AIR MASS - A large expanse of
air having similar temperature and humidity at
any given height.
ANEMOMETER - An instrument used
to measure wind speed
ANTICYCLONE - A region of high
atmospheric pressure around which winds move in
a clockwise direction in the northern hemisphere.
Generally associated with warm and dry weather.
ANVIL - The cirrus outflow forming
the top of a large thunderstorm giving it (the
storm) an anvil shape.
ATMOSPHERE - The gaseous fluid
surrounding a planet. The earth's atmosphere consists
primarily of oxygen and nitrogen; however it is
the water vapor content that is responsible for
most of the weather.
ATMOSPHERIC PRESSURE - The downward
force of the air per unit area, averaging 101.3
kpa at sea level (1013.25 millibars or 29.92 inches
AWIPS - An acronym for Advanced
Weather Interactive Processing System. This computer
system is now the backbone of the NWS; it replaces
the old AFOS computer network. It consists of
powerful minicomputers linked in a nationwide
network to weather service offices and national
centers. AWIPS workstations are able to simultaneously
display real-time radar and satellite imagery
as well as high-resolution model output graphics
- all on the same screen!
BACKING - A counterclockwise
shift in wind direction (for example, south winds
shifting to the east).
BLIZZARD - A very cold wind
with blowing snow. Sustained winds (or frequent
gusts) reach or exceed 40 mph, and visibilities
are restricted to 1/4 mile or less by the blowing
CAPE - Convective Available
Potential Energy - The amount of
buoyant energy available to accelerate an air
parcel vertically; it is a measure of the potential
intensity of deep,moist convection. An atmosphere
with high CAPE values has the potential to develop
strong, tall and violent thunderstorms, whereas
an environment with little or no CAPE has little
chance of developing a thunderstorm - or if they
do form, they will be weak and low topped..
CELSIUS SCALE - The standard
temperature scale in the metric system which sets
the freezing point at 0 and the boiling point
at 100 (compared to 32 and 212 degrees respectively
in the more familiar Fahrenheit scale).
CIRRUS - High clouds (generally
above 20,000 feet) which are composed of ice crystals.
CLIMATE - This refers to the
average state of the atmosphere over a long period
of time, taking into account temperature, wind,
humidity and rainfall. To put it succinctly, "Climate
is what we expect - weather is what we get".
CLIMATOLOGY - A branch of meteorology
that deals with the long-term characteristics
COLD ADVECTION - This occurs
as a cold air mass replaces a warm air mass.
COLD FRONT - The leading edge
of a cold air mass. Cold frontal passages are
usually associated with sharp wind shifts, pressure
rises, showery or stormy conditions, and rapid
CONTRAILS - A trail of clouds,
usually cirrus, that forms behind aircraft as
they travel through the atmosphere.
CONVECTION - Rising air currents
due to heating as sunlight warms the ground and
the lower atmosphere. Warm air rises because it
is less dense after it expands. Convection is
the process that leads to the formation of thunderstorms.
CORIOLIS ACCELERATION - The
earth's rotation causes moving objects, unattached
to the earth, to accelerate to the right of their
path of motion in the northern hemisphere.
CUMULONIMBUS CLOUD - A cloud
with considerable vertical growth and precipitation;
a thunderstorm. Generally contains an anvil cloud
at the top.
CUMULUS - Clouds that exhibit
vertical growth and are caused by convective processes.
CYCLOGENESIS - Deepening of
a low pressure system associated with the continuing
development of a mid-latitude cyclone (storm).
CYCLONE - An area of low pressure
around which winds blow in a counter-clockwise
fashion in the northern hemisphere. Generally
associated with cool, wet and unsettled weather.
CYCLONIC - Counter-clockwise
airflow around a low pressure system in the northern
hemisphere (clockwise in the southern).
DEW POINT TEMPERATURE - The
temperature of the air when it is cooled to saturation.
(When saturation is reached, the air cannot hold
any additional moisture, and any excess moisture
condenses as dew on surface features such as grass
DIURNAL TEMPERATURE RANGE -
The temperature difference between the minimum
at night (low) and the maximum during the day
DOPPLER RADAR - Radar that employs
the shift in frequency of radio waves returned
from moving objects in order to give the objects'
speed. Thus, the radar can detect the motion of
individual air parcels, raindrops, fronts, or
DRY ADIABATIC LAPSE RATE - The
rate of cooling (by expansion) as an air parcel
rises, if no other additions of heat occur. It
is approximately 10 degrees Celsius per kilometer,
or 5 degrees Fahrenheit per thousand feet. It
is also the rate of heating (by compression) as
an air parcel descends.
DRY LINE - The leading edge
of an advancing hot dry air mass of continental
tropical origin, the Desert Southwest for the
United States. The dry line usually acts as a
focusing mechanism for strong thunderstorms during
the summer convective season.
DUST DEVIL - A small surface
generated vortex storm (or whirlwind) that occurs
in deserts and other locations where the ground
is very hot. Dust devils resemble tiny tornados,
but are caused by convective processes, and they
produce significantly less damage than tornados
- in many cases they cause no damage of consequence.
DYNAMIC LIFTING - The forced
uplifting of air from various atmospheric processes,
such as weather fronts, and cyclones.
ENVIRONMENTAL LAPSE RATE - the
measured lapse rate (rate of decrease in temperature
with height) in the atmosphere surrounding a rising
bubble of air. It usually is around 3-4 degrees
Fahrenheit per thousand feet.
EQUINOX - The time when the
sun crosses the plane of the Earth's equator occurring
about March 21 and September 22. During the Equinox,
the day will have equal number (12) of daylight
and nighttime hours.
FOG - Suspension of liquid water
droplets in the air near the earth's surface that
reduces visibility to less than 1 kilometer (5/8
mile). This occurs when the air is saturated,
or when the temperature is the same as the dewpoint.
It also generally occurs with light winds.
FREEZING RAIN - Rain that turns
to ice as it strikes cold objects or the cold
FUNNEL CLOUD - A ROTATING vortex
extending from the base of a thunderstorm (or
developing convective cloud) that would become
a tornado IF it touched the ground.
GEOSTROPHIC WIND - The calculated
or measured wind that occurs when the pressure
gradient acceleration equals the Coriolis acceleration.
The geostrophic wind is seen above the boundary
layer, in the upper layers of the atmosphere.
GRADIENT WIND - Wind blowing
along curved isobars with a balance of centrifugal,
Coriolis and pressure gradient accelerations.
GRAUPEL - A type of winter precipitation
formed by packed snow flakes or ice crystals after
riming has given it the appearance of small soft
hail. Otherwise known as snow pellets.
GREENWICH MEAN TIME (GMT) -
Since the prime meridian of 0 degrees longitude
passes through Greenwich, England, this location
is used for a universal time throughout the world.
Each of the time zones are measured outward 15
degrees from the prime meridian. Thus, 1400 (2
PM) GMT is 0900 EST, 0800 CST, 0700 MST etc. GMT
minus 7 hours equals MST. GMT is also called "Z"
time and most NWS products, especially model guidance
products, use this time.
GUST FRONT - A miniature cold
front created by the downdraft of a large thunderstorm
as the dense, rain-cooled air spreads outward
beneath the thunderstorm. Gust fronts can produce
strong, gusty, damaging surface winds, and often
serve as a triggering mechanism for new thunderstorms.
HAIL - Solid precipitation in
the form of chunks or balls of ice that occurs
with strong convection, generally in the form
HOOK ECHO - The shape of a severe
thunderstorm echo on a radar screen which denotes
the presence of a tornado. The echo is in the
form of a hook, or appendage extending off of
the main storm echo. Most often seen in the radar
imagery of Midwest or great plains storms - very
rarely seen in Arizona radar imagery.
HORIZONTAL CONVERGENCE - A net
flow of air towards a given location, that occurs
from converging air currents or changes in wind
speed. Convergence in the lower atmosphere results
in rising air, and the formation of clouds and
HORIZONTAL DIVERGENCE - A net
flow of air outward from a given location, that
occurs from diverging air currents or changes
in wind speed. Divergence in the lower atmosphere
results in sinking air, and generally warm and
HURRICANE - An atmospheric vortex
storm of tropical origin that is intermediate
in size between a tornado and a mid-latitude cyclone.
A tropical storm is classified as a hurricane
when its sustained winds reach or exceed 74 mph.
HYGROMETER - An instrument used
to measure atmospheric humidity.
INVERSION - A region in the
atmosphere where the temperature increases with
height. The presence of an inversion creates a
very stable atmosphere; when it occurs at the
surface it leads to very little mixing and a trapping
of pollutants in the lower atmosphere.
INSTABILITY - The atmosphere
is in an unstable state when a displaced parcel
of air continues to move either up or down, away
from its initial location. Instability is necessary
for convection to occur, and for the formation
ISOBARS - Lines of constant
pressure. On surface meteorological charts, these
contours are used to delineate areas of low and
ISOTHERMS - Lines of constant
temperature on a weather map.
JET STREAM - A stream of air,
about 12 km (8 miles) high, with an average speed
of 100 km/hr (67 mph) at 30 to 50 degrees north
latitude. Jet stream winds are from a general
westerly direction, and since mid latitude storm
systems follow the jet stream, it is also known
as the "storm track".
LAPSE RATE - The change in temperature
with altitude. A positive lapse rate occurs when
the temperature decreases with altitude (the normal
occurrence in the lower atmosphere).
LIFTED INDEX - A calculated
index used to measure atmospheric stability. Positive
lifted indices denote a stable atmosphere, negative
numbers an unstable atmosphere; the greater the
negative number the more unstable the atmosphere,
and the greater the chances for thunderstorm development.
LOW-LEVEL JET - A stream of
air moving northward from the Gulf of Mexico,
generally lower in altitude (~ 2 km or 1.3 miles)
than the jet stream. Wind speeds are lower than
the polar jet stream, and the low level jet generally
transports significant moisture from the gulf
into the central and northern United States.
LOW PRESSURE SYSTEM - A region
of relatively lower atmospheric pressure in the
midlatitudes, generally accompanied by cloudy,
windy and wet weather.
MESOCYCLONE - The cyclonic,
rotating part of a large thunderstorm. Tornados
are usually formed in association with the mesocyclone.
MESOSCALE - A scale used in
meteorology that extends from approximately 1
to 100 km (.7 to 67 miles). Mesoscale features
include such things as thunderstorms, sea breezes,
gust fronts and macrobursts.
MICROSCALE - A scale used in
meteorology that extends from about 1 centimeter
to a kilometer (.67 miles). Microscale processes
include such things as dust devils, tornados and
MILLIBAR - A unit of pressure
in the metric system. One atmosphere equals 1013
millibars. The 500 millibar level, a standard
level used in meteorology and in weather forecast
maps, is located about 18000 feet above sea level;
approximately half of the earth's atmosphere is
located below this height!
MIXING RATIO - The mass of water
vapor per unit mass of dry air, in grams/kilogram.
Higher values of mixing ratio imply a more humid
MOIST ADIABATIC LAPSE RATE -
The expansion cooling rate of rising saturated
air (from 4 to 10 degrees Celsius per kilometer
- or 2 to 5 degrees Fahrenheit per thousand feet).
As air rises it expands, and cools. However, as
saturated air rises, the cooling forces moisture
to condense out of the air. As the vapor condenses
it releases heat into the atmosphere, which partially
offsets the expansional cooling.
MONSOON - A seasonal shift in
the winds. For Arizona, the monsoon results in
westerly winds shifting to the south or southeast;
this shift brings considerable moisture into the
state from the Gulf of California and the Gulf
OCCLUDED FRONT - A front that
is formed as a cold front overtakes a warm front.
POLAR JET STREAM - A stream
of air blowing from a westerly direction at a
height of about 12 km (8 miles) over midlatitude
locations. It generally separates polar air masses
from warmer air; storms generally move with the
jet stream and thus it is also known as the "storm
PRECIPITABLE WATER - The depth,
or amount, of water that would result if all of
the water vapor in the atmosphere above a certain
location were condensed into liquid water. NWS
graphics show precipitable water in terms of inches
and percentage of normal (for the given day).
PRECIPITATION PROBABILITY -
the degree of certainty, or chance, of rainfall
at a given location (from 0 to 100 percent). In
the NWS, the percent figure represents the chance
of getting measurable rainfall (0.01 inch or more)
in the official rain gauge at a site.
PRESSURE GRADIENT FORCE - The
force exerted on air due to a difference in pressure
between two points. One of the forces that causes
wind; as the TV meteorologists often say "wind
blows from the high pressure to the low pressure
RADAR - An instrument useful
for remote sensing of meteorological phenomena.
It operates by sending radio waves and monitoring
those returned by reflecting objects such as raindrops,
cloud droplets, dust particles, blowing dirt and
sand grains, or birds. When the reflecting particles
are raindrops, the higher the energy returned
to the radar, the larger the drops and the heavier
RADIATION - Electromagnetic
energy emitted by all objects (including the sun,
of course) in amounts related to the temperature
of the object.
RADIATION FOG - Fog formed as
the ground (and objects on the ground) cool by
losing long- wave infrared radiation. Conditions
favorable for formation of radiation fog include
light winds, clear skies, and high humidity at
the surface (near or at 100 percent).
RADIOSONDE - The instrument
package carried by weather balloons to measure
the temperature, humidity and pressure of the
atmosphere at multiple levels (up to at least
30000 feet in altitude).
REFLECTION - Light that bounces
off an object at the same angle and wavelength
as that of the incident (initial) light ray.
REFRACTION - The bending of
light at a particular angle as it passes through
a transparent medium (such as glass or water).
Rainbows are causes by light refracting and reflecting
(twice) inside raindrops.
RELATIVE HUMIDITY - The amount
of water vapor in the air divided by the amount
of water vapor at saturation, expressed as a percentage.
100 percent humidity denotes saturated air. Warm
air can hold more moisture than cold air, so for
a given amount of water vapor, the higher the
temperature, the lower the relative humidity.
RIDGE - A region of anticyclonic
curvature of winds in the atmosphere located around
a high pressure area. A ridge in the upper atmosphere
is generally associated with dry and warm weather.
SANTA ANA WIND - Strong down
slope winds that blow through the mountain passes
in southern California. These winds, which can
easily exceed 40 mph, are warm and dry and can
severely exacerbate brush or forest fires, especially
under drought conditions.
SATURATION - The condition when
the atmosphere is holding the maximum amount of
water vapor that it can for a given temperature.
When the air is saturated, the temperature will
equal the dew point temperature.
SEVERE THUNDERSTORM - Thunderstorms
with damaging winds or hail. The NWS defines a
severe storm as one having winds greater than
57 mph or hail 3/4 inch in diameter or larger
(dime size or bigger).
SNOW PELLETS - Precipitation
of white or opaque spherical ice, usually 5mm
in diameter or less. Snow pellets, also known
as graupel, are softer than hail or ice pellets.
SOLSTICE - Either of the two
times of the year when the sun is the greatest
distance from the celestial equator, occurring
about June 22 and December 22. The summer solstice
marks the day containing the maximum hours of
daylight, whereas the winter solstice has the
fewest hours of daylight (the longest night).
STABLE ATMOSPHERE (STABILITY)
- The condition when little or no vertical mixing
occurs due to the nature of the temperature change
with height (the temperature either becomes warmer
with height, or cools at a low rate). Under stable
conditions, convection (and thunderstorms) are
inhibited, winds are generally on the light side,
and pollution is easily trapped in the lower layers
of the atmosphere (near the ground).
STATIONARY FRONT - A surface
boundary between air masses (such as a cold or
warm front) that has stalled and has little or
STRATUS - Low clouds that have
a stratified form, containing liquid water droplets.
STREAMLINE - A line on a weather
map showing the path a parcel of air would follow
as it moves through the atmosphere.
SUBTROPICAL JET STREAM - A stream
of air from the west, at about 14 km (9.3 miles)
height and about 30 degrees north latitude. This
jet stream is weaker than the polar jet stream
and usually separates equatorial air masses from
cooler air to the north.
SUPERCELL - The largest thunderstorms
that approach a steady-state internal structure
for a few hours and are able to produce large
hail and tornados. The NWS defines supercells
as thunderstorms that have a persistent, rotating
updraft as part of their structure - these rotating
updrafts have the potential to spawn deadly tornados.
SUPERCOOLED - Liquid water droplets
between 0 and -40 degrees Celsius that would freeze
immediately if particles were present to start
the solidifying process.
SYNOPTIC - Deals with the gathering
of meteorological data over a large area (such
as the United States) at a specified instant in
time, for the purpose of projecting the data into
the future (to give weather forecasts).
SYNOPTIC CHART - A weather map
produced by plotting synoptic meteorological data
(such as winds, temperature or pressure).
THERMAL LOW - An area of low
atmospheric pressure produced from continued surface
heating as in the Desert Southwest. As the air
is heated, it expands, becomes less dense and
thus the pressure becomes lower.
THERMAL UPDRAFT - A rising air
current in a large thunderstorm that is buoyant
because of its warm temperature compared to surrounding
air. "Thermals" are not always associated
with thunderstorms; they are useful to soaring
pilots and hang gliders, as the rising air currents
are responsible for keeping the planes/gliders
THUNDER - The audible sound
waves produced as a lightning discharge heats,
and expands, a conducting column of air.
TORNADO - The most violent vortex
storm on earth. Most commonly, tornados originate
within a thunderstorm, although other processes
can produce weaker tornados. Tornado strength
can be classified on the Fujita scale of intensity
which numbers from 0 to 5. F5 tornados are the
most destructive known, with winds over 250 possible.
F0 tornados are the weakest, though they can still
produce significant damage.
TROPICAL DEPRESSION - A stormy
region in the tropics more intense than a tropical
disturbance but less intense than a tropical storm.
Wind speeds in a tropical depression are less
than 39 mph.
TROPICAL DISTURBANCE - A region
of cloudy, relatively disorganized, weather in
the tropics These sometimes develop into a tropical
storm (and later into a hurricane).
TROPICAL STORM - A vortex storm
that occurs over the tropical ocean. In a more
specific sense, it is a stage in hurricane development
where the intensity is slightly less than a hurricane.
Sustained wind speeds in a tropical storm are
39 mph or greater.
TROUGH - A region of cyclonic
(counter-clockwise) curvature of winds in the
atmosphere, located around a low-pressure area.
Troughs are generally associated with cloudy,
cool, unsettled, and wet weather.
TURBULENCE - Very unorganized
atmospheric motion, including gusts and lulls
in the wind. It is responsible for random shaking
motion experienced in an airplane during flight,
and in extreme cases can damage or destroy an
UPPER AIR SUPPORT - Strength
provided to surface low-pressure systems by horizontal
divergence in the upper atmosphere, or to surface
high-pressure areas by horizontal convergence
in the upper air.
URBAN HEAT ISLAND - The increased
temperatures in urban areas compared with surrounding
rural areas. This is due to the significant amount
of brick, concrete, asphalt, etc, in a city which
absorbs significant amounts of heat during the
day and releases it gradually at night, keeping
the urban areas warmer than the surrounding rural
VEERING - A clockwise shift
in wind direction, such as when the winds shift
from the south to the west. Winds will veer during
a cold frontal passage, as the southerly or southeast
flow ahead of the front quickly shifts to the
west or northwest following the front.
VORTICITY - The amount of rotation,
or spin, of the air - also defined as circulation
per unit area. Cyclonic vorticity (counter-clockwise
spin) is essential for the creation of upper air
divergence, rising air, and the development of
surface low pressure systems.
WARM FRONT - The leading edge
of a warm air mass. Warm fronts are usually associated
with temperature rises, cloudy and wet weather.
Clouds in a warm front are generally layered,
and less convection (thunderstorm activity) is
seen along warm fronts, as compared to cold fronts.
WARNING - A warning issued by
the National Weather Service means the hazardous
weather is IMMINENT or OCCURRING. Warnings are
issued for severe thunderstorms, tornadoes, flash
floods, blizzards, hurricanes, and other phenomena.
WATCH - A watch issued by the
National Weather Service means that conditions
are favorable for the formation of the phenomena
mentioned in the watch, such as flash floods or
WIND CHILL - The air temperature
we FEEL that is corrected for the influence of
wind speed. In MOST cases, the wind chill temperature
will be lower than the actual temperature, but
in certain cases of high temperature and humidity,
wind will actually make the perceived temperature
hotter than the actual temperature! Above a certain
wind speed, additional wind will not lower the
wind chill value.
WIND SHEAR - A difference in
wind speed OR direction between two wind currents
in the atmosphere. Strong vertical wind shear
(where the wind changes speed or direction significantly
from the surface to some point in the middle or
upper atmosphere) is essential for the formation
of tornadoes, assuming that the atmosphere is
sufficiently unstable for the generation of strong