Marcia Bromfield., age 9, of Dorchester, Mass., for her question:
Why are thunderstorms always on summer days?
At this moment, about 1800 thunderstorms are happening somewhere in the world. On an average, 44,000 of them burst forth in scattered places over the earth every year. Most, but not all, of them happen in summery weather, If you lived in the tropical regions of Java or Panama, you could expect 200 thunderstorms to crackle over your head every year. Our Gulf states get between 70 to 90 a year.
It is true that New England and most other land area get their thunderstorms, or most of them, on summer days. But over the oceans, most of them crack down at night and in the winter. In the Southland and the Southwest, some of the most violent thunderstorms happen in winter. And in the Midwest they tend to happen on summer nights. In fact, this helps produce the rich crops of these area, For rain falling at night has a better chance of seeping into the soil than rain falling during a hot day.
This round‑the‑world survey seems to show that a thunderstorm can burst forth just where and when it pleases. But this is not so. It can only happen when certain weather conditions in the atmosphere are just right. Two distinct layers of contrasting air are absolutely necessary. The layer of air near the ground must be light and rising. Above it, there must be a layer of much, much cooler air. A steep contrast in the temperatures of the two layers of air is necessary to make things possible for a thunderstorm.
On a summer day in New England, the scorching ground may warm the; air above it to 90 degrees or more. But the warmed layer of air near the ground does not pass on its heat to the layer of air above it. In fact, it cools itself as it expands and rises. The breezes blowing aloft, two or three miles above the ground, may be below freezing point, Such a steep contrast in the temperature of the atmosphere is sufficient to cause a summer day thunderstorm.
In the Midwest, certain weather conditions keep the cool breezes aloft at bay until after sunset. Then the contrast of temperatures upstairs builds up and we get a summer night thunderstorm. The ocean holds its summer heat far into the winter. When the sun is not shining, the air high over the ocean loses its heat faster than the air near the warmish water. Here a contrast of temperatures sufficient to produce a thunderstorm builds up on winter nights.
The contrast need not always be between hot and cold. Sometimes it may be between merely cool and bitter cold layers of air. These conditions sometimes happen in midwinter when a layer of cold, cold air frc)m Canada clashes with coolish air flowing up from the Gulf of Mexico. Some of the most violent thunderstorms occur as a result of these huge, winter cold fronts. Whenever the difference in temperature between the lower air and the higher air is steep enough, conditions are right for thunderstorms, be it day or night, winter or summer.