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Question: Why are there few, if any, thunderstorms in the winter?
Your question, like many of those before yours, points to one of the primary functions of science. That is, science attempts to find patterns in nature and account for these patterns in terms of broad reaching, interconnected conceptual theories and models. Uncovering underlying, explanatory mechanisms often requires refining our original questions and seeking additional observations. In the case of thunderstorms, we might rephrase your question in two parts: (1) What is a thunderstorm?
(2) Why are thunderstorms most common in the spring and summer months?
Thunderstorms are relatively localized (i.e., typically within a 15 mile diameter), short duration (i.e., an average of 30 minutes) rain (and/or hail) storms that produce lightning and thunder that occur most frequently over warmed land masses. Approximately 100,000 thunderstorms occur each year in the United States and some 16 million around the world.
Like food recipes, different meteorological phenomena require a specific combination of ingredients. Thunderstorms require three ingredients: (a) warm, moist air at lower atmospheric levels, (b) colder, dryer air at upper elevations (i.e., at or below 0 Celsius at 2-3 miles above the ground), and (c) a force to help lift the warm, moist air up to the colder, dryer, higher regions. This lift force may be provided by a ground level cold front, sea breezes, mountains or the sun's heat during a hot spring or summer day.
When all three ingredients are present (which occurs most commonly in the spring and summer versus the colder, dryer fall and winter seasons), the warm, moist air will form a rising column of air that begins to cool. At that point, several events occur that accelerate the rising column of air. First, gaseous water vapor begins condensing into liquid water. In accordance with the Law of Conservation of Energy, each gram of water vapor that condenses, releases the same amount of heat energy as required to convert a gram of liquid water into water vapor (water has a very high "latent heat" of vaporization/condensation, approximately 540 calories per gram). Second, this condensation makes the air warmer, dryer and less dense, causing it to rise faster via convection currents, even without additional outside lift forces. The tremendous heat energy released, the turbulent, rapidly rising air, and the downdraft caused by the falling hail or rain are converted into strong winds and the build up and release of static charges we experience as lightning and thunder.
During the pre-scientific era and even today in some primitive cultures, lightning and thunder are "explained" in terms of "angry gods." In fact, the release of visible light is caused by positively and negatively charged particles flowing back together through moist, electrically conducting air (at as great a charge separation as 100 million volts). The lightning stroke superheats the air (as high as 90,000 Celsius) along its path and the heated air rapidly expands against the cooler, surrounding air, creating a shock wave of sound energy that we call thunder. Given that a single lightning stroke between different parts of the same cloud, different clouds, or a cloud and the ground can extend over as much as a five miles in length and the fact that sound travels so much slower than light, the sound may be heard as either a loud clap or a rolling, rumbling sound depending on how close you are to the lightning stroke.
In any case, thunderstorms are truly awesome phenomenon when enjoyed from a safe location. To learn more about the science of thunderstorms and safety issues, or meteorology in general, visit the following Internet sites:
American Red Cross: http://www.redcross.org/services/disaster/0,1082,0_590_,00.html
National Climatic Data Center: http://www.ncdc.noaa.gov
National Oceanic and Atmospheric Administration: http://www.noaa.gov
National Severe Storms Laboratory: http://www.nssl.noaa.gov/
National Weather Service-Forecast Office, Binghamton Airport: http://www.nws.noaa.gov/er/bgm/
Weather Channel: http://www.weather.com
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