Thunderstorms are one of the most common types of hazardous weather you will experience if you live in the United States. On average, we see 100,000 thunderstorms develop in the U.S. each year, according to the National Severe Storms Laboratory. So how do they form? You really need three things: moisture, instability, and lift. Each are kind of a prerequisite for the next, in a way.
Without an abundance of moisture in the atmosphere, it’s really hard to get precipitation. So, it makes sense that this is the first step in the process. You need moisture in the air in order to condense it into water droplets that will be heavy enough to fall from the sky. In the United States (particularly in the eastern half of the country), moisture is fairly easy to come by in the spring and summer months thanks to the Gulf of Mexico. Whenever there is a wind out of the south/southwest, warm and moist air is easily transported north.
Instability is a huge piece of the puzzle for thunderstorms. Without it, you just have humid air trapped near the surface! When air rises, if it is warmer than the air that surrounds it, it will keep rising. This is what we call “unstable” air. If the air is stable, it means that it is cooler and heavier than the air that surrounds it. If it's cooler and heavier than the air around it, the air won’t rise, but sink. Think about how you can sometimes see the cold air flowing out of your freezer. That’s this concept in action!
This is a great time to start talking about the last piece of the puzzle: lift. Without something to “force” or “lift” the air upward, it’s pretty difficult to get it to move on its own. But, once it gets a push, it will rise with ease. There are quite a few different types of lift that we can talk about, but the most common ones are:
- Fronts (warm, cold, or occluded)
- Smaller-scale boundaries (sea-breeze fronts, gusty winds that rush out ahead of an existing thunderstorm)
- Terrain (mountains and hills)
- Upper-level disturbances
- Or just on a warm, humid day… because warm air rises!
Any one of these types of “lift” will be sufficient, but the most common you will hear about is a front.
When you have those ingredients together... moisture, instability, and a source of lift… you have the necessary requirements for a thunderstorm to develop. You can think of their development in three stages: the developing or cumulus stage, the mature stage, and the dissipating stage.
Illustration of thunderstorm stages from NOAA
In the cumulus stage, warm, moist air is rising, causing the moisture in the air to condense. This will continue as more air is fed upward.
In the mature stage, the moisture in the air will continue to condense, and eventually it will be too heavy to remain suspended in the air. This is when the rain begins to fall out of the storm! The heavy rain will drag down cool air with it, creating a downdraft. This is where the storm can become strong due to additional factors like wind shear that can help maintain its life cycle.
In the absence of other factors to help it along, the storm will then reach the dissipating stage. In the dissipating stage, the downdrafts in the cloud become stronger than the updraft. This overwhelms the feed of warm, moist air, and the storm gradually weakens with lighter rain until it dissipates. A gust front rushes out in front of the storm, and further prevents warm, moist air from being driven into the cell. Lightning does remain a concern until the cell dissipates.
So, with moisture, instability, and lift all combining together, thunderstorm development is a good bet. There are other factors that can limit or enhance the development of storms, but this blog just focuses on basic thunderstorm development. There is certainly more to think about once storms enter severe criteria!