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Advisory: Weather that does not meet severe weather criteria, but does pose a hazard to the public.
Average Temperature: The average of the high and low temperatures for that time period.
Equation: Avg. Temp.= (High temp.+ Low temp.)/2
Cooling Degree Day (CDD): Computed by taking the difference between the daily mean temperature and 65°F. CDD = 0 if the daily average temperature is colder than 65°F. Summed over the cold season (January 1 through December 31).
Equation: CDD = Avg. Temp. - 65
Dew Point Temperature: The temperature at which the current amount of moisture in the air would saturate it (100% relative humidity). This is a better measure of moisture than relative humidity because dew point temperature only changes when the amount of moisture in the air changes.
Fall (Meteorological): September 1 through November 30
Heating Degree Day (HDD): Computed by taking the difference between the daily mean temperature and 65°F. HDD = 0 if the daily average temperature is warmer than 65°F. Summed over the cold season (July 1 through June 30).
Equation: HDD = 65 - Avg. Temp.
Heat Index: Apparent temperature that combines the effect of temperature and humidity to show heat stress on the body. Conditions usually require a temperature of 90°F and a dew point temperature of at least 65°F
Chart: Weatherimages.org NWS Headquarters NWS Little Rock
Heat Safety Brochure: National Weather Service Source Number 2
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Infrared (satellite image): Satellite images that detect infrared radiation. Since the radiation depends on temperature, these images are color coded according cloud-top, or surface (in clear area), temperatures. Colors or brighter-white areas usually mean colder temperatures or higher clouds. During the summer, cold cloud tops usually mean thunderstorms, but this can be misleading during the winter or in areas with only high cirrus clouds.
Relative Humidity: The amount of moisture the air contains divided by the moisture capacity of the air at that temperature. Relative humidity is a poor measurement of the amount of moisture in the air since the capacity of the air roughly doubles for every temperature increase of 18°F. Dew point temperature is preferred by meteorologists.
Equation: RH = (content/capacity) x 100%
Severe Thunderstorm: Any thunderstorm that produces hail of at least 3/4 inch in diameter, wind gusts of 55 MPH, and/or a tornado.
Spring (Meteorological): March 1 through May 31
Summer (Meteorological): June 1 through August 31 (the three warmest months of the year)
Trace (rain): Rainfall that amounts to less than 0.01 inch accumulation
Trace (snow): Snowfall that amounts to less than 0.1 inch accumulation
Visible (satellite imagery): Satellite view of visible light. The image is not quite visible light, since the amount of "brightness" over a 1-km, 4-km, or 16-km grid is shown.
Warning: Severe weather is imminent (within the hour for thunderstorm, tornado, or flood; within a day for winter weather or hurricane)
More details: See Michael Branick's Severe Weather Glossary
Watch: Severe weather condition is possible within the 3-6 hrs (thunderstorm, tornado, or flood) or within the next 1-2 days (winter weather or hurricane)
More details: See Michael Branick's Severe Weather Glossary
Water Vapor (satellite image): Satellite sensor that detects water vapor in the middle of the troposphere (400-700 mb). Milky white areas show regions of high middle-atmosphere moisture. The steering wind flow can also be seen by circulation patterns. Bands of moisture adjacent to dry areas depict the jet stream.
Wind Chill: Index that represents the rate of heat loss from the human body due to the combined effect of cold temperature and wind.
Chart: NWS Sioux Falls wind chill chart NWS Sioux Falls Wind Chill Page
Winter weather safety brochure: National Weather Service Winter Weather Awareness Page
Winter (Meteorological): December 1 through February 28 or 29 (the three coldest months of the year)
Z-time: Zulu time or Greenwich Mean Time. The standard time along 0° Longitude. This time differs from Central Time as follows:
CST = Z-time - 6 hours
CDT = Z-time - 5 hours
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Other Sources
Atmospheric Electricity Glossary by Richard E. Orville Texas A&M University
Climate and Weather Terms from Desert Research Institute
Forecast Terminology from NWS Louisville, KY
Meteorological Abbreviations and Acronyms from NWS Hydrologic Prediction Center
Meteorological Acronyms by Dan McCarthy, NWS Storm Prediction Center
Meteorology Glossary for Teachers from EarthStorm Program, Oklahoma Climatological Survey, University of Oklahoma
NWS Glossary from NWS Headquarters
Severe Weather Glossary by Mike Branick NWS Norman, OK
Severe Weather Terms from NWS Louisville, KY
Storm Prediction Center Frequently Asked Questions
Tropical Prediction Center Glossary
Tropical Prediction Center Frequently Asked Questions
Weather World 2010 from The University of Illinois
