precipitation and Winter Weather
Falling drops of water larger than 0.02 inch in diameter. In forecasts,
"rain" usually implies that the rain will fall steadily over a period of
time. (See "showers" below).
Falls at the rate of 0.10 inch or less an hour.
Moderate rain: Falls at the rate of 0.11 to 0.30 inch an
Falls at the rate of 0.30 inch an hour or more.
Falling drops of water smaller than 0.02 inch in
diameter. They appear to float in air currents, but unlike fog, do fall to
Drizzle with visibility of more than 5/8 of a mile.
Drizzle with visibility from 5/16 to 5/8 of a mile.
Drizzle with visibility of less than 5/16 of a mile.
Rain that falls intermittently over a small area. The rain from an
individual shower can be heavy or light, but doesn't cover a large area or
last more than an hour or so.
Falling ice composed of crystals in complex hexagonal
forms. Snow forms mainly when water vapor turns directly to ice without
going through the liquid stage, a process called sublimation.
Aggregations of snow crystals.
Light showers of snow that do not cover large areas and do not fall steadily
for long periods of time.
Very small snow crystals. The ice equivalent of drizzle.
White, opaque ice particles that form as ice crystals fall through cloud
droplets that are below freezing but still liquid (supercooled). The cloud
droplets freeze to the crystals forming a lumpy mass. Scientists call snow
pellets "graupel." Such pellets falling from thunderstorms are often called
Drops of rain or drizzle that freeze into ice as they fall. They are usually
smaller than 0.30 inch in diameter. Official weather observations list sleet
as "ice pellets." In some parts of the country "sleet" refers to a mixture
of ice pellets and freezing rain.
Freezing rain or drizzle:
Falling rain or drizzle that cools below 32°F, but does not turn to ice in
the air. The water is "supercooled." When the drops hit anything they
instantly turn into ice.
A storm with large amounts of freezing rain that coats trees, power lines
and roadways with ice. Often the ice is heavy enough to pull down trees and
Falling ice in roughly round shapes at least 0.20 inch in diameter. Hail
comes from thunderstorms and is larger than sleet. Hailstones form when
upward moving air -- updrafts -- in a thunderstorm keep pieces of graupel
from falling. Drops of supercooled water hit and freeze to the graupel,
causing it to grow. When the balls of ice become too heavy for the updrafts
to continue supporting them, they fall as hailstones. Sleet, in contrast,
consists of raindrops that freeze on the way down.
A rain or snow shower in which there is lightning. Thunder is always caused
by lightning. In general, the upward and downward winds, updrafts and
downdrafts, in thunderstorms are more violent than those in ordinary
A thunderstorm with snow instead of rain falling on the ground.
A thunderstorm with winds of 57 mph or faster or hail more than 3/4 inch in
diameter reaching the ground. Severe thunderstorms can also produce
Source: The USA TODAY
National Weather Service Watches, warnings and
- Blizzard warning: Sustained winds or
frequent gusts of 35 mph occurring in combination with considerable
falling and/or blowing snow for a period of at least three hours.
Visibilities will frequently be reduced to less than one-quarter mile and
temperatures will often remain extremely cold in a blizzard.
- Heavy snow warning: Snow accumulations
are expected to approach or exceed six inches in 12 hours but will not be
accompanied by significant wind. A heavy snow warning could also be issued
if eight inches or more of accumulations are expected in a 24 hour period.
In addition, during a heavy snow warning, freezing rain and sleet are not
- Ice storm warning: A significant coating
of ice, one-quarter inch or more, is expected.
- Wind chill warning: Life-threatening wind
chills reach minus 50 or lower. Criteria varies across the nation.
- Winter storm watch:
A significant winter storm may affect your area, but its occurrence,
location and timing are still uncertain. A winter storm watch is issued to
provide 12 to 36 hours notice of the possibility of severe winter weather.
A watch will often be issued when neither the path of a developing winter
storm nor the consequences of the weather event are as yet well defined.
Ideally, the winter storm watch will eventually be upgraded to a warning
when the nature and location of the developing weather event becomes more
apparent. A winter storm watch is intended to provide enough lead time so
those who need to set plans in motion can do so.
- Winter storm warning:
Issued when hazardous winter weather is occurring, imminent or likely. A
warning is used for winter weather conditions posing a threat to life and
property. A winter storm warning are usually issued for heavy snow
approaching or exceeding six inches, ice accumulations, dangerous wind
chills, or a combination of the three. Warnings can be issued for lesser
amounts of snow, say 3 to 6 inches, if the snow occurs with strong winds
in excess of 20 miles an hour and/or significant sleet or heavy ice
accumulations from freezing rain. Expected snow accumulation during a
winter storm warning is four inches or more in 12 hours or six inches or
more in 24 hours at low, flat areas such as the Plains or South. For
mountainous areas less than or equal to 7,000 feet, a snowfall of six
inches or more in 12 hours or 10 inches or more in 24 hours would prompt a
warning. For elevations greater than 7,000 feet, snowfall of eight inches
or more in 12 hours or 12 inches or more in 24 hours would qualify for a
- Blizzard: Winds of 35 mph or more along
with considerable falling and/or blowing snow reducing visibility to less
than one-quarter mile for three or more hours. Extremely cold temperatures
often are associated with dangerous blizzard conditions, but are not a
formal part of the definition. The hazard created by the combination of
snow, wind and low visibility significantly increases, however, with
temperatures below 20 degrees.
- Blowing snow: Wind driven snow that
reduces visibility to six miles or less causing significant drifting.
Blowing snow may be snow that is falling and/or loose snow on the ground
picked up by the wind.
- Drifting snow: Uneven distribution of
snowfall caused by strong surface winds. Drifting snow does not reduce
- Flurries: Light snow falling for short
durations. No accumulation or just a light dusting is all that is
- Freeze: Occurs when the surface air
temperature is expected to be 32 degrees Fahrenheit or below over a
widespread area for a significant period of time.
- Freezing rain or drizzle: Occurs when
rain or drizzle freezes on surfaces such as trees, cars and roads, forming
a coating or glaze of ice. Temperatures above the ground are warm enough
for rain to form, but surface temperatures are below 32 degrees
Fahrenheit, causing the rain to freeze on impact. Even small accumulations
of ice can be a significant hazard.
- Frost: Describes the formation of thin
ice crystals on the ground or other surfaces. Frost develops when the
temperature of the earth's surface falls below 32 degrees Fahrenheit, but
because frost is primarily an event that occurs as the result of
radiational cooling, it frequently occurs with air temperatures in the
- Graupel: Small pellets of ice created
when supercooled water droplets coat, or rime, a snowflake. The pellets
are cloudy or white, not clear like sleet, and often are mistaken for
- Heavy snow: Depending on the region of
the USA, this generally means that four or more inches of snow has
accumulated in 12 hours, or six or more inches of snow in 24 hours.
- Ice storm: An ice storm is used to describe
occasions when damaging accumulation of ice are expected during a freezing
rain situation. Significant accumulations of ice are defined as
one-quarter inch or greater. This can cause trees, utility and power lines
to fall down causing the loss of power and communication.
- Sleet:Rain drops that freeze into ice
pellets before reaching the ground. Sleet usually bounces when hitting a
surface and does not stick to objects. However, it can accumulate like
snow and cause a hazard to motorists. Heavy sleet occurs when a half of an
inch of sleet accumulates.
- Snow showers: Snow falling at varying
intensities for brief periods of time. Some accumulation is possible.
- Snow squalls: Intense, but of limited
duration, periods of moderate to heavy snowfall, accompanied by strong,
gusty surface winds and possible lightning.
- Watch: A watch is used when the risk of a
hazardous weather event has increased significantly, but the occurrence,
location and timing are still uncertain.
- Warning/Advisory: These products are
issued when a hazardous weather event is occurring, is imminent or has a
very high probability of occurrence. A warning is used for conditions
posing a threat to life or property. Advisories are for less serious
conditions that cause significant inconvenience and, if caution is not
exercised, could lead to situations that may threaten life and property.
- Whiteout: A condition caused by falling
and/or blowing snow that reduces visibility to nothing or zero miles;
typically only a few feet. Whiteouts can occur rapidly often blinding
motorists and creating chain-reaction crashes involving multiple vehicles.
Whiteouts are most frequent during blizzards.
- Wind chill: The wind
chill is based on the rate of heat loss from exposed skin caused by the
combined effects of wind and cold. As the wind increases, heat is carried
away from the body at an accelerated rate, driving down the body
temperature. This temperature is the reading the body "feels" given the
combination of wind and air temperature. At wind speeds of four mph or
less, the wind chill temperature is the same as the actual air
Wind chill chart)