Weather Glossary
A-Z Weather and Climate Words
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 SOURCES
- acid rain:
- Rain having a
pH lower than 5.6 (the pH of natural
rainwater), usually caused by sulfuric acid and/or nitric acid from air
pollution.
- advection:
- Horizontal transport of temperature, pressure, moisture or
some other atmospheric attribute solely by the wind.
- aerosol:
- Particulate matter put into the
atmosphere. Examples are smoke and dust.
- air mass:
- A body of air that has the same
temperature, moisture, and stability properties.
- altimeter:
- An instrument that measures the altitude
of something with respect to a set level.
- altitude:
- The vertical distance above a certain
level, usually sea level or ground level.
- altocumulus:
- A middle cloud type that is
white and/or gray in color and occurs as a layer or patch with a waved aspect.
Generally have shadowed parts.
- altostratus:
- A middle cloud type that appears
as a gray or bluish sheet of stretched out, fibrous, or uniform appearance.
Overcast clouds.
- anemometer:
- An instrument that measures wind speed.
- anticyclone:
- A body of air that circulates
clockwise and outward in the Northern Hemisphere
and counterclockwise and outward in the Southern
Hemisphere, and is associated with a high atmospheric pressure.
- anvil:
- The top part of a cumulonimbus
cloud that is flat and spread out, with a shape that looks like a
blacksmith's anvil.
- aphelion:
- The point where the earth is farthest
(152 million kilometers) from the sun while in its orbit. Currently, this occurs
on July 5.
- atmosphere:
- Air enveloping earth due to earth's
gravitational pull. Can be divided into layers radiating out from the surface in
the following order- troposphere, stratosphere,
mesosphere, thermosphere.
- aurora:
- Appears as greenish-white and sometimes pink light in the night
sky over the middle and high latitudes. Called the aurora borealis in the Northern
Hemisphere and the aurora australis in the Southern Hemisphere. Caused by
electrons interacting with each other in the upper atmosphere.
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- barometer:
- Instrument that measures atmospheric pressure.
- Bermuda High:
- A subtropical anticyclone
that occurs over the North Atlantic Ocean.
- biosphere:
- The zone between the earth and the atmosphere
in which most life forms reside.
- black frost:
- When vegetation is frozen on the inside during a frost in
which there is no moisture. A black frost will almost always kill the plant.
- boiling point:
- The temperature at which a
substance will change from a liquid state to a gas state. The boiling point of
water is 100°C and 212°F.
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- calm:
- When the air conditions have no movement or wind.
- calorie:
- The amount of heat required to raise the temperature of one gram
of water one degree Celsius.
- ceilometer:
- An instrument used to measure cloud height.
- Celsius:
- Term used to describe temperature based
on the boiling point (100°) and the freezing
point (0°) of pure water at sea level.
- cirriform:
- Describes high
clouds formed with small, mostly ice particles that result in white,
transparent clouds that often produce a halo when the sun
shines through them.
- cirrus:
- A high cloud made
up of ice crystals that appear fibrous and silky.
- climatology:
- Scientific study of the climate, the causes of a climate,
and the use of data to solve climate related problems.
- cold front:
- When a colder, higher
pressure air mass moves so that cold air replaces warmer air. Storm activity
is often associated with the moving in of a cold front.
- condensation:
- When a vapor becomes a liquid
due to some change. Occurs often when the temperature becomes cooler, the humidity
becomes greater, or the air pressure changes.
- conduction:
- Transfer of energy through a body by particles or molecules
within that body. Energy flows from the hotter parts to the cooler parts.
- contrail:
- A trail of condensed water vapor formed by jet aircraft flying
at high altitudes. This trail is formed by the water
vapor added to the air by engine exhaust, as well as particles in the exhaust
that act as cloud nuclei.
- convection:
- Vertical energy transfer in the atmosphere.
- cooling degree day:
- A measurement used to determine
how much fuel will be needed to cool indoor environments. There is one cooling
degree-day unit for each degree that the mean daily
temperature departs from 65 °F or 18 °C.
- cumulonimbus:
- A dense cloud with a lot of
vertical development that looks like a large tower. Often the top is flattened,
and spreads out above the cloud in an anvil shape. This is
a storm cloud, and is often associated with precipitation
and severe weather.
- cumulus:
- A low cloud that develops from the
bottom up. Appear white and puffy, with a cauliflower-like top. Small ones are
associated with fair weather(cumulus humilis), but as they grow vertically, they
can produce rain showers (cumulus congestus).
- current :
- A horizontal movement of water or air.
- cyclone:
- A body of air that circulates counter
clockwise and inward in the Northern Hemisphere
and clockwise and inward in the Southern
Hemisphere, and is associated with a low atmospheric
pressure .
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- degree:
- 1) A unit of temperature, describing an
interval on a scale of Fahrenheit or Celsius..
2) A unit of angular distance, used to describe distance of latitude
and longitude.
- degree-day unit:
- A measure of how much the mean daily temperature differs
from a base temperature in Fahrenheit and Celsius.
Used to determine how much fuel will be needed to heat or cool indoor
environments.
- desertification:
- When an area begins to develop desert-like conditions
due to lack of water, deforestation, overgrazing and over cropping.
- dew:
- When liquid water from the atmosphere
condenses on plants and objects near the ground.
- dewpoint:
- The temperature that air must reach before condensation
can occur. The air is saturated at this point.
- diffuse sky radiation:
- The energy from the sun that is scattered by
molecules, clouds and aerosols in the atmosphere.
- direct radiation:
- The energy from the sun that hits the earth without
being obstructed by clouds, molecules, or aerosols. It is
the sunlight that creates a shadow at the surface.
- diurnal:
- Referring to daily phenomena.
- drought:
- A long period of time where there is abnormally dry weather due
to little or no precipitation.
- dry adiabatic lapse rate:
- The rate at which
an unsaturated parcel of air will decrease in
temperature as it moves through the atmosphere. The
rate is a decrease of about 9.767°C per kilometer upwards.
- dry season:
- In certain climates, it is a period
of one month or more where precipitation is at a
minimum.
- dryline:
- A boundary that separates warm, dry air from warm, moist air.
This boundary usually represents a zone of instability where
thunderstorms often develop due to the mixing of the different types of air.
- dust devil:
- A small rotating column of wind that can be seen due to the
dust and debris that it picks up.
- duststorm:
- When strong winds blow over a dusty area, blowing the dust
into the air and reducing visibility significantly.
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- easterlies:
- Winds that come from the East. Examples are equatorial
easterlies, tropical easterlies (trade winds),
and polar easterlies.
- eclipse:
- When one celestial body, such as the sun or the moon, hides
another from view.
- El Nino :
- Warming of ocean waters in the eastern
tropical pacific, resulting in warmer waters off the coasts of Ecuador and
northern Peru. Associated with heavy rainfall in those regions, and also affects
weather patterns in North America as well. Usually begins to occur around
Christmas, and is so named for El Nino, the Spanish name for the Christ Child.
- electromagnetic radiation:
- Form
of energy emitted through space and reaches the earth. Solar
electromagnetic radiation comes from the sun and drives all energy processes
on earth.
- electromagnetic radiation
spectrum:
- All the types of radiation put in order from smallest
wavelength to largest wavelength. Please see table* below.
From: MicroWorlds.
- elevation:
- Vertical distance from sea
level to a point affixed to the ground. The elevation of a mountain, for
example, may be 10,000 feet.
- ENSO Cycle:
- El Nino Southern Oscillation Cycle.
The ocean-atmosphere interactions in the tropical and equatorial Pacific region
involving abnormal sea surfae temperatures and atmospheric pressures. These
abnormalities often result in changes in the trade winds.
Occurs at 2-7 year intervals and may last up to two years.
- environmental lapse rate:
- The rate of decrease
of air temperature as altitude increases. Is compared to
the dry adiabatic lapse rate or moist
adiabatic lapse rate to determine vertical motions of an air parcel.
- equator:
- Imaginary circle having 0° latitude
that runs horizontally around the center of the earth. Is equally far from both
the North and South poles.
- equinox:
- Either of two points where the sun's rays
will shine directly over the equator at noon, and all
locations on earth will theoretically experience twelve hours of daylight. The
vernal equinox occurs on March 21 and the autumnal equinox occurs on September
22 in the Northern Hemisphere. The dates are
reversed for the Southern Hemisphere.
- erosion:
- When rock and soil are moved from one place to another by
running water, precipitation, ice, or wind.
- evaporation:
- The change of state from a liquid
to a vapor.
- evapotranspiration:
- The combined water vapor put into the air through evaporation
from water on earth's surface and plants giving off water to the atmosphere.
- exosphere:
- The outermost portion of the atmosphere.
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- Fahrenheit:
- Temperature scale where water
freezes at 32°F and water boils at 212°F at sea level
air pressure.
- flash flood:
- A flood that rises and falls rapidly
with little or no advance warning, usually due to intense rainfall over a small
area.
- flood:
- When water overflows the confines of a body
of water and gathers over low-lying areas.
- fog:
- A visible gathering of water droplets suspended
in the air near the earth's surface.
- freezing point:
- The temperature at which a
substance changes from a liquid to a solid. For water, it is 0°C or 32°F.
- front:
- Boundary where two air
masses with different temperatures and moisture content meet.
- frost:
- 1) A cover of ice crystals produced when
atmospheric water vapor is deposited directly on a surface when the temperature
is below freezing., 2) The condition that exists when the temperature of the
earth's surface and earth-bound objects falls below freezing. An example is a
killing frost, where a frost will end the growing season in agriculture because
it freezes crops and they die.
- frost point:
- Temperature where air is cooled enough to become saturated,
but forms ice instead of dew.
- Fujita Tornado Scale:
- Classifies tornadoes from F0
(wind speeds of 40-72 mph) to F5 (261-318 mph) on the basis of rotational speeds
derived from the damage they cause. Developed by T. Theodore Fujita.
- funnel cloud:
- A quickly rotating column of
air coming down from a cloud base but not reaching the ground. When it touches
the ground it becomes a tornado.
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- greenhouse gas:
- Gases in the atmosphere
that are more transparent to the short
wavelength radiation (mostly visible light)
from the sun, than to the long wavelength
radiation (infrared radiation) leaving the
Earth. These gases then reemit the trapped energy, partly toward the Earth's
surface. Examples are carbon dioxide, methane, CFCs, and nitrous oxides.
- greenhouse effect:
- A natural occurrence caused by certain greenhouse
gases in the Earth's atmosphere. Because greenhouse gases trap longwave
energy and reemit it back to the Earth's surface, the global temperature is
changed 33 degrees Celsius, or 59 degrees Fahrenheit warmer than it would
otherwise be. Some scientists think that the increase in greenhouse gases in the
atmosphere (caused by pollution- mainly the burning of fossil fuels and
deforestation) may be causing a rise in global temperature.
- Gulf Stream:
- A warm ocean current that flows from
the Gulf of Mexico and the Carribean Sea north along the North American coast,
and northeast across the Atlantic Ocean.
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- hail:
- Frozen precipitation
that looks like balls or lumps of ice. Usually produced by cumulonimbus
clouds.
- halo:
- A ring of light that appears to encircle the
sun or moon. This occurs when the light from the sun or moon shines through cirrus
clouds.
- Heat Index:
- Temperature that describes how hot it feels taking into
account high air temperatures and high humidity levels.
- heating degree day:
- A measurement used to determine how
much fuel will be needed to heat indoor environments. There is one heating
degree-day unit for each degree that the mean daily
temperature departs from 65 °F or 18 °C.
- high clouds:
- Clouds found between 3-8
kilometers (10000-25000 feet) in polar regions and 6-18 kilometers (20000-60000
feet) in tropical regions. Contains cirriform clouds (cirrus,
cirrocumulus, and cirrostratus).
- high pressure system:
- Area of maximum air
pressure and anticyclonic air movement.
- humidity:
- The measure of how much water there is
in the air.
- hurricane:
- A severe tropical cyclone
with maximum 1-minute sustained surface wind speeds greater than 74 mph.
- hydrologic cycle:
- The exchange of water between the earth, the oceans,
and the atmosphere. Also called the water cycle.
- hydrosphere:
- The part of planet earth that is covered with water and ice.
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- Indian summer:
- Occurring during mid to late autumn after the first frost,
a period of unusually warm weather with clear skies and cool nights.
- infrared radiation:
- The portion of
energy that has a wavelength between 0.8 micrometers (just above visible
radiation) to 0.1 millimeters (microwave radiation).
- instability:
- A state of the atmosphere
when a parcel of air will keep moving either up or down from its starting
position when moved. An unstable atmosphere is often associated with severe
weather.
- Intertropical Convergence Zone (ITCZ):
- Belt of clouds and thunderstorms
that runs parallel to the equator and marks where the trade
winds of both hemispheres come together. Shifts seasonally, and is
associated with monsoons and dry
seasons in some regions.
- isobar:
- A line of equal pressure
resembling a contour line on a map.
- isotherm:
- A line passing through points of equal temperature on a map.
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- jet stream:
- Narrow river of very strong horizontal winds usually located
in the upper troposphere along fronts.
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- Kelvin:
- The SI (Standard International) unit
of temperature. A degree symbol is not used. One Kelvin equals one degree Celsius,
but the base temperature of Kelvin is absolute zero (-273° C), where there is
absolutely no movement of molecules at all.
- Knot:
- A measure of speed equal to the velocity of one nautical mile
traveled in one hour. Usually used in marine studies. One knot equals 1.151
miles per hour or 1.852 kilometers per hour.
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- La Nina:
- A period of strong trade winds
and abnormally low sea surface temperatures in the central and eastern tropical
Pacific. It is the opposite of an El Nino. Spanish for
little girl child.
- latent heat:
- Heat released or absorbed per
unit of mass when a substance changes phase. When water evaporates
from liquid to steam, it absorbs heat. When liquid water freezes, it releases
latent heat.
- latitude:
- The distance in degrees
on the earth measured from the equator, running
horizontally. Lines of latitude are described as North or South of the equator,
and are parallel to each other.
- lee:
- The side of an object that is shielded from the wind.
- lithosphere:
- The outer solid portion of the planet earth, made up of the
earth's crust and upper mantle. All the rock on earth.
- longitude:
- The distance in degrees
on the earth measured East and West from the Prime Meridian (in Greenwich
England). Lines of a certain degree of longitude are called meridians,
and because they run vertically over the earth, they are not parallel. Meridians
converge at the North and South Poles.
- low clouds:
- Clouds found between the earth's
surface and 2 kilometers (0-6500 feet) in all geographic regions. Contains
stratocumulus and stratus clouds, and sometimes nimbostratus,
cirrus and cumulonimbus
clouds.
- low pressure system:
- Area of minimum air
pressure, and cyclonic air movement.
- lunar eclipse:
- When the earth is in a direct
line between the sun and the moon. Because the moon reflects the sun's light and
it cannot do this when the earth is in the way, it may appear dim or even orange
in color.
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- meridian:
- If a circle is drawn vertically around
Earth at its largest point and ran through both poles, one half of that circle
running from the North pole to the South pole would be a meridian. Describes a
line of longitude.
- mesosphere:
- Part of the atmosphere
that has an altitude of between about 50 kilometers and
about 70-80 kilometers.
- meteorology:
- Study of the phenomena of the atmosphere.
Includes physics, chemistry and dynamics of the atmosphere as well as the direct
effects upon the earth's surface, oceans and life in general. Oriented towards
developing a complete understanding, accurate prediction, and artificial control
of atmospheric occurrences.
- middle clouds:
- Clouds found between 2-4
kilometers (6500-13000 feet) in polar regions and 2-8 kilometers (6500-25000
feet) in tropical regions. Contains altocumulus and altostratus clouds, and
sometimes nimbostratus.
- moist adiabatic lapse rate:
- The decrease
in temperature as a saturated parcel of air moves up
through the atmosphere. This rate is 0.55°C per 100
meters.
- monsoon:
- When the principal wind direction changes
noticeably with the season. The main cause is the heating
and cooling of a nearby ocean depending on summer and winter, such as the Indian
Ocean and the India monsoon for example.
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- nimbostratus:
- Usually considered a middle
cloud, is gray and often dark. The edges are not easy to see because of
continuously falling rain, snow, or sleet. This cloud does not have lightning,
thunder or hail.
- nimbus:
- A rain cloud.
- Northern Hemisphere:
- The half of the
earth located above the equator, from 0° to the North
Pole.
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- occluded front:
- When a cold front overtakes
a warm front, and three air masses
with different temperatures conflict. The resulting frontal boundary type will
depend on how they meet.
- orographic:
- Weather phenomena caused by mountains.
- orographic cloud:
- A cloud whose form is changed as high terrain, like a
mountain, changes the airflow over the area. These clouds are often irregularly
shaped, like a lens or a banner.
- ozone layer:
- A place in the stratosphere
(about 10-50 kilometers up) where the high ozone (O3 molecules) concentration
shields the Earth from harmful ultraviolet radiation.
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- Pascal:
- The SI (Standard International)unit of
pressure equal to the pressure resulting from a force of one Newton acting over
an area of one square meter.
- perihelion:
- Opposite of aphelion.
The point where the earth is nearest to the sun (147 million kilometers) while
in its orbit. Currently, this occurs on January 3, but varies when there is a
leap year.
- permafrost:
- Layer of soil or bedrock beneath earth's surface that is
permanently frozen and has been for thousands of years.
- pH scale:
- A measure of the range of acidity and
alkalinity of a substance, on a scale from 0-14, with acids being on the lower
end and bases on the higher end. A value of 7 is neutral.
- precipitation:
- Any form of water particles
that fall from the clouds and reach the ground. Examples are rain, snow, sleet,
and hail. Fog, dew, frost,
and clouds are not forms of precipitation.
- psychrometer:
- Instrument used to measure the water vapor content of the
air.
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- radar:
- An electronic instrument that broadcasts and receives microwave
signals back from precipitation areas, and
determines their location, height, movement, and intensity.
- rainbow:
- An arc displaying all colors in the visible
light spectrum. Formed when light from the sun is reflected
and refracted through water droplets. Always appears
on the side of the sky opposite of the sun.
- reflectivity:
- A measure of how well a surface
turns the part of the radiation it
receives back in the direction from which the radiation came.
- refraction:
- Bending of light as it passes
through something with different properties, such as a light beam bending as it
travels through glass.
- relative humidity:
- Ratio of the actual vapor pressure of the air to the
saturation vapor pressure for the surrounding air temperature. In other words,
how much water the air is holding divided by how much water the air is capable
of holding.
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- Saffir-Simpson Scale:
- A hurricane intensity
scale created to quantify the amount of damage they will cause upon landfall.
The scale is derived using wind speeds and the central atmospheric pressure of
the storm, from 1 (minimal damage) to 5 (intense).
- saturation:
- When the air is holding the maximum
amount of water that it can hold. If any more water is added to the air, the
water will condense and leave the air in the form of
liquid.
- sea level:
- The height or level of the sea, used
as a reference for elevation. "Above" or
"below" sea level.
- season:
- A way of dividing up the calendar year
according to regular occurrences such as the equinoxes
and solstices. For example, in the Northern
Hemisphere, from the vernal equinox to the summer solstice is Spring (March
21 to June21). From that solstice to the autumnal equinox is Summer
(June21-September 22). From that equinox to the winter solstice is Autumn
(September 22-December 22). From that solstice to the vernal equinox is Winter
(December 22-March 21).
- SI Unit:
- Short for International System of Units,
and denotes the standard unit used internationally for different measures of
variables.
- solar eclipse:
- When the moon is in a direct
line between the earth and the sun, and the sun appears obscured. The moon's
shadow will be cast on the earth, making some parts of the earth dark for a
short length of time.
- solar radiation:
- electromagnetic
waves that come from the sun. Examples are UV rays,
visible light, and infrared
radiation.
- solstice:
- Point in the earth's orbit of the sun
where the sun's rays are shining directly as far north or as far south as they
possibly can shine, due to the tilt of the earth on its axis. In the Northern
Hemisphere, the summer solstice is June 21, and the winter solstice is
December 22. The dates are reversed for the Southern
Hemisphere.
- Southern Hemisphere:
- The half of the
earth located below the equator, extending from 0° to
the South pole.
- stability:
- A state of the atmosphere
when a parcel of air will return to its starting position when moved up or down.
This is because the parcel of air is denser than its surrounding air. Stability
is often associated with fair weather.
- storm surge:
- A rise in sea level that comes with
a hurricane or other intense storm.
- stratosphere:
- The layer between the troposphere
and the mesosphere in the earth's atmosphere.
It is located at an altitude between 10 and 50
kilometers, and is the area where most beneficial ozone formation occurs.
- stratus clouds:
- A low cloud
type that forms a gray layer with a uniform base. Does not usually produce precipitation.
Overcast skies.
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- temperature inversion:
- When a layer of air has a temperature increase
instead of decrease with altitude.
- thermal pollution:
- An increase in air or water temperature caused by heat
from man-made sources.
- thermocline:
- A temperature gradient within layer of a body of water that
is notably greater than the temperature gradients above and below it.
- thermometer:
- An instrument used for measuring temperature.
- thermosphere:
- The atmospheric shell that goes
from the top of the mesosphere to outer space. The
temperature increases with altitude, and its height is
about 80 kilometers.
- tidal wave:
- 1) The periodic variations of sea level
caused by the moon's pull, and also caused by earthquakes. 2) In everyday
language, referring to any unusually high and destructive water level along a
shore.
- tornado:
- Small mass of air that spins rapidly
about an almost vertical axis and forms a funnel cloud
that contacts the ground. Comes down from a cumulonimbus
cloud and is considered probably the most destructive of all weather systems.
- trade winds:
- Wind system in the tropics that
blows from the subtropical high pressure areas to
the equatorial low pressure area. They are northeast
winds in the Northern Hemisphere and
southeast winds in the Southern Hemisphere.
- transpiration:
- Process where water in plants is transferred to water
vapor in the atmosphere through the leaf pores.
- Tropic of Cancer:
- Furthest north parallel (latitude
line) where the sun's rays will shine directly overhead during the summer solstice,
located at 23°27' N.
- Tropic of Capricorn:
- Furthest south parallel (latitude
line) where the sun's rays will shine directly overhead during the summer solstice,
located at 23°27' S.
- tropopause:
- Boundary between the troposphere
and the stratosphere where there is a change in the lapse
rate of the atmospheric temperature.
- troposphere:
- The layer of the atmosphere
that goes from the earth's surface to the tropopause.
The lowest 6-16 kilometers (4-10 miles), where all the earth's weather takes
place.
- typhoon:
- A severe tropical cyclone with sustained
surface wind speeds greater than 74 mph in the western North Pacific. Same as a hurricane,
but given a different name depending on the location of the storm.
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- ultraviolet radiation:
- electromagnetic
radiation with wavelengths ranging from 1-300 nanometers. Can be divided
into three types-UVA (responsible for tanning the skin and some types of skin
cancers), UVB (responsible for sunburn and skin cancers), and UVC (potentially
lethal radiation, but does not reach the earth's surface because of protection
from the ozone layer).
- unsaturated:
- Is able to hold more water than
it currently is holding.
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- visible radiation:
- Part of the electromagnetic
radiation spectrum with wavelengths between 0.39 and 0.76 micrometers
(between ultraviolet and infrared).
The normal human eye is sensitive to this radiation, it is the light that we
see.
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- warm front:
- Any front that moves so that warm
air replaces colder air.
- warning:
- For a tornado, thunderstorm, flood, or hurricane.
Indicates that the event is definitely going to happen or is in the process of
occurring. Issued by the U.S. National Weather Service.
- watch:
- For a tornado, thunderstorm, flood, or hurricane.
Indicates that the conditions are ideal for one of the aforementioned events to
occur. Issued by the U.S. National Weather Service so that the public can take
the necessary precautions or even leave the area before the event occurs.
- westerlies:
- The dominant west-to-east motion of the atmosphere
centered over the middle latitudes (35°-65°) of both
hemispheres.
- wind chill equivalent temperature:
- Air temperature index that attempts to
quantify how cold the air feels when skin is exposed to low air temperature and
wind. The faster the wind blows, the colder the air will feel.
- wind shear:
- The rate of change of wind speed or wind direction over a
short period of time and a short distance. Can occur vertically or horizontally.
- wind vane:
- Instrument used to measure wind direction.
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- zephyr
- Any soft, gentle breeze.
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