Glossary of Weather Terms

Advisory - Advisories are issued for weather situations that cause significant inconveniences but do not meet warning criteria and, if caution is not exercised, could lead to life -threatening situations. Advisories are issued for significant events that are occurring, are imminent, or have a very high probability of occurrence.

Altitude - Height expressed as the distance above a reference point, which is normally sea level or ground level.

Anemometer - An instrument that measures wind speed.

Barometer - An instrument for measuring atmospheric pressure.

Barometric pressure - The actual pressure value indicated by a pressure sensor.

Beaufort Scale - A scale that indicates the wind speed using the effect wind has on certain familiar objects.

Celsius - a temperature scale in which zero is the freezing point of water and one hundred is the boiling point.

Clear - Sky condition of less than 1/10 cloud coverage.

Climate - The prevalent long term weather conditions in a particular area. Climatic elements include precipitation, temperature, humidity, sunshine and wind velocity and phenomena such as fog, frost, and hail storms. Climate cannot be considered a satisfactory indicator of actual conditions since it is based upon a vast number of elements taken as an average.

Climate change - This strictly refers to all forms of climatic inconsistency. But it is often used in a more restricted sense to imply a significant change. Within the media, climate change has been used synonymously with global warming. Scientists, however, use the term in a wider sense to include past climate changes also.

Cloud - A visible cluster of tiny water and/or ice particles in the atmosphere.

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 7/10ths or more of the sky is covered by clouds.

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.

Cooling Degree Day - A form of degree day used to estimate the required energy for cooling. one cooling degree day occurs for each degree the daily mean temperature is above 65 degrees Fahrenheit.

Degree Day - a measure of the departure of the daily mean temperature from the normal daily temperature; heating and cooling Degree Days are the departure of the daily mean temperature from sixty -five degrees Fahrenheit.

Dense Fog - a fog in which the visibility is less than one -quarter mile.

Depression - a region of low atmospheric pressure that is usually accompanied by low clouds and precipitation.

Dew - Moisture from water vapor in the air that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature.

Dew Point - The temperature to which the air must be cooled for water vapor to condense and form fog or clouds.

Drifting snow - Uneven distribution of snowfall caused by strong surface winds. Drifting snow does not reduce visibility.

Drizzle - Small, slowly falling water droplets, with diameters between .2 and .5 millimeters.

Drought - Abnormally dry weather in a region over an extended period sufficient to cause a serious hydrological (water cycle) imbalance in the affected area. This can cause such problems as crop damage and water -supply shortage.

Fahrenheit - the standard scale used to measure temperature in the United States; in which the freezing point of water is thirty -two degrees and the boiling point is two hundred and twelve degrees.

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.

Fog - Water that has condensed close to ground level, producing a cloud of very small droplets that reduces visibility to less than one km (three thousand and three hundred feet).

Forecast - A forecast provides a description of the most significant weather conditions expected during the current and following days. The exact content depends upon the intended user, such as the Public or Marine forecast audiences.

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 - The change in a substance from a liquid to a solid state.

Freezing fog - A suspension of numerous minute ice crystals in the air, or water droplets at temperatures below 0 Celsius, based at the Earth's surface, which reduces horizontal visibility; also called ice fog.

Freezing Rain - 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.

Frost - The formation of thin ice crystals on the ground or other surfaces. Frost develops when the temperature of the exposed surface falls below 32 degrees Fahrenheit and water vapor is deposited as a solid.

Gale - Sustained wind speeds from 34 to 47 knots (39 to 54 mph).

Greenhouse Effect - The warming of the atmosphere by the trapping of longwave radiation (heat) being radiated to space. The gases most responsible for this effect are water vapor and carbon dioxide.

Growing Degree Day - A form of degree day 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.

Growing Season - The period of time between the last killing frost of spring and the first killing frost of autumn.

Gust - A brief sudden increase in wind speed. Generally the duration is less than 20 seconds and the fluctuation greater than 10 mph.

Hail - Precipitation in the form of balls or irregular lumps of ice produced by liquid precipitation, freezing and being coated by layers of ice as it is lifted and cooled in strong updrafts of thunderstorms..

Heat Index - An index that combines air temperature and humidity to give an apparent temperature (how hot it feels). Here is a heat index formula originally from Weatherwise magazine. It gives valid results above 70 deg. F.
( -42.379+2.04901523*t+10.14333127*r -.22475541*t*r -(6.83783e -3)*t^2 -(5.481717e -2)*r^2+(1.22874e -3)*t^2*r+(8.5282e -4)*t*r^2 -(1.99e -6)*t^2*r^2)
t=temp deg f and r=%rel hum

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.

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.

High - An area of high pressure, usually accompanied by anticyclonic and outward wind flow. Also known as an anticyclone.

Humidity - The amount of water vapor in the atmosphere.

Hurricane - A severe tropical cyclone with sustained winds over 74 mph (64 knots). Normally applied to such storms in the Atlantic Basin and the Pacific Ocean east of the International Date Line.

Hygrometer - An instrument used to measure humidity.

Indian Summer - An unseasonably warm period near the middle of autumn, usually following a substantial period of cool weather.

Isobar - A line of equal barometric pressure on a weather map.

Isotherm - A line of equal temperature on a weather map.

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 systems.

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 latitude.

Lightning - Any form of visible electrical discharges produced by thunderstorms.

Low - An area of low pressure, usually accompanied by cyclonic and inward wind flow. Also known as a cyclone.

Maximum Temperature - The highest temperature during a specified time period.

Mean Temperature - The average of a series of temperatures taken over a period of time, such as a day or a month.

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.

Meteorologist - A person who studies meteorology. Some examples include research meteorologist, climatologist, operational meteorologist, TV meteorologist.

Meteorology - The study of the physics, chemistry, and dynamics of the atmosphere and the direct effects of the atmosphere upon the Earth's surface, the oceans, and life in general.

Microclimate - A local climate that differs from the main climate around it.

Millibar - A metric unit of atmospheric pressure. 1 mb = 100 Pa (pascal). Normal surface pressure is approximately 1013 millibars.

Minimum Temperature - The lowest temperature during a specified time period.

Mist - Consists of microscopic water droplets suspended in the air which produce a thin grayish veil over the landscape. It reduces visibility to a lesser extent than fog.

Muggy - Colloquially descriptive of warm and especially humid weather.

NOAA - National Oceanic and Atmospheric Administration. A branch of the U.S. Department of Commerce, NOAA is the parent organization of the National Weather Service.

Normal - The long -term average value of a meteorological element for a certain area. For example, "temperatures are normal for this time of year" Usually averaged over 30 years.

Overcast - Sky condition when greater than 9/10 of the sky is covered by clouds.

Partly Cloudy - Sky condition when between 3/10 and 7/10 of the sky is covered. Used more frequently at night.

Partly Sunny - Similar to partly cloudy. Used to emphasize daytime sunshine.

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 pressure.

Radar - An instrument used to detect precipitation by measuring the strength of the electromagnetic signal reflected back. (RADAR= Radio Detection and Ranging)

Rain - Liquid water droplets that fall from the atmosphere, having diameters greater than drizzle (0.5 mm).

Rain Gauge - An instrument used to measure rainfall amounts.

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).

Ridge - An elongated area of high pressure in the atmosphere. Opposite of a trough.

Scattered - A cloud layer that covers between 3/8ths and 1/2 of the sky.

Shower - Precipitation that is intermittent, both in time, space or intensity.

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.

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. Forms when snow enters a warm layer of air above the surface and melts and then enters a deep layer of sub freezing air near the surface and refreezes.

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 accumulation.

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.

Soil Moisture Deficit - Soil Moisture Deficit (SMD) is the amount of water needed to bring the soil moisture content back to field capacity, which is the amount of water the soil can hold against gravity. For example, say you have a pot plant. field capacity is the amount of water you can give the pot plant without it leaking out of the bottom. Too much water and you get a negative SMD (the pot plant leaks), too little and you have a positive SMD (the pot plant needs watering). Just enough and the SMD = 0.

Station Identifier
- A group of four alphabetic characters used to identify a location that makes weather observations.

Station Pressure - The pressure that is read from a barometer but is not adjusted to sea level.

Storm - In marine usage, winds 48 knots (55 mph) or greater.

Temperature - a measure of the warmth or coldness of an object or substance with reference to a standard value.

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 temperature.

Thermometer - An instrument for measuring temperature.

Thunder - The sound caused by a lightning stroke as it heats the air and causes it to rapidly expand.

Thunderstorm - A storm with lightning and thunder, produced by a cumulonimbus cloud, usually producing gusty winds, heavy rain and sometimes hail.

Tornado - A violent rotating column of air, in contact with the ground, pendant from a cumulonimbus cloud. A tornado does not require the visible presence of a funnel cloud. It has a typical width of tens to hundreds of meters and a lifespan of minutes to hours.

Trough - An elongated area of relatively low atmospheric pressure surface or aloft. Usually not associated with a closed circulation, and thus used to distinguish from a closed low. The opposite of ridge.

UKMET - United Kingdom forecast model.

Visibility - The greatest horizontal distance an observer can see and identify a prominent object.

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.

Weather - State of the atmosphere with respect to heat or cold, wetness or dryness, calm or storm, clearness or cloudiness. Also, weather is the meteorological day -to -day variations of the atmosphere and their effects on life and human activity. It includes temperature, pressure, humidity, clouds, wind, precipitation and fog.

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.

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).

For a complete list of terms used by meteorologists, forecasters, weather observers, and in weather forecasts, click here.