As a kid, you may have learned that counting the seconds between seeing lightning and hearing thunder can tell you how far away the storm is. Maybe you still find yourself habitually counting between a lightning strike and a thunderclap.
But it’s now time to advance your thunderstorm knowledge a little further. What causes these dramatic natural events, and how can you predict when the next one might arrive? Let’s take a look.
The Ingredients of a Storm
To make a thunderstorm, you start with unstable air. “Unstable air” refers to when moist warm air sits below dry cold air. When the atmosphere is stable, the weather tends to be calm. But unstable air, which moves and rises quickly, can lead to rapid changes in weather and dramatic weather events like thunderstorms.
As you may have learned before, warm air rises and cold air sinks. The rise of warm moist air is what creates clouds. However, if that warm air rises fast enough, it can lead to more exciting events like thunderstorms. The warmer the low air is and the colder the high air is, the more instability you get—and the more likely you are to see a thunderstorm.
Moisture is also a key ingredient here. Without it, rain and clouds can’t form. That’s why it’s usually humid when you see a thunderstorm.
Single-Cell, Multicell, and Supercell Storms
As the warm moist air rises, the cold air it meets cools it down, and the moisture in the air condenses into water droplets. As this air cools, it drops a bit, then warms up and rises again, creating a cycle of rising warm air and falling cool air called a convection cell.
You often encounter atmospheric convection cells in the form of wind. Convection cells also create clouds, as the cooled water droplets condense and become visible in the air. If enough moisture and air are involved in the convection cell, it can turn into a thunderstorm.
A single convection cell can create a normal-sized thunderstorm. These small storms don’t usually last much more than an hour. The rising and falling air create incredibly tall dark clouds in the sky, while the moisture in the cell often comes down as rain or hail.
However, sometimes multiple convection cells join together and make a multicell storm. Each cell of the storm will only last about an hour, but together the multicell storm can stretch for several hours and may cause more damage than a single-cell storm.
Sometimes, these storms also arrange themselves into a “squall line”—a long narrow thunderstorm along a line where the conditions for a storm are just right.
The scariest type of thunderstorm, though, is the supercell. To form a supercell, the rising air has to tilt and spin, creating what’s called a mesocyclone. If it becomes powerful enough, that spinning updraft can become a tornado.
Even without tornadoes, supercells are incredibly powerful storms. They last longer than an hour, can be up to 10 miles across and may extend 50,000 feet into the air. An anvil-shaped cloud is one of the classic marks of a supercell. Luckily, supercells are the least common type of thunderstorm.
Where Do Thunder and Lightning Come From?
Lightning begins with a process that you know best as static electricity.
The activity of the wind and moisture inside a thundercloud creates a static charge. The atmosphere usually acts as an insulator, but if the charge builds up enough, the insulation no longer works, and you get lightning.
You won’t see every lightning strike in a thunderstorm. Many of them happen inside the cloud. Yet, when an electric charge from the ground reaches toward the electrical charge in the sky, the two can meet as a visible lightning strike.
When lightning strikes, it heats the nearby air to ridiculously high temperatures (up to 54,000 degrees). This heated air expands into a shockwave that creates a thunderclap. Then, the cooling and contracting air makes a rumbling sound—so when you hear thunder, you’re really hearing the air shake from the rapid heating and cooling.
And that counting trick you may have learned as a kid really does work. Light travels faster than sound, and sound can travel one mile in about five seconds. So, if you hear thunder five seconds after you see lightning, the storm is about a mile away.
Why Are Thunderstorm Clouds So Dark?
Lightning and thunder aren’t the only impressive features of a thunderstorm, though. These storms are also characterized by intimidating dark clouds. Sometimes, you might even see unusual colors, like green, within the clouds.
These storm clouds are so dark because they’re tall enough to block sunlight from reaching the ground or the inside of the cloud. Thunderstorm clouds extend far higher into the atmosphere than typical clouds do, and they’re packed with moisture, which blocks sunlight even better and creates a dark cloud.
And if you see a green stormcloud? Make sure to keep far, far away from it: it’s a sign that a tornado could be on the way.
The water content of a stormcloud means that most of the light under the cloud is blue. Severe thunderstorms usually happen late in the day, after the sun has warmed the air enough to create unstable air conditions. The yellowish light of the late sun hits the blue light of the storm’s moisture, and our eyes pick it up as a green tinge. Not all green stormclouds create thunderstorms, but it’s a sign that you should stay out of that storm’s way.
The vast majority of thunderstorms are relatively harmless single-cell affairs. However, the occasional supercell storm can do incredible amounts of damage with rain, hail, and wind, even if a tornado isn’t present. Small or large, thunderstorms are fascinating natural events and seeing one might be even more interesting now that you know how they’re made. Just make sure to watch from a safe place, and leave the storm-chasing up to the professionals.