More Weather Terms & Definitions
COMBINED SOURCE WX GLOSSARY
Air Quality Index (AQI) - The United States Environmental
Protection Agency (EPA) has developed an Air Quality Index
that is used to report air quality. This AQI is divided into six
categories indicating increasing levels of health concern. An
AQI value over 300 represents hazardous air quality and
below 50 the air quality is good.
The AQI is based on the five "criteria" pollutants regulated
under the Clean Air Act: ground-level ozone, particulate
matter, carbon monoxide, sulfur dioxide, and nitrogen
dioxide. The EPA has established National Ambient Air
Quality Standards (NAAQS) for each of these pollutants in
order to protect public health. An AQI value of 100 generally
corresponds to the level of the NAAQS for the pollutant. The
Clean Air Act (USA) (1990) requires the EPA to review its
National Ambient Air Quality Standards every five years to
reflect evolving health effects information. The Air Quality
Index is adjusted periodically to reflect these changes.
Clear - In terms of octants, with the sky having 1/8th or less
of opaque cloud coverage.
Cloud - A visible cluster of tiny water and/or ice particles in
Cloud Base - For a given cloud or cloud layer, it is the lowest
level in the atmosphere where cloud particles are visible
Cloudy - The state of the sky when When 7/8ths to 8/8ths of
the sky is covered by clouds. Aviation forecasts utilize the
term OVC (“Overcast”) with 8/8ths of opaque sky coverage.
Cold Front - A narrow transition zone separating advancing
colder air from retreating warmer air. The air behind a cold
front is cooler and typically drier than the air it is replacing.
Dense Fog - a fog in which the visibility is less than one-
Drizzle - Small, slowly falling water droplets, with diameters
between 0.2 and 0.5 millimeters. Precipitation consisting of
numerous minute droplets of water less than 0.5 mm (500
micrometers) in diameter.
Fair - describes weather in which there is less than 4/10ths
of opaque cloud cover, no precipitation, and there is no
extreme visibility, wind or temperature conditions.
Flurries - Snow flurries are an intermittent light snowfall of
short duration (generally light snow showers) with no
measurable accumulation (trace category).
Freezing Drizzle - A drizzle that falls as a liquid but freezes
into glaze or rime upon contact with the cold ground or
Freezing Rain - Rain that falls as a liquid but freezes into
glaze upon contact with the ground. Rain that freezes on
objects such as trees, cars and roads, forming a coating or
glaze of ice. Temperatures at higher levels are warm enough
for rain to form, but surface temperatures are below 32
degrees Fahrenheit, causing the rain to freeze on impact.
Front - The boundary or transition zone between two
different air masses. The basic frontal types are cold fronts,
warm fronts and occluded fronts. Occluded fronts usually
form around mature low pressure areas. There are two types
of occlusion, warm and cold: In a cold occlusion, the cold air
mass overtaking the warm front is colder than the cool air
ahead of the warm front, and plows under both air masses.
In a warm occlusion, the cold air mass overtaking the warm
front is warmer than the cool air ahead of the warm front,
and rides over the colder air mass while lifting the warm air.
Gust - A brief sudden increase in wind speed. Generally the
duration is less than 20 seconds and the fluctuation greater
than 10 mph.
Heating Degree Day - A form of degree day used to estimate
the required energy for heating. One heating degree day
occurs for each degree the daily mean temperature is below
65 degrees Fahrenheit.
Hail - Showery precipitation in the form of irregular pellets
or balls of ice more than 5 mm in diameter, falling from a
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. This generally means...
• snowfall accumulating to 4" or more in depth in 12 hours or
• snowfall accumulating to 8" or more in depth in 24 hours or
In forecasts, snowfall amounts are expressed as a range of
values, e g.. "8 to 12 inches." However, in heavy snow
situations where there is considerable uncertainty
concerning the range of values, more appropriate phrases
such as "...up to 12 inches..." or alternatively "...8 inches or
Isobar - A line of equal barometric pressure on a weather
Jet Stream - Strong winds concentrated within a narrow
band in the upper atmosphere. It normally refers to
horizontal, high -altitude winds. The jet stream often
"steers" surface features such as front and low pressure
Knot - A measure of speed. It is one nautical mile per hour
(1.15 mph). A nautical mile is one minute of one degree of
Low - An area of low pressure, usually accompanied by
cyclonic and inward wind flow. Also known as a cyclone.
METAR - A weather observation near ground level. It may
include date and time, wind, visibility, weather and
obstructions to vision, sky condition, temperature and dew
point, sea level pressure, precipitation amount and other
data used for aircraft operations.
Millibar - A metric unit of atmospheric pressure. 1 mb = 100
Pa (Pascal). Normal surface pressure is approximately 1013
Mostly Clear - When 1/8th to 3/8ths of the night sky is
covered by with opaque (not transparent) clouds. The sky is
referred to as “Mostly Sunny” if this condition is present
during daylight hours. Aviation terminology for this sky
condition is FEW (“trace”)
Mostly Cloudy - When 5/8ths to 7/8ths of the night time sky
is covered by with opaque (not transparent) clouds. Daylight
hours would us the term “Considerable Cloudiness.” Aviation
terminology for this degree of sky coverage is BKN
Overcast - Aviation term for a sky condition having 9/10 or
more of the sky covered with opaque clouds. Abbreviated as
OVC. An official sky cover classification for aviation weather
observations, when the sky is completely covered by an
obscuring phenomenon. This is applied only when obscuring
phenomenon aloft are present—that is not when obscuring
phenomenon are surface-based, such as fog. Corresponds
with NWS definition of “Cloudy,” with 8/8ths coverage.
Partly Cloudy - When the night time sky is covered with from
3/8ths to 5/8ths of opaque (not transparent) clouds. The
daytime term used to describe this sky condition is “Partly
Sunny.” Aviation sky conditions are described as SCT
(“scattered”) with sky coverage from 3/8ths to 4/8ths.
Partly Sunny - Similar to partly cloudy. Used to emphasize
Precipitation - Liquid or solid water that falls from the
atmosphere and reaches the ground.
Pressure - The force exerted by the interaction of the
atmosphere and gravity. Also known as atmospheric
Relative Humidity - The amount of water vapor in the air,
compared to the amount the air could hold if it was totally
saturated. (Expressed as a percentage).
Sky Condition - The state of the sky in terms of such
parameters as sky cover, layers and associated heights,
ceiling, and cloud types.
Sky Cover - The amount of the sky which is covered by clouds
or obscurations in contact with the surface.
Sky Condition - Used by NWS in a forecast to describe the
predominant average sky condition based upon octants
(eighths) of the sky covered by opaque (not transparent)
Clear / Sunny
0/8 to 1/8
Mostly Clear/Mostly Sunny
1/8 to 3/8
Party Cloudy/Partly Sunny
3/8 to 5/8
Mostly Cloudy/Considerable Cloudiness
5/8 to 7/8
7/8 to 8/8
Fair (mainly for night)
Less than 4/10
opaque clouds, no
Sleet - (PL) - Sleet is defined as pellets of ice composed of
frozen or mostly frozen raindrops or refrozen partially
melted snowflakes. Transparent or translucent precipitation
in the form of pellets of ice that are smaller than 5 mm (.2
in). These pellets of ice usually bounce after hitting the
ground or other hard surfaces. “Heavy” sleet is a relatively
rare event and can be defined as an accumulation of ice
pellets covering the ground at a rate of 0.30”/hr or more.
Snow - Frozen precipitation composed of ice particles in
complex hexagonal patterns. Snow forms in cold clouds by
the direct transfer of water vapor to ice.
Snow Depth - The vertical height of frozen precipitation on
the ground. For this purpose, frozen precipitation includes
ice pellets, glaze, hail, any combination of these, and sheet
ice formed directly or indirectly from precipitation.
Snow Flurries - Light snow showers, usually of an
intermittent nature and short duration with no measurable
Snow Shower - Snow falling at varying intensities for brief
periods of time. Some accumulation is possible.
Snowfall - The depth of new snow that has accumulated
since the previous day or since the previous observation.
Station Pressure - The pressure that is read from a
barometer but is not adjusted to sea level. Atmospheric
Temperature-humidity-sun-wind (THSW) index - Like Heat
Index, the THSW Index uses humidity and temperature to
calculate an apparent temperature. In addition, THSW
incorporates the heating effects of solar radiation and the
cooling effects of wind (like wind chill) on our perception of
Warm Front - A narrow transitions zone separating
advancing warmer air from retreating cooler air. The air
behind a warm front is warmer and typically more humid
than the air it is replacing.
Warning - Forecast issued when a particular weather or
flood hazard is "imminent" or already occurring (e.g.,
tornado warning, flash flood warning). A warning is used for
conditions posing a threat to life or property.
Wind - Air in motion relative to the surface of the earth.
Wind Chill - The additional cooling effect resulting from wind
blowing on bare skin. The wind chill is based on the rate of
heat loss from exposed skin caused by the combined effects
of wind and cold. The (equivalent) wind chill temperature is
the temperature the body "feels" for a certain combination
of wind and air temperature
Wind Direction - The direction from which the wind is
blowing, the compass direction from which the wind is
Wind Speed - The rate at which air is moving horizontally
past a given point. It may be a 2 -minute average speed
(reported as wind speed) or an instantaneous speed
(reported as a peak wind speed, or gust).