incoming ultraviolet
radiation. It is located between the troposphere and the stratosphere,
around 9.5 to 12.5 miles (15 to 20 kilometers) above the earth's surface.
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.
PREVAILING
WIND
A wind that blows from one direction more frequently than any other during
a given period, such as a day, month, season, or year.
PSYCHROMETER
An instrument used to measure water vapor content of the atmosphere. It consists
of two thermometers, a wet bulb and dry bulb. May also be referred to as
a sling psychrometer.
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.
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 imsfimagestely after a rain shower. The bow is always observed
in the opposite side of the sky from the sun.
RELATIVE HUMIDITY
A type of humidity that considers the ratio of the actual vapor pressure of
the air to the saturation vapor pressure. It is usually expressed in percentage.
SATURATE
To treat or charge something to the point where no more can be absorbed, dissolved,
or retained. In meteorology, it is used when discussing the amount of water
vapor in a volume of air.
SATURATION
POINT
The point when the water vapor in the atmosphere is at its maximum level for
the existing temperature.
SEA BREEZE
A diurnal coastal breeze that blows onshore, from the sea to the land. It is
caused by the temperature difference when the surface of the land is warmer
than the adjacent body of water. Predominate during the day, it reaches
its maximum early to mid afternoon. It blows in the opposite direction
of a land breeze.
SHOWER
Precipitation from a convective cloud that is characterized
by its sudden beginning and ending, changes in intensity,
and rapid changes in the appearance of the sky. It occurs
in the form of rain (SHRA), snow (SHSN), or ice (SHPE).
It is reported as "SH" in an observation and on the METAR.
SKY
The vault-like apparent surface against which all aerial objects are seen from
the earth.
SLEET
Also known as ice pellets, it is winter precipitation in the
form of small bits or pellets of ice that rebound after striking
the ground or any other hard surface. It is reported as "PE" in
an observation and on the METAR.
SLUSH
Snow or ice on the ground that has been reduced to a softy watery mixture by
rain and/or warm temperatures.
SNOW
Frozen precipitation in the form of white or translucent ice
crystals in complex branched hexagonal form. It most often
falls from stratiform clouds, but can fall as snow showers
from cumuliform ones. It usually appears clustered into snowflakes.
It is reported as "SN" in an observation and on the METAR.
SPRING
The season of the year which occurs as the sun approaches the summer solstice,
and characterized by increasing temperatures in the mid-latitudes. Customarily,
this refers to the months of March, April, and May in the North Hemisphere,
and the months of September, October, and November in the Southern Hemisphere.
Astronomically, this is the period between the vernal equinox and the summer
solstice.
STATION PRESSURE
The absolute air pressure
at a given reporting station. The air pressure is directly proportional
to the combined weight of all air in the atmosphere located in a column
directly above the reporting site. Consequently, the station pressure
may vary tremendously from one location to another in mountainous
regions due to the strong variation of atmospheric pressure with height.
Vertical variations of pressure range up to 150 mb per mile whereas
horizontal variations are usually less than .1 mb per mile.
SUMMER
Astronomically, this is the period between the summer solstice and the autumnal
equinox. It is characterized as having the warmest temperatures of the
year, except in some tropical regions. Customarily, this refers to the
months of June, July, and August in the North Hemisphere, and the months
of December, January, and February in the Southern Hemisphere.
TEMPERATURE
The measure of molecular motion or the degree of heat of a substance. It is
measured on an arbitrary scale from absolute zero, where the molecules
theoretically stop moving. It is also the degree of hotness or coldness.
In surface observations, it refers primarily to the free air or ambient
temperature close to the surface of the earth.
THAW
A warm spell of weather when ice and snow melt. To free something from the
binding action of ice by warming it to a temperature above the melting
point of ice.
THERMOMETER
An instrument used for measuring temperature. The different scales used in
meteorology are Celsius, Fahrenheit, and Kelvin or Absolute.
THUNDER
The sound emitted by rapidly expanding gases along the channel of a lightning
discharge. Over three-quarters of lightning's electrical discharge is used
in heating the gases in the atmosphere in and imsfimagestely around the
visible channel. Temperatures can rise to over 10,000 degrees Celsius in
microseconds, resulting in a violent pressure wave, composed of compression
and rarefaction. The rumble of thunder is created as one's ear catches
other parts of the discharge, the part of the lightning flash nearest registering
first, then the parts further away.
THUNDERSTORM
Produced by a cumulonimbus cloud, it is a microscale event of relatively short
duration characterized by thunder, lightning, gusty surface winds, turbulence,
hail, icing, precipitation, moderate to extreme up and downdrafts, and
under the most severe conditions, tornadoes.
TIDE
The periodic rising and falling of the earth's oceans and atmosphere. It is
the result of the tide-producing forces of the moon and the sun acting
on the rotating earth. This propagates a wave through the atmosphere and
along the surface of the earth's waters.
TORNADO
A violently rotating column of air in contact with and extending between a
convective cloud and the surface of the earth. It is the most destructive
of all storm-scale atmospheric phenomena. They can occur anywhere in the
world given the right conditions, but are most frequent in the United States
in an area bounded by the Rockies on the west and the Appalachians in the
east.
TSUNAMI
An ocean wave with a long period that is formed by an underwater earthquake
or landslide, or volcanic eruption. It may travel unnoticed across the
ocean for thousands of miles from its point of origin and builds up to
great heights over shallower water. Also known as a seismic sea wave, and
incorrectly, as a tidal wave.
TWISTER
A slang term used in the United States for a tornado.
TYPHOON
The name for a tropical cyclone with sustained winds of 74 miles per hour (65
knots) or greater in the western North Pacific Ocean. This same tropical
cyclone is known as a hurricane in the eastern North Pacific and North
Atlantic Ocean, and as a cyclone in the Indian Ocean.
ULTRAVIOLET
Electromagnetic radiation that has a wavelength shorter than visible light
and longer than x-rays. Although it accounts for only 4 to 5 percent of
the total energy of insolation, it is responsible for many complex photochemical
reactions, such as fluorescence and the formation of ozone.
UPDRAFT
A small scale current of air with vertical motion. If there is enough moisture,
then it may condense, forming a cumulus cloud, the first step towards thunderstorm
development. Contrast with a downdraft.
VAPOR PRESSURE
The pressure exerted by the molecules of a given vapor. In meteorology, it
is considered as the part of total atmospheric pressure due to the water
vapor content. It is independent of other gases or vapors.
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.
WARNING
A forecast issued when severe weather has developed, is already occurring and
reported, or is detected on radar. Warnings state a particular hazard or
imminent danger, such as tornadoes, severe thunderstorms, flash and river
floods, winter storms, heavy snows, etc.
WATER
Refers to the chemical compound, H2O, as well as its liquid form. At atmospheric
temperatures and pressures, it can exist in all three phases: solid (ice),
liquid (water), and gaseous (water vapor). It is a vital, life- sustaining
part of life on earth.
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.
WEATHER VANE
Originally used as a wind vane, it is an instrument that indicates the wind
direction. The name developed based on observations on what kind of weather
occurred with certain wind directions. Creative designs often adorn the
tops of barns and houses.
WET BULB DEPRESSION
Dependent on the temperature and the humidity of the air, it is the difference
between the dry bulb and the wet bulb readings.
WET BULB THERMOMETER
A thermometer used to measure the lowest temperature in the ambient atmosphere
in its natural state by evaporating water from a wet muslin- covered bulb
of a thermometer. The wet bulb temperature is used to compute dew point
and relative humidity. One of the two therometers that make up a psychrometer.
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.
WIND CHILL
INDEX
The calculation of temperature that takes into consideration the effects of
wind and temperature on the human body. Describes the average loss of body
heat and how the temperature feels. This is not the actual air temperature.
For an example, check out the wind chill chart.
WIND DIRECTION
The direction from which the wind is blowing. For example, an easterly wind
is blowing from the east, not toward the east. It is reported with reference
to true north, or 360 degrees on the compass, and expressed to the nearest
10 degrees, or to one of the 16 points of the compass (N, NE, etc.).
WIND SPEED
The rate of the motion of the air on a unit of time. It can be measured in
a number of ways. The Kestrel units measure in
mph (Mile per Hour) / m/s (Meters per Second), ft/m (Feet per Minute),
km/h (Kilometers per Hour), kt (Knots), B (Beaufort Force).
WINTER
Astronomically, this is the period between the winter solstice and the vernal
equinox. It is characterized as having the coldest temperatures of the
year, when the sun is primarily over the opposite hemisphere. Customarily,
this refers to the months of December, January, and February in the North
Hemisphere, and the months of June, July, and August in the Southern Hemisphere.
YEAR
The interval required for the earth to complete one revolution around the sun.
A sidereal year, which is the time it take for the earth to make one absolute
revolution around the sun, is 365 days, 6 hours, 9 minutes, and 9.5 seconds.
The calendar year begins at 12 o'clock midnight local time on the night
of December 31st-January 1st. Currently, we operate under the Gregorian
calendar of 365 days, with 366 days every four years, a leap year. The
tropical year, also called the mean solar year, is dependent on the seasons.
It is the interval between two consecutive returns of the sun to the vernal
equinox. In 1900, that took 365 days, 5 hours, 48 minutes, and 46 seconds,
and it is decreasing at the rate of 0.53 second per century.
YELLOW SNOW
Snow that is given golden, or yellow, appearance by the presence of pine or
cypress pollen in it.
ZULU TIME
One of several names for the twenty-four hour time which is used throughout
the scientific and military communities. Other names for this time measurement
are Universal Time Coordinate (UTC) or Greenwich Mean Time (GMT).
Sources
Geer, Ira (ed). Glossary of Weather and Climate. Boston. American Meteorological Society, 1996.
Huschke, R.E. (ed.). The Glossary of Meteorology. Boston, Massachusetts. American Meteorological Society Press, 1980.
National Weather Service Observing Handbook No. 7, Surface Weather Observations and Reports. U.S. Department of Commerce, 1996.
Schneider, Stephen H. (ed.). Encyclopedia of Climate and Weather. New York. Oxford University Press, 1996.
The Weather Chanel, URL: "http://www.weather.com/glossary"
