National Weather Service United States Department of Commerce




Anvil - The spreading out (by strong winds) of the upper portion of the thunderstorm. It usually has a fibrous or smooth appearance. With long-lasting thunderstorms, the anvil may spread more than 100 miles downwind.

Cumulus cloud - a cauliflower-shaped cloud with a flat base and sharp edges. Tufts are rising columns of air condensing. As the cloud and cloud droplets grow in size, the base will begin to gray.

Downburst - A sudden rush of cool air toward ground that can impact with speeds greater than 70 mph and produce damage similar to that of a tornado. It usually occurs near the leading edge of the storm or may occur in heavy rain. Viewing the damage from the air does not reveal evidence of a twisting motion or convergence toward a central track, like it would for a tornado.

Downdraft - A column of cool air that sinks toward the ground. It is most often accompanied by rain.

Flanking line - A line of cumulus clouds connected to and extending outward from the most active portion of a parent cumulonimbus, usually found on the southwest (right, rear) side of a storm. The cloud line has roughly a stair-step appearance with the taller clouds adjacent to the parent cumulonimbus. It is most frequently associated with strong or severe thunderstorms.

Funnel cloud - a funnel-shaped cloud extending from a towering cumulus or thunderstorm. 
It is associated with a rotating column of air that has condensed to form a cloud.

Gust front - the leading edge of the thunderstorm's downdraft of air as it spreads out away from the storm. It is usually felt as a change to gusty cool winds and often precedes the thunderstorm's rain by several minutes.

Hail - Precipitation in the form of balls or clumps of ice.

Hook echo - A radar pattern sometimes observed in the southwest (right, rear) quadrant of a tornadic thunderstorm. The rain echo forms the hook pattern as air rotates around the strong updraft. The updraft is the hollow portion of the hook (looks like a backwards "J" or a 6) and is where the tornado would most likely be found (if the storm were to produce one). This signature is in the radar reflectivity field; Doppler radar's velocity information can help confirm the presence of a tornado, especially when a hook echo exists in the reflectivity field.

Macroburst - A larger downburst affecting an area greater than 2.5 km in diameter.

Mammatus (or mamma clouds) - these clouds appear to be hanging, rounded protuberances or pouches on the underside of the cloud. With thunderstorms, they are usually seen under the anvil and often accompany severe thunderstorms.

Microburst - A small downburst affecting an area less than 2.5 km in diameter.

Precipitation shaft - a visible column of rain or hail falling from the base of the cloud.

Rain-free base - the dark underside of a cloud (its base) that has no visible precipitation falling from it. This marks the updraft of a thunderstorm.

Roll cloud - on rare occasions, a shelf cloud may turn into a roll cloud. The motions of the warm air riding up and over the cool air moving down and under creates a swirling of air or an eddy. The cloud takes on the shape of a horizontal tube that appears to be rolling. It is detached from the thunderstorm on its leading edge.

Scud clouds - Low cloud fragments often seen in association with and behind thunderstorm gust fronts. These clouds are ragged and wind torn and are not usually attached to the thunderstorm. By the untrained, these can be misinterpreted as tornadoes, since they can hang low to the ground.

Severe thunderstorm - A thunderstorm producing damaging winds (trees down, etc.) or winds 58 mph or more and/or hail three-quarter of an inch or greater in diameter.

Shelf cloud - a low-level, wedge-shaped cloud attached to the thunderstorm. It forms above the gust front as warm air ahead of the storm rides over the cool outflow from the thunderstorm.

Squall line - a solid line or band of active thunderstorms.

Thunderstorm (cumulonimbus) - the towering cumulus cloud has continued to grow in height and width and now lightning is occurring. The storm may extend 5 to 10 miles high into the atmosphere and 5 to 25 miles across. Heavy rain and gusty winds often accompany the storms.

Tornado - a violently rotating column of air in contact with the ground and extending to the thunderstorm base, often seen extending from near the wall cloud. It can be a few yards across to a mile wide.

Towering cumulus cloud - a cumulus cloud that continues to grow so that its height is taller than or equal to its width. It is the first stage toward growing into a thunderstorm. It may be producing a shower.

Updraft - Warm, moist, rising air. As the air rises, it condenses into a visible cumulus or cumulonimbus cloud. The updraft fuels the storm. In an ordinary thunderstorm, air rises at 40 mph and in a severe thunderstorm speeds may reach more than 100 mph.

Wall cloud - this cloud appears as an abrupt lowering of the cloud base from the relatively flat rain-free base. It is attached to a thunderstorm and may be rotating. This is the portion of the thunderstorm from which the tornado often descends.




Bankfull - the maximum height of the river before it overflows its banks. Being above bankfull does not necessarily mean that it causes damage (it depends on the particular river and what development exists along it).

Coastal Flood - high tides, persistent onshore winds, or a hurricane storm surge can cause flooding along coastal areas.

Flash Flood - a flood that occurs suddenly during or shortly following heavy rains or from a sudden release of water (as in a dam break). Small streams and creeks usually react the fastest to heavy rains and rise several feet in hours or even minutes. Flash floods can also be caused by ice jams.

Flood Crest - the highest height that the river reaches during a flood event.

Flood Stage - the height of the river at which property damage begins to occur. Often differs from bankfull. The river may overflow its banks into flood plain without reaching flood stage.

River Flood - a flood on a large river such as the Connecticut takes a tremendous amount of rain and usually develops over a period of one to two days. Rain water first runs into the small streams which flow into the larger branches and eventually end up in the main stem of the river.

Urban Flood - pavement which causes rapid runoff (rain can't soak into the ground so it runs downhill) and poor drainage can lead to flooded roadways and underpasses and even become deadly.




Blizzard - strong winds (greater than 35 mph) and heavy snow or blowing snow combine to produce very poor visibility. There no longer is a temperature criterion for a blizzard.

Blowing snow - Wind-driven snow that causes reduced visibility and sometimes significant drifting. Blowing snow may be snow that is falling or snow that was once loose on the ground and picked up by the wind.

Drifting snow - winds are strong enough to blow falling snow or loose snow on the ground into mounds causing uneven snow depths. The wind carries the snow near the ground causing no restriction to visibility.

Freeze - used when temperatures at or near the surface (ground) are expected to be 32 F degrees or colder. Sometimes used with adjectives "Killing," "severe," or "hard". A freeze may or may not be accompanied by frost.

Freezing drizzle - drizzle that falls onto a surface with a temperature below freezing causing it to freeze to the surface forming a thin coating of ice or rime. Drizzle is a very light precipitation with little accumulation, but even a small amount of ice can sometimes cause a problem.

Freezing rain - rain that falls onto a surface with a temperature below freezing causing it to freeze to the surface, forming a coating of ice or glaze.

Frost - the formation of ice crystals in the forms of scales, needles, feathers, or fans, which develop under conditions similar to dew, except that the minimum temperature has dropped to at least 32 F degrees. (The 32 F sometimes is only at the plant level, while shelter temperatures (higher up) reporting 35 F, for example.)

Frostbite - frozen body tissue.

Heavy snow - Snow accumulating to at least 6 inches in 12 hours. These values are less couple (4 inches in 12 hours or 6 inches in 24 hours) in more southern portions of the country, where snowfall is not as common as in New England.
Hypothermia - a medical emergency: when the body temperature drops below 95 F.

Ice storm - significant and possibly damaging accumulations of ice are expected during freezing rain situations. Significant ice accumulations are usually accumulations of ½ inch or greater, but may vary from region to region across the country.

Sleet - ice pellets or granules of frozen rain. Occurs when rain falls into a layer of air with temperatures below freezing. Sleet usually bounces when hitting a surface and does not stick, but can accumulate on roadways causing a hazard to motorists.

Snow - A prediction of snow indicates a steady fall of snow for several hours or more. It may be modified by terms such as "light," "intermittent," or "occasional" to indicate lesser intensity or periodic snow.

Snow flurries - light snow falling for short durations. No accumulation to a light dusting (or trace) is expected.

Snow showers - snow falling at varying intensities for brief time periods. Some accumulation is possible.

Snow squalls - brief, intense snow showers, accompanied by strong, gusty winds. Accumulations may be significant.

Wind chill (Wind chill index) - an apparent temperatures which takes into account the combined effect of lowering temperatures and the rate of heat loss from a human body, caused by the wind. As the wind increases, heat is carried away from a person's body at a more accelerated rate, driving down the body temperature.