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| This web site is a joint venture of the
Fairbanks Museum & Planetarium and
Vermont Public Radio with support from the Partnership
for a Nation of Learners, a Corporation for Public Broadcasting and Institute of Museum and Library
Services leadership initiative. |
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- An area of high pressure.
Winds circulate around the center of an anticyclone in a clockwise
direction in the Northern Hemisphere and in a counter-clockwise
direction in the Southern Hemisphere. Fair weather is almost always
associated with an anticyclone.
- A cold front that moves
east to west in direction rather than the normal west to east movement.
In New England, such fronts often move in from the northeast.
- A wind that changes its
direction in a counter- clockwise motion over time or height. An
example over time would be a Northwest wind changing to a West wind.
- A scale classifying
wind strength in terms of observable effects both on the sea and
over land. See http://en.wikipedia.org/wiki/Beaufort_scale for definition.
- The height of the lowest layer
of clouds.
– The boundary where
a cold airmass pushes into a warmer airmass.
– The vertical transport
of heat and moisture, especially by updrafts and downdrafts in an
unstable atmosphere. The terms "convection" and "thunderstorms"
often are used interchangeably, although thunderstorms are only
one form of convection.
- An atmospheric condition
that exists when the winds cause a horizontal net inflow of air
into a specified region. When this occurs at the surface, rising
motion occurs in the center of convergence.
- a pattern of wind flow in
which air flows inward toward an axis oriented parallel to the general
direction of flow; the opposite of diffluence. Confluence is not
the same as convergence.
- An upper level low pressure
system that is no longer in the normal west to east upper level
air flow. In the northern hemisphere, usually a cut-off low will
lie to the South of the northern jet stream.
- An area of low pressure around
which winds blow in a counter clockwise direction in the Northern
Hemisphere and a clockwise direction in the Southern Hemisphere.
Inclement weather is often associated with a cyclone.
– A parameter developed
by heating and cooling engineers to relate energy use to temperature.
May be indexed to any temperature; the most common is 65°F.
One degree day occurs when the daily mean temperature is above or
below 65°F.
- a pattern of wind flow in
which air moves outward (in a "fan-out" pattern) away
from a central axis that is oriented parallel to the general direction
of the flow; opposite of confluence; diffluence is not the same
as divergence
- The condition that exists
when the distribution of winds within a given area results in a
net horizontal outflow of air from the region. In divergence at
upper levels, the resulting deficit is compensated by an upward
movement of air from below, favoring cloud formation and precipitation.
- A strong downward rush of
air which produces a blast of damaging wind on or close to the surface.
They are usually associated with severe thunderstorms.
- Air that descends from
higher terrain, and consequently warms and dries. Downslope winds
often produce fair weather conditions.
- A cloud, with its base on the surface,
reducing visibility. If visibility is frequently reduced to 1/4
of a mile or less, the fog is termed ‘dense’.
- A shallow layer of fog (less
than 20 feet thick) on the ground that reduces visibility more in
the horizontal than in the vertical.
- An area
where the Northern and Southern Hemispheric trade winds converge,
usually located between 10 degrees North and South of the equator.
It is a broad area of low pressure where both the Coriolis force
and the low-level pressure gradient are weak; tropical disturbances
occasionally form within it. It fluctuates in location, following
the sun's rays, so that during the Northern Hemisphere summer, the
ITCZ moves northward over the southern North Atlantic and southern
Asia.
A layer of the atmosphere where the temperature increases with height
rather than decreasing. One common type of inversion is called ‘surface-based’.
This forms most often during long nights when the ground radiates
heat into space, cooling the air directly above it.
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A line on a weather map that connects points of equal pressure.
On a weather map, isobars extend around centers of high and low
pressure.
- Relatively strong winds that are concentrated in a narrow band
in the atmosphere. Jet Streams are usually thousands of kilometers
long, hundreds of kilometers wide but only a few kilometers thick.
They are usually found between 6 and 10 miles above the surface.
- The side of an
object that is facing away from the wind.
- A cloud that generally has the form of a smooth lens. They usually
appear in formation as winds blowing over mountains set up vertical
wave motion. The clouds are at the crest of each wave.
- A large downdraft of air with a surface outflow diameter of 2.5
miles or greater and damaging winds lasting from 5 to 20 minutes.
May reach tornado intensity. Usually caused by large thunderstorms,
or groups of thunderstorms.
- A small downdraft of air with a surface outflow diameter of less
than 2.5 miles with the peak winds lasting from 2 to 5 minutes.
Usually caused by thunderstorms.
– The front
that results when a cold front catches up to and overtakes a warm
front, thus pinching the warm air between two colder, denser air
masses. The warm air is therefore cut off, or occluded, from the
surface.
- A condition that exists when a relatively warm, less dense air
mass moves up and over a cooler, denser air mass on the surface.
The result is usually low clouds, fog and steady, light precipitation.
- The amount of water vapor present, expressed as a percentage of
the maximum that could be present at the same temperature and pressure.
- An elongated axis of high
pressure.
– The condition in which evaporation and condensation or crystallization
are in balance-- i.e. taking place at an equal rate—over a
flat surface of water or ice.
- Air that is colder than its surroundings and as such is resistant
to upward movement.
- An elongated axis of low pressure.
- Air that is warmer than its surroundings and as such tends to
rise, leading to the formation of clouds and possibly precipitation.
- Air that flows up ascending terrain. May lead to low clouds, fog
and precipitation, or enhancement of ongoing precipitation produced
through other means.
- A wind that changes its direction in a clockwise motion over either
time or height. An example over time would be a west wind changing
to a northwest wind.
- Precipitation that falls from clouds but evaporates in dry air
beneath the cloud before reaching the ground.
- A measure of the amount of "spin" (or rotation) in the
atmosphere. Positive vorticity causes air to rise, which can result
in clouds and precipitation, given sufficient moisture.
- The boundary that forms where a warm air mass pushes into a colder
air mass.
- The side of an
object or terrain feature (such as a mountain) that is facing into
the wind. |
| Spring & Summer |
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See Heat Index.
– A rise in sea
level that floods coastal areas. Causes are prolonged strong onshore
flow of wind and/or high astronomical tides.
– A parameter
used to estimate the required energy for cooling. May be indexed
to any temperature; the most common is 65° F. One additional
cooling degree day occurs for each degree the daily mean temperature
is above 65° F.
- Water that forms on an object close to the ground when its temperature
falls below the dew point of the surface air.
t
- The temperature to which air must be cooled for saturation to
occur. See ‘saturation’.
- Precipitation in the form of liquid drops, less than .5 mm in
diameter.
- A boundary which separates warm, dry desert air from warm, moist
maritime tropical air. The differences in the two air masses may
be significant. The dry line is usually a boundary of instability
along which thunderstorms form. In the US, a dry line is situated
semipermanently across western Texas.
- A small but rapidly
rotating column of wind of short duration that is made visible by
dust, sand, and debris picked up from the ground. Its diameter usually
ranges from 10 to 100 feet and it develops most often on clear,
dry, hot afternoons. Fairly uncommon in this part of the country.
- Issued within 12 hours of the onset of the heat conditions listed
in the excessive heat watch.
- Issued
for the potential of the following conditions within 12 to 36 hours:
heat index of at least 105 degrees Fahrenheit for more than 3 hours
per day for 2 consecutive days or heat index more than 115 degrees
Fahrenheit for any period of time.
- A flood which is caused by heavy or excessive rainfall in a short
period of time, generally under 6 hours, leading to water that rises
and falls quite rapidly. The term may also be used to alert the
public of non life threatening flooding of small streams, streets,
storm drains, and low lying urban areas. A flash flood can also
be caused by the failure of a dam or from ice jams on waterways.
- Issued to inform the public, emergency management, and other cooperating
agencies that flash flooding is in progress, imminent, or highly
likely.
- Issued to indicate current or developing hydrologic conditions
that are favorable for flash flooding in and close to the watch
area, but the occurrence is neither certain or imminent.
- The condition that occurs when water overflows the artificial
or natural boundaries of a stream, river, or other body of water.
Also issued for the ponding of water at or near where the rain is
falling or has fallen. The term may also be used to alert the public
of non life threatening flooding of small streams, streets, storm
drains and low lying urban areas. It may also be used if small streams
in rural areas reach or exceed bankfull.
- Maximum height of a flood wave as it passes a certain location.
- The level at which a stream, river or other body of water begins
to or will begin to leave its banks.
- Issued to inform the public, emergency management, and other cooperating
agencies that flooding is in progress, imminent, or highly likely.
- A scale correlating the damage from a tornado with its wind speed.
A copy of the Fujita Tornado Damage Scale is provided with this
terminology guide.
- A rapidly rotating column of air extending from a cumulonimbus
cloud with a circulation that does not reach the ground. Once a
funnel cloud reaches the ground it is then called a tornado.
- A form of degree day used to estimate the approximate dates when
a crop will be ready to harvest. One growing degree day occurs when
the daily mean temperature is one degree above the minimum temperature
required for the growth of that specific crop.
- Precipitation in the form of transparent or partially opaque balls
or irregular lumps of concentric ice. Hail is normally defined as
having a diameter of 5 millimeters or more and is produced by thunderstorms.
- Issued within 12 hours of the onset of the following conditions:
heat index of at least 105 degrees but less than 115 degrees for
less than 3 hours per day. Nighttime lows remain above 80 degrees
for 2 consecutive days.
- (Apparent Temperature) - This index is an accurate measure of
how hot it feels (in degrees) when moisture (relative humidity)
is added to the actual air temperature.
- Precipitation in the form of liquid drops the diameter of which
must be .5 millimeters or greater.
- A thunderstorm with winds of 58 mph or greater and/or with hail
3/4 inches diameter or larger.
- Issued to warn the public, emergency management and other cooperating
agencies when a severe thunderstorm is forecast to occur or is occurring.
The warning will include where the storm was occurring, its direction
of movement and the primary threat from the storm.
- Issued
when conditions are favorable for the development of severe thunderstorms
in and close to a defined area.
-
Intermittent rainfall of short duration that falls from a cumulus
cloud. Heavy precipitation is possible. Implies more of a scattered
rainfall.
e
- A solid or nearly solid line of thunderstorms or strong winds
that might extend for several hundred miles.
- A severe thunderstorm whose updrafts and downdrafts are in near
balance allowing the storm to maintain itself for several hours.
Supercells often produce large hail and tornados.
- A rapidly rotating column of air extending from a cumulonimbus
cloud with a circulation that reaches the ground.
- Issued to warn the public, emergency management and other cooperating
agencies when a tornado is forecast to occur or is occurring. The
warning will include where the storm was occurring and its direction
of movement.
h
- Issued when conditions are favorable for the development of severe
thunderstorms and possible tornados in and close to a defined area.
- An area of clouds that extends beneath a severe thunderstorm.
If a wall cloud rotates, it might precede tornado development.
- A rapidly rotating column of air extending from a cumulonimbus
cloud with a circulation that reaches the surface of the water,
(i.e. a tornado over water). |
| Fall & Winter |
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Sustained winds or frequent
gusts of 35 mph (56 km/h) or greater, considerable falling and/or
blowing snow reducing visibility frequently to 1/4 mile (0.4 km)
or less for a period of three hours or more. There are no temperature
criteria in the definition of a blizzard but freezing temperatures
and 35 mph winds will create sub-zero wind chills.
Issued when widespread blowing
snow with winds 25 to 35 mph (40 to 56 km/h), occasionally reducing
visibility to 1/4 mile (400 m) or less, is expected.
- An uneven distribution of snowfall or existing
snow caused by strong surface wind. Drifting snow may occur during
or after a snowfall. If the snow is lifted more than 6 feet above
the ground, it is termed ‘blowing snow’.
- A condition occurring over a large area when the
surface air temperature remains below 32 degrees Fahrenheit for
an extended period of time.
- Issued during the growing season when
surface temperatures are expected to drop below freezing over a
large area for an extended period of time, regardless if frost develops
or not.
- When freezing rain or
freezing drizzle is forecast but a significant amount of it is not
expected. However, even small amounts of freezing rain or freezing
drizzle may cause traveling problems.
- Drizzle that falls in liquid form and
then freezes upon impact with the ground or an item with a temperature
of 32 degrees Fahrenheit or less. Even in small amounts, freezing
drizzle may cause traveling problems.
- Rain that falls in liquid form and then
freezes upon impact with the ground or an item with a temperature
of 32 degrees Fahrenheit or less. Even in small amounts, freezing
rain can cause traveling problems. Large amounts can pull down power
lines and tree branches.
- A covering of ice that forms on exposed surfaces
when the air temperature falls below the frost point.
- Issued during the growing season when widespread
frost formation is expected over an extensive area. Air temperatures
are usually in the mid 30s Fahrenheit at eye level.
- When the temperature to which air must be
cooled to saturation is below freezing.
– Dew that changes into tiny beads of
ice when the temperature of the surface on which it formed drops
to or below the freezing point.
– An parameter used to estimate
the required energy for heating. It may be indexed to any temperature;
the most common is 65° F. One additional heating degree day
occurs for each degree the daily mean temperature is less than 65°
F.
– In northern New England, issued
when snowfall totaling 7 inches or more in 12 hours or less is expected.
Also issued when snowfall totaling 9 inches or more in 24 hours
or less is expected.
– According to the Glossary Of Meteorology,
2nd Edition, “any one of a number of macroscopic, crystalline
forms in which ice appears, including hexagonal columns, hexagonal
platelets, dendritic crystals, ice needles, and combinations of
these forms.”
- Fog composed of ice crystals instead
of water droplets.
- Issued when damaging accumulations
of ice—at least _”-- are expected during a freezing
rain event.
- Precipitation composed of white or translucent
ice crystals, through direct phase change of water vapor to ice.
- White ice crystals that have combined
in a complex branched hexagonal form.
- Issued when snowfall is expected to exceed
2 inches but not expected to exceed 5 inches.
- Intermittent snowfall of short duration.
No measurable accumulation of snow occurs.
r - A short duration of light to moderate snowfall.
Accumulations of snow are possible.
- Precipitation consisting of transparent pellets
of ice, 5 millimeters or less in diameter. Also called ice pellets.
Forms when snow enters a warm layer of air above the surface and
melts and then enters a deep layer of sub freezing air on the surface
and refreezes.
- Issued when accumulation of sleet in excess
of 1/2 inch is expected. Relatively rare. Usually issued as a winter
storm warning for heavy sleet.
– An “apparent temperature”
which describes the cooling effect on exposed skin through the combination
of air temperature and wind. Increased wind speed will accelerate
the loss of body heat.
- Issued when winds of 10 mph or greater
are expected to create wind chill factors of 30 degrees below zero
or more.
- Issued when hazardous winter weather
is occurring, imminent or highly likely over part or all of the
area. Hazardous winter weather includes but is not limited to heavy
snow, blizzards, ice storms, freezing rain or drizzle and sleet.
- Issued for winter weather situations
that may cause significant inconvenience and, if caution is not
exercised, could lead to life threatening situations. Issued when
events are occurring, imminent or highly likely to occur. | | | | |
