Thunderstorms are small-scale severe weather events associated with frequent lightning, high winds, and heavy rainfall. They can and do occur at any time of the year, but are most likely to happen during the afternoon and evening hours and during the spring and summer seasons.
Thunderstorms are so called because of the thunderous loud noise they make. Since the sound of thunder comes from lightning, all thunderstorms have lightning. If you've ever seen a thunderstorm off in the distance but not heard it, you can rest assured there's a thunderclap -- you're simply too far away to hear its sound.
Besides looking at the weather radar, another way to detect a growing thunderstorm is to look for cumulonimbus clouds. Thunderstorms are created when air near the ground is heated and is transported upward into the atmosphere -- a process that's known as "convection." Since cumulonimbus clouds are clouds that extend vertically up into the atmosphere, they're often a sure-fire sign that strong convection is taking place. And where there's convection, storms are sure to follow.
One point to remember is that the higher the top of a cumulonimbus cloud, the more severe the storm.
Contrary to what you may think, not all thunderstorms are severe. The National Weather Service does not call a thunderstorm "severe" unless it's capable of producing one or more of these conditions:
Severe thunderstorms often develop ahead of cold fronts, an area where warm and cool air strongly oppose. Vigorous rising occurs at this opposition point and produces stronger instability (and therefore more intense weather) than the everyday lift that feeds local thunderstorms.
Thunder (the sound made by a lightning flash) travels approximately one mile per 5 seconds. This ratio can be used to estimate how many miles away a thunderstorm may be. Simply count the number of seconds ("One-Mississippi, Two-Mississippi...) between seeing a lightning flash and hearing a thunderclap and divide by 5!
info by www.thoughtco.com