APPENDIX B
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GLOSSARY OF WEATHER TERMS


Weather terms are listed by topic.

THUNDERSTORM TERMS:

1. 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.

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

3. 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 rains and gusty winds often accompany the storms.

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

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

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

7. 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 100 miles downwind.

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

9. 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.

10. 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.

11. 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.

12. 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.

13. 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.

14. 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.

15. 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.

16. 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.

17. 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).

18. 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. 19. 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 over 100 mph.

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

21. Downburst - A sudden rush of cool air toward ground that can impact with speeds over 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.

22. Microburst - A small downburst effecting an area less than 2.5 Km in diameter.

23. Macroburst - A larger downburst effecting an area greater than 2.5 Km in diameter.

24. Severe thunderstorm - A thunderstorm producing damaging winds or winds greater than 58 mph and/or hail three-quarter of an inch or greater.
 

HURRICANE TERMS:

1. Tropical disturbance - A moving area of thunderstorms in the Tropics that maintains its identity for 24 hours or more.

2. Tropical depression - A cluster of thunderstorms in the tropics that maintains its identity and shows rotary circulation at the surface with constant wind speeds of 38 mph or less.

3. Tropical storm - Evolves from a tropical depression or may be a hurricane in its dissipating stage. Rotary circulation is distinct and constant wind speeds range from 39 to 73 mph.

4. Hurricane - Evolves from a tropical storm. Rotary circulation has become pronounced and an eye is detectable. Constant wind speeds are 74 mph or greater.

5. Eye - The center of the hurricane where winds are light and skies are clear to partly cloudy. The eye is rimmed by massive thunderstorms producing torrential rains and extreme winds.

6. Eye wall - A wall of thunderstorms around the eye.

7. Spiral bands - Bands of thunderstorms that appear to spiral in toward the hurricane's center.

8. Storm surge - a dome of water often 50 miles wide that comes sweeping across the coastline near the area where the eye of the hurricane makes landfall.
 

FLOOD TERMS:

1. 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.

2. River flood - a flood on large river such as the Potomac take 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. A flood crest from heavy rains in West Virginia may take 2 days or more to reach Washington, D.C.

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

4. 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.

5. Bankfull - the maximum height of the river before it overflows its banks.

6. 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.

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

WINTER WEATHER TERMS:

1. Snow - Refers to a steady fall and accumulation of snow for several hours or more. It may be modified by terms such as "light," "intermittent," or "occasional" to indicate intensity or periodic snow.

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

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

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

5. 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.

6. 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.

7. Heavy snow - Snow accumulating to six inches in 24 hours. These values will be a couple inches higher for mountainous regions, New England, or near the Great Lakes where higher snowfall is more common.

8. Blizzard - strong winds (greater than 35 mph) and heavy snow or blowing snow combine to produce very poor visibility.

9. 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.

10. 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.

11. 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.

12. Ice storm - significant and possibly damaging accumulations of ice are expected during freezing rain situations. Significant ice accumulations are usually accumulations of .25 inches or greater, but may vary from region to region across the country.

13. Wind chill (Wind chill factor) - combines the rate of heat loss caused by wind and lowering temperature. As the wind rises, heat is carried away from a person's body at a more accelerated rate driving down the body temperature.

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

15. 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 temperature has dropped to at least 32F.

16. Hypothermia - when the body temperature drops below 95F.

17. Frost bite - frozen body tissue.

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