appenidx b
APPENDIX B - GLOSSARY
OF WEATHER TERMS
THUNDERSTORM TERMS
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.
FLOOD TERMS
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.
WINTER WEATHER TERMS
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.
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