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Terms

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 from NWS: Heat Index calculated from RH

Calculator using Dew Point from NWS: Heat Index Calculator
Heat Safety Brochure: National Weather Service 

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 temperatures. Brighter-white areas usually mean colder temperatures or higher 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 since capacity doubles every ~18°F increase. Dew point temperature is preferred.
Equation: RH = (content / capacity) × 100%

Severe Thunderstorm: Produces hail ≥ 3/4 inch, wind gusts ≥ 55 MPH, and/or a tornado.

Spring (Meteorological): March 1 through May 31.

Summer (Meteorological): June 1 through August 31.

Trace (rain): Rainfall less than 0.01 inch.

Trace (snow): Snowfall less than 0.1 inch.

Visible (satellite imagery): Satellite view of visible light.

Warning: Severe weather is imminent.
More details: NWS Binghamton, NY Severe Weather Glossary

Watch: Severe weather is possible.
More details: NWS Binghamton, NY Severe Weather Glossary

Water Vapor (satellite image): Satellite sensor that detects mid-tropospheric moisture.

Wind Chill: Index representing heat loss from cold/wind.
Chart: NWS Wind Chill Chart 

Winter (Meteorological): December 1 through February 28/29

Z-time: Zulu or GMT.
CST = Z-time - 6 hours
CDT = Z-time - 5 hours

Other Sources

• Forecast Weather Terms from NWS Binghamton, NY
• Full Weather Glossary from NWS Spokane, WA
• Meteorological Abbreviations and Acronyms from NWS 
• Meteorological Acronyms and Severe Weather Glossary from NWS St. Louis
• National Weather Service Education site
• NWS Glossary from NWS Headquarters
• NWS Safety Pages (see Materials for Use in each category)
• NWS Storm Prediction Center Educational Resources (use Outreach tab)
• NWS Storm Prediction Center Frequently Asked Questions
• NWS Tropical Prediction Center (use Educational Resources)
• Tropical Prediction Center Frequently Asked Questions
• Weather Study Guides and Real-Time Weather Links from American Meteorological Society
• Weather Teacher Guides from NWS Shreveport, LA
• Weather Terms from NWS Columbia, SC
• Weather World 2010 from The University of Illinois

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