GENERAL WEATHER TERMS

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Following are weather terms frequently used by the National Weather Service. All temperatures are in Fahrenheit.


A

ADVECTION
The horizontal movement of an air mass that causes changes in the physical properties of the air such as temperature and moisture.
ADVISORY
Highlights special weather conditions that are less serious than a warning. They are for events that may cause significant inconvenience , and if caution is not used, could lead to situations that may threaten life and/or property.
AIRMASS
A large body of air that has nearly uniform conditions of temperature and humidity.
AIR STAGNATION
A meteorological situation in which there is a major buildup of air pollution in the atmosphere. This usually occurs when the same air mass is parked over the same area for several days. During this time the light winds cannot "cleanse" the buildup of smoke, dust, gases, and other industrial air pollution.
ALBERTA CLIPPER
A low pressure system that moves out of southwest Canada and mainly affects the Plains, Midwest, and Great Lakes region. Usually accompanied by light snow, strong winds, and colder temperatures. Another variation of the same system is called a Saskatchewan Screamer.
AVALANCHE
A mass of snow, rock, and/or ice falling down a mountain or incline. In practice it usually refers to the snow avalanche. In the U.S. the term snowslide is commonly used to mean a snow avalanche.

B

BAR
An obstacle formed at the shallow entrance to the mouth of a river or bay which empties into the ocean.
BEAUFORT SCALE
A numerical scale used to estimate the force of the wind.

 

Wind Speed (mph) Designation Description

<1

calm

smoke rises vertically, trees do not move

1-3

light air

smoke drift indicates wind direction

4-7

light breeze

weather vane moves, leaves rustle

8-12

light breeze

leaves and twigs in constant motion

13-18

moderate breeze

dust and loose paper raised, small branches move

19-24

fresh breeze

small trees sway

25-31

strong breeze

large branches move, wind whistles wires

32-38

moderate gale

whole trees move, walking affected

39-46

fresh gale

twigs break off trees, walking difficult

47-54

strong gale

slight structural damage occurs, branches break

55-63

whole gale

trees uprooted, considerable structural damage

64-74

storm

widespread damage

75+

hurricane

severe and extensive damage

BLOWING DUST
Wind-driven dust that significantly reduces surface visibility to less than seven miles.
BLOWING SNOW
Wind-driven snow that significantly reduces surface visibility to less than seven miles.

C

CHINOOK WIND
A warm, dry wind that descends the eastern slopes of the Rocky mountains. The warmth and dryness of this wind can quickly melt and evaporate a snowcover. Another name for this type of wind is foehn.
CIRRUS CLOUD
A wispy, cloud that is composed of ice crystals and is formed at altitudes of 20,000 to 40,000 feet above the ground.
CUMULONIMBUS CLOUD
A cumulus cloud that is vertically developed and often has an anvil shaped top. Generally associated with lightning, thunder, heavy showers, and occasionally hail and strong winds.
CUMULUS CLOUD
A cloud that has a flat base with an upper portion that is billowy or heaping.
COMBINED SEAS
The combination or interaction of wind, waves and swells.
CYCLONE
An area of low atmospheric pressure that has a closed circulation. Cyclones (or more commonly called 'low pressures') usually bring about marked changes in the weather.

D

DEGREE-DAY
(heating-cooling) Gauges the amount of heating or cooling needed for a building using 65 degrees as a baseline. To compute heating/cooling degree-days, the average temperature is taken and referenced to a base line of 65 degrees. An average temperature of 50 yields 15 heating degree-days while an average of 75 would yield 10 cooling degree-days. Electrical, natural gas, power, heating, and air conditioning industries utilize heating and cooling degree information to calculate their needs.
DEGREE-DAY
(growing) To compute growing degree-days one would use various base-line references: 40 degrees for canning purposes; 45 for potatoes; and 50 for sweet corn, snap beans, lima beans, tomatoes, grapes, and field corn. Every degree that the average temperature is above the base line value becomes a growing degree day. Agricultural-related interests use growing degree-days to determine planting times.
DEW
Water droplets that form upon surfaces on or near the ground when air is cooled toward its dewpoint.
DEWPOINT
The temperature to which air must be cooled, at constant pressure and moisture content, in order for saturation to occur. The higher the dew point, the greater the amount of water vapor in that vicinity. Dewpoints in the 70's make people feel uncomfortable.
DOPPLER WEATHER RADAR
A new Weather Surveillance Radar (WSR-88D) system developed in 1988. Ultimately, about 120 systems will be installed at Weather Forecast Offices, with an additional 24 systems at Department of Defense (Air Force Bases) sites. This powerful and sensitive Doppler system generates many useful products for meteorologists, among them: standard reflectivity 'echoes', wind 'velocity' or atmospheric air motion pictures, and areal 1-hour, 3-hour, or storm-total precipitation images.
DOWNBURST
A strong downdraft, initiated by a thunderstorm, that includes an outburst of damaging winds on or near the ground. Downbursts may last for anywhere from a few minutes in small scale microbursts on up to 20 minutes in larger, longer lived microbursts. One example of a downburst, called straight-line winds, can reach speeds of 110-150 mph, or squarely in the range of a strong tornado. Downburts are further detailed as either: Microburst: a convective downdraft with an affected outflow area of less than 2.5 miles wide and peak winds lasting less than 5 minutes. They can create dangerous vertical/horizontal wind shears which can adversely affect aircraft performance and cause property damage. Macroburst: a convective downdraft with an affected outflow area of at least 2.5 miles wide and peak winds lasting between 5 and 20 minutes. Intense macrobursts may cause tornado-force damage.
DOWNSLOPE/UPSLOPE FLOW
Air that descends down a mountain chain or over sloping terrain (pressurized air moving from high pressure to low pressure), resulting in subsequent drying, and in some cases, dramatic warming of air that can quickly melt a snowcover. Local names for downslope winds, or "foehn" winds in the western USA are Chinook Wind, East Winds, North Winds, and Mono Winds. Usually associated with little or no clouds. On the other hand, upslope flow is representative of air being lifted by rising terrain and is normally associated with extensive clouds and/or precipitation.
DRIZZLE
Water drops that are very small and fine. For the most part, drizzle falls from stratus clouds and is usually accompanied by low visibility and fog.
DRYLINE
A narrow zone of extremely sharp moisture gradient often parallel to the Rocky Mountains during the spring and early summer. It separates moist unstable air flowing off the Gulf of Mexico from dry air flowing off the semi-arid high plateau regions of Mexico and southwest U.S.A. Severe weather can be associated with the dryline. It is also know as a dewpoint front or dry front .
DUST DEVIL
A small, vigorous whirlwind, usually of short duration, rendered visible by dust, sand, and debris picked up from the ground. They range from 10 feet to greater than 100 feet in diameter, and can extend up to 1000 feet above the ground. They form in response to intense surface heating, which causes rapidly rising warm air to produce a mini low-pressure system. They are usually found in desert or dry climatic regions where dust and dirt can easily be lifted. Only rarely do they cause any damage. Wind speeds can reach 30 to 60 mph.
DUST STORM
Severe weather condition marked by strong winds and dust-filled air over an extensive area. Visibility is reduced to 1/2 mile or less.

E

EBB CURRENT
A tidal current that is receding or declining.
EL NINO
Significant warming of the waters in the eastern Pacific Ocean, usually off the coast of South America, which results in shifts of world-wide weather patterns. Can cause prolonged periods of drought or floods.

F

FATHOM
A unit of length equal to six feet which is used to measure the depth of water.
FETCH
An area from which waves are generated by a wind that is nearly constant in direction and speed.
FLASH FLOOD
A dangerous and sudden flood that threatens lives and property and usually occurs after heavy rain. May also occur after an ice jam breaks up or after a dam fails.
FOG BOW
A nebulous arc or circle of white or yellowish light sometimes seen in fog.
FLURRIES
Light snowfall that generally does not produce measurable accumulation.
FREEZING DRIZZLE or RAIN
Describes the effect of drizzle or rain freezing upon impact on objects that have a temperature of 32 degrees or below.
FREEZING LEVEL
The point in the atmosphere where temperatures are at 32 degrees.
FRONT
The boundary between two different air masses, ie. cold front, warm front, stationary front.
FROST
A covering of small ice crystals that forms on or near the ground when temperatures approach or drop below 32.
FUNNEL CLOUD
A rotating, visible extension of cloud, pendant to a cumulus or cumulonimbus with circulation not reaching the ground.
FUJITA SCALE
A scale used to classify tornadoes based on wind damage and was developed by Theodore Fujita (University of Chicago).

 

F Scale

Wind Speed (mph)

Damage

F0 40-72 Light
F1 73-112 Moderate
F2 113-157 Major
F3 158-206 Severe
F4 207-260 Devestating
F5 261-318 Incredible
F6 319 or greater Catastrophic



G

GROUND FOG
Fog of little vertical extent, usually 20 feet or less.
GUST FRONT
The leading edge of a downdraft associated with a thunderstorm which is marked by a sudden wind shift, sharply falling temperatures and possibly heavy downpours and/or hail.
GUSTNADO
A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. It is not associated with the storm-scale rotation found in severe thunderstorms.

H

HABOOB
A violent dust storm or sand storm as of northern Africa, India, or the southwestern United States.
HAIL
Precipitation in the form of balls or lumps usually consisting of concentric layers of ice. A thunderstorm is classified as severe when it produces hail 3/4 of an inch or larger in diameter.
HALOS
Rings or arcs that encircle the sun or moon which are caused by refraction of light through ice crystals that make up cirrus clouds.
HAZE
Fine particles of dust, smoke or water droplets suspended in the air that reduces visibility.
HEAT INDEX
The apparent temperature that describes the combined effect of high temperatures and high levels of humidity.
HEAVY SNOW
In the western Great Lake region, defined as snowfall accumulations of 6 inches or more in 12 hours, or 8 inches or more in 24 hours.
HEAVY SURF
Large waves breaking on or near the shore resulting from swells spawned by a distant storm.
HUMIDITY
Amount of water vapor in the atmosphere.
HURRICANE
A dangerous tropical cyclone with winds speeds of 64 knots (74 mph), or higher. (typhoon in western Pacific)

I

ICE STORM
A freezing rain event that produces damaging ice accumulations of 1/4 inch or greater.
INVERSION
A situation where the temperature increases with height instead of decreases, which is usually the case.
INSTABILITY (UNSTABLE AIR)
A state of atmosphere in which the vertical distribution of temperature allows rising, warm air to continue to rise and accelerate. This kind of motion is conducive for thunderstorm development.
ISOBAR
Lines of equal barometric pressure as shown on a weather map.

J

JET STREAK
A concentrated region within the jet stream where the wind speeds are the strongest. The jet streak sets up unique wind currents in its vicinity which either enhance or diminish the likelihood of clouds and precipitation. The jet streak will propagate downstream along the jet stream axis.
JET STREAM
A narrow band of strong winds in the atmosphere that controls the movement of high and low pressure systems and associated fronts. Jet streams meander from time to time. Wind speeds can reach 200 mph or higher in certain cases. It is usually found at 30,000 to 40,000 feet above the earth's surface. The jet stream owes its existence to the large temperature contrast between the polar and equatorial regions.

K

KNOT
Unit of speed used in aviation and marine activities, which is equal to about 1.15 statue miles an hour.

L

LAKE/LAND BREEZE
A lake breeze occurs when prevailing winds blow off the water, while a land breeze indicates winds blowing from land to sea. Both are caused by the difference in surface temperature (heating) of the land and water. As a result, a lake breeze occurs during the day while a land breeze happens at night.
LAKE-EFFECT SNOW SQUALL (LAKE SNOW)
A local intense, narrow band of moderate to heavy snow that can extend long distances inland, persist for many hours, and may be accompanied by strong, gusty surface winds and possibly lightning. Accumulations can be 6 inches or more in 12 hours.
LEEWARD/WINDWARD
Leeward is on the side or facing the direction toward which the wind is blowing . On the other hand, windward is on the side or facing the direction away from which the wind is blowing.
LIGHTNING
A sudden visible flash of energy and light caused by electrical discharges from thunderstorms.

M

MILLIBAR
Unit of atmospheric pressure.
MINOR TIDAL OVERFLOW
Minor flooding caused by high tides that results in little if any damage.

N

NAUTICAL MILE
A unit of distance used in marine navigation and forecasts, equal to 1.15 statute miles.
NEAP TIDE
A tide of minimum range occurring at the first and third quarters of the moon.
NEARSHORE WATERS
The waters of Lake Superior or Lake Michigan extending out to five miles from shore.
NEXRAD
An acronym that stands for NEXt generation of weather RADar.
NOR'EASTER
A strong low pressure system that affects the Mid-Atlantic and New England States. It can form over land or over the coastal waters. It usually produces heavy snows, flooding rains, strong northeast winds, and coastal flooding, and beach erosion.

O

OFFSHORE/ONSHORE FLOW
Offshore flow occurs when air moves from land to sea, while onshore flow is when air over the water advances across land. Offshore flow is usually associated with dry weather, while onshore flow indicates an increase in moisture and resultant precipitation probabilities.
OFFSHORE (OPEN) WATERS
The waters extending from 5 miles to the midpoint of the Lake.
OROGRAPHIC UPLIFT (UPSLOPE FLOW)
Occurs when air is forced to rise and cool due to terrain features such as hills or mountains. If the cooling is sufficient, water vapor condenses into clouds. Additional cooling results in rain or snow. It can cause extensive cloudiness and increased amounts of precipitation in higher terrain.
OZONE
A nearly colorless (but faintly blue) gaseous form of oxygen, with a characteristic odor like that of weak chlorine. Its formula is O3. It is usually found in trace amounts in the atmosphere, but is primarily found at 30,000 to 150,000 feet above the ground. Its production results from a photochemical process involving ultraviolet radiation. Because it absorbs harmful ultraviolet radiation at those heights, it is a beneficial gas. However, photochemical processes involving industrial/vehicle emissions can produce ozone near the ground, in which case it can be harmful to people with respiratory or heart problems.

P

PAN HANDLE HOOK
Low pressure systems that originate in the panhandle region of Texas and Oklahoma, which initially move east and then "hook" or recurve more northeast toward the upper Midwest or Great Lakes region. In winter these systems usually deposit heavy snows north of their surface track. Thunderstorms may be found south of the track.
PUGET SOUND CONVERGENCE ZONE
A situation where wind forced around the Olympic Mountains converges over the Puget Sound. Causes extreme variability in weather conditions around Seattle, WA with some areas in sunshine and others in clouds and rain.

R

RADIATIONAL COOLING
The cooling of the earth's surface. At night, the earth suffers a net heat loss to space due to terrestrial cooling.
RAIN
Indicates a nearly steady and uniform fall of liquid precipitation (rain) over an area for several hours, as opposed to the term "showers", which implies intermittent and scattered precipitation of a more unstable, convective nature.
RAINBOW
An arc that exhibits in concentric bands the colors of the spectrum and is formed opposite the sun by refraction and reflection of the sun's rays in raindrops.
RAINSHADOW
Areas on the leeward side of a mountain or mountain range which often receive much less rain than the windward side.
RELATIVE HUMIDITY
The ratio of the amount of moisture in the air to the amount which the air could hold at the same temperature and pressure if it were saturated; usually expressed in percent.
RIDGE
An elongated area of high pressure in the atmosphere; the opposite of trough.
RIP CURRENT (or RUNOUT)
A strong, narrow current of surface-water that flows seaward through the surf into deeper water. Waves approaching the shoreline create a water buildup, which results in a return flow. This return flow (rip current) transports the excess water into deeper waters. Bubbles and debris usually float on the surface of the rip current.
ROLL CLOUD
A turbulent cloud formation that resembles a roller. This cloud can be found in the lee of some mountains. The air in the cloud rotates around an axis parallel to range of mountains. It is also sometimes found along the leading edge of a thunderstorm cloud; formed by rolling action in the wind shear region between cool downdrafts and warm updrafts.

S

SAFFIR/SIMPSON HURRICANE SCALE
A scale ranging from one to five based on a hurricane's present intensity. This can be used to give an estimate of the potential property damage and flooding expected. In practice, wind speed is the parameter that determines the category since storm surge is highly dependent on the slope of the continental shelf.

 

Scale

Wind Speed (mph)

Storm Surge (ft)

Damage

1 74-95 4-5 Minor
2 96-110 6-8 Moderate
3 111-130 9-12 Major
4 131-155 13-18 Severe
5 Greater than 155 Greater than 18 Catastrophic

SANTA ANA WIND
A strong, hot, dry foehn-like wind that blows from the north, northeast or east into southern California.
SEICHE
A standing wave oscillation in any enclosed lake which continues after the forcing mechanism has ceased. In the Great Lakes, this forcing mechanism may be either strong winds blowing along the axis of a lake or a pressure jump, or down draft winds associated with fast moving squall lines over a lake. In either case, water is piled up at one end. The water then sloshes from one end of the lake to the other causing fluctuations of perhaps several feet before damping out.
SEVERE THUNDERSTORM
A thunderstorm that produces either of the following: damaging winds of 58 miles an hour or greater, hail 3/4 of an inch in diameter or larger, or a tornado. Severe thunderstorms can result in the loss of life and property.
SLEET
Describes solid grains of ice formed by the freezing of raindrops or the refreezing of largely melted snowflakes.
SNOW
A steady fall of snowflakes for several hours over the same area.
SNOWPACK
The combined layers of snow and ice on the ground at any one time. Also called the "snowcover".
SNOW SHOWERS
Snow that starts and stops suddenly and is characterized by rapid changes in both intensity and visibility. There is normally measurable accumulation.
SOUTHERN OSCILLATION
A periodic, large scale atmospheric oscillation of the large scale distribution of sea level pressure, and air and water temperature that originates over the southern hemisphere. Consequently, there is an associated change in the surface winds, and some storms become stronger than normal. This oscillation is on the scale of a year or a few years, and has global implications such as widespread drought or flooding. Oceanic fishing is also disrupted.
SPRING TIDE
A tide of greater-than-average range around the times of new and full moon.
SQUALL LINE
A broken or solid line of thunderstorms that may extend across several hundred miles, ahead or along an advancing cold front.
STRATUS
Low clouds that are flat and gray, usually covering most of the sky.
SUSTAINED WIND
Wind speed determined by averaging observed values over a 1-minute period.
SWELL
A long, often massive and crestless wave or succession of waves that continues well beyond or after its cause, such as with hurricanes.

T

THERMAL
A relatively small-scale, rising air current produced when the earth's surface is heated. Thermals are a common source of low level turbulence for aircraft.
TIDAL PILING
Abnormally high water levels caused by an accumulation of successive incoming tides that do not completely drain due to opposing strong winds and/or waves.
TORNADO
A violently rotating column of air, usually pendant to a cumulonimbus, with circulation reaching the ground. The visible condensation (cloud) may not reach the ground, but if the lower circulation, marked by dust, dirt, and/or debris, reaches the ground, it is classified as a tornado. It nearly always starts off as a funnel cloud and may be accompanied by a loud roaring noise. Tornadoes are classified into 3 main groups: weak - wind speeds up to 110 mph; strong - wind speeds 110 to 205 mph; violent - wind speeds 205 to perhaps 320 mph.
TROPICAL CYCLONE
A cyclone originating over tropical or subtropical waters with organized convection and a definite cyclonic surface wind circulation.
TROPICAL or SUBTROPICAL DEPRESSION
Cyclones that have maximum sustained winds of 33 knots (38 mph) or less. These are referred to as low pressure systems in public advisories and statements.
TROPICAL DISTURBANCE
An area of organized convection which originates in the tropics or subtropics and maintains its identity for 24 hours or more. In successive stages of intensification, it may be subsequently classified as a tropical wave, tropical depression, tropical storm, or hurricane.
TROPICAL STORM
Tropical cyclone that has maximum sustained winds from 34 to 63 knots (39 to 73 mph) inclusive.
TROPICAL WAVE
A trough or cyclonic curvature maximum in the trade wind easterlies and is not classified as a tropical cyclone.
TROUGH
An elongated area of low pressure in the atmosphere; the opposite of a ridge.
TSUNAMI
An ocean wave produced by a sub-marine earthquake, landslide, or volcanic eruption. These waves may reach enormous dimensions and have sufficient energy to travel across entire oceans.

U

URBAN/SMALL STREAM FLOODING
Flooding that occurs after heavy rains of relatively short duration and is generally not life-threatening. Causes ponding of water in urban areas, especially in low places, and results in minor flooding of small streams and creeks.
UPPER-LEVEL DISTURBANCE
A disturbance of the flow pattern in the upper atmosphere, which is usually associated with clouds and precipitation. This disturbance is characterized by distinct cyclonic flow, a pocket of cold air; and sometimes, a jet streak. These features make the air aloft more unstable and conducive to clouds and precipitation.

V

VOLCANIC ASH
Fine particles of mineral matter from a volcanic eruption, which can be dispersed long distances by winds aloft. The chemical composition and abrasiveness of the particles can seriously affect aircraft and also machinery on the ground.
VIRGA
Wisps or streaks of rain or snow falling out of a cloud, but evaporating before reaching the ground.

W

WALL CLOUD
A local, abrupt lowering of a rain-free cumulonimbus base forming a low hanging accessory cloud that is usually 1 to 4 miles in diameter. The wall cloud is usually situated in the right-rear quadrant of the cumulonimbus with respect to storm motion, below an intense updraft associated with a strong or severe thunderstorm. Rotating wall clouds often precede tornado development.
WARNING
Indicates that a hazardous weather element is imminent or has a very high probability of occurrence.
WATCH
Alerts the public to the possibility of severe weather, or some other hazardous weather element. It is intended to provide enough lead time so those who need to set their plans in motion can do so.
WAVE
An identifiable, periodic disturbance or motion in a medium that shows displacement. The most commonly referred medium is water, followed by the atmosphere which is a "fluid." The forecasted heights of waves on the Great Lakes or in the oceans are those heights expected at the end of the fetch for that body of water.
WATERSPOUT
A violently rotating column of air, usually pendant to a cumulus or cumulonimbus cloud, over a body of water, with circulation reaching the water.
WET BULB TEMPERATURE
The temperature an air parcel would have if cooled to saturation at a constant pressure by evaporation of water into it.
WIND WAVES
Waves generated from the action of wind on a water surface, as opposed to swell.
WIND CHILL
An apparent temperature that describes the combined effect of wind and low air temperatures on exposed skin.

Z

ZEPHYR
A breeze from the west or a gentle breeze.

 


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