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Weather Resources - Glossary

Wondering what “graupel” is? Or how about “virga”? You have come to the right place. This section contains the definitions of several hundred weather related terms.

Please contact us if you have a suggestion for additional terms.

We have also developed a special glossary section with winter weather terms only.

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GFS - Global Forecast System. (formally known as the MRF- Medium Range Forecast model); one of the operational forecast models run at NCEP. The GFS is run four times daily, with forecast output out to 240 hours (10 days).

Global warming (or Global Climate Change) - The majority of climate scientists are in agreement that the overall warming of the planet (about 1 degree Farhenheit since 1900), has been caused in part by mankind. This warming is likely due to the increase of so called "greenhouse gases" - such as CO2, methane and CFCs (chloro-fluorocarbons). These gases absorb outgoing heat from our planet and "reflect" it back to Earth. When this happens, energy from the Sun is trapped in our atmosphere and warms our climate.

The Greenhouse Effect is normal and natural, in fact if not for this effect, the Earth would be about 60 degrees Farenheit colder - a lifeless ice planet. The problem we face is that the delicate balance of temperature may be upset by a change in atmospheric chemistry. In the past 200 years (since the Industrial Revolution) the increased burning of fossil fuels has released vast amounts of carbon dioxide into the atmosphere. The concentration of CO2 has risen about 25% in the past two centuries from 280 parts per million to nearly 360 parts per million. Everyday human activity releases about seven billion metric tons of carbon dioxide into the air every year - adding to the 750 billion metric tons that are already there. Of the 7 billion tons, only about three billion tons stays in the atmosphere; the rest is absorbed by plants and the oceans. This "carbon sink" capacity complicates the issue of global warming, because the oceans may continue to have a vast holding capacity for CO2. Some scientists feel that the increase in atmospheric CO2 will be offset by the ability of plants and the oceans to absorb this gas. In fact, some experts believe that the increase in CO2 will be a good thing - improving crop yields and making more parts of the world able to support crops. At the same time, others worry that warming will cause more severe droughts in key agricultural areas.

Graupel -Also known as “snow pellets”, graupel is a form of frozen precipitation consisting of snowflakes and supercooled water droplets frozen together. It is softer than hail and forms through a complex process when water condenses on a snowflake causing the snowflake to become a small ball of ice. Graupel is most common in Colorado during a “thundersnow” event because the static buildup from falling graupel pellets can cause lightning. Colorado sees more graupel each year than almost anywhere else in the country because our high elevation can set up the necessary atmospheric conditions for graupel to develop.

Ground Clutter - A pattern of radar echoes from fixed ground targets (buildings, hills, etc.) near the radar. Ground clutter may hide or confuse precipitation echoes near the radar antenna.

Gulf Stream - A narrow current of warm ocean water that flows along the east coast of the United States. This is the breeding grounds for some of the intense winter storm that take place in the Northeastern United States.

Gust Front - The leading edge of the cool downdraft of air that comes out of the bottom of a thunderstorm. As a gust front passes a location it resembles a cold front. The leading edge of gusty surface winds from thunderstorm downdrafts; sometimes associated with a shelf cloud or roll cloud. See also downburst, gustnado, outflow boundary.

Gustnado (or Gustinado) - [Slang], gust front tornado. A small tornado, usually weak and short-lived, that occurs along the gust front of a thunderstorm. Often it is visible only as a debris cloud or dust whirl near the ground. Gustnadoes are not associated with storm-scale rotation (i.e. mesocyclones); they are more likely to be associated visually with a shelf cloud than with a wall cloud.

Portions of the 9NEWS Weather Glossary were taken from the second edition of the Glossary of Meteorology published by the American Meteorological Society (AMS). © 2009 American Meteorological Society