What Causes Thunder And Lightning? How Far Away Is a Storm?

A flash of lightning followed by a rumble of thunder is one of the most dramatic signs that a thunderstorm is nearby. But lightning and thunder aren't separate events—they're two parts of the same phenomenon.

Understanding what causes them also explains why you see lightning before you hear thunder and how you can use that delay to estimate how far away a storm is.

What Causes Lightning?

Lightning begins inside a thunderstorm, where powerful updrafts and downdrafts keep water droplets, ice crystals and small pieces of hail moving rapidly through the cloud.

As these particles collide, electrical charges become separated. In a typical thunderstorm, positive charges tend to accumulate near the top of the cloud while negative charges become concentrated lower down.

The growing electrical imbalance creates an increasingly strong electric field.

Eventually, the difference in charge becomes great enough to overcome the insulating properties of the surrounding air. Electricity suddenly travels through the atmosphere, producing a lightning discharge.

Lightning can occur within a single cloud, between clouds or between a cloud and the ground. In fact, much of the lightning produced by thunderstorms never reaches the ground at all. And lightning can occur even when no rain is falling where you are.

What Causes Thunder?

Thunder is the sound created by lightning. A lightning channel heats the surrounding air extremely rapidly, reaching temperatures of roughly 50,000°F (about 28,000°C)—several times hotter than the surface of the Sun.

That sudden heating causes the air around the lightning channel to expand explosively. The resulting pressure wave travels outward through the atmosphere as sound.

That's thunder.

Why Do You See Lightning Before You Hear Thunder?

Lightning and thunder occur essentially at the same time, but light travels enormously faster than sound.

Light reaches you so quickly that, at ordinary storm distances, the delay is effectively unnoticeable. Sound is much slower. Under typical conditions, it travels through air at roughly 1,100 feet per second, or about one-fifth of a mile per second. That's why you see the flash first and hear the thunder several seconds later.

The farther away the lightning is, the longer the delay.

How to Tell How Far Away a Thunderstorm Is

You can use the time between lightning and thunder to make a quick estimate of the lightning's distance.

When you see a flash of lightning, start counting the seconds until you hear thunder. Then divide the number of seconds by 5 to estimate the distance in miles.

For example:

• 5 seconds = about 1 mile away
• 10 seconds = about 2 miles
• 15 seconds = about 3 miles
• 30 seconds = about 6 miles

This is often called the flash-to-bang method.

It's only an estimate because the speed of sound varies somewhat with atmospheric conditions, but it's accurate enough to give you a useful idea of how close the lightning is.

Why Does Thunder Sometimes Crack and Sometimes Rumble?

If lightning is nearby, thunder may sound like a sudden, sharp crack or boom. More distant lightning often produces a long, rolling rumble.

One reason is that a lightning bolt can stretch for miles. Sound from different portions of that long channel reaches you at slightly different times. The sound can also reflect and bend as it travels through layers of air with different temperatures and densities, adding to the rolling effect.

A single lightning discharge can therefore produce thunder that lasts for many seconds.

Can You Have Lightning Without Thunder?

If you can see lightning but can't hear thunder, the lightning is usually simply too far away for the sound to reach you clearly.

This is sometimes called heat lightning, especially when distant flashes are visible on warm summer nights. Heat lightning isn't a special type of lightning caused by heat. It's ordinary lightning from a thunderstorm far enough away that its thunder is no longer audible.

How Far Away Can You Hear Thunder?

Thunder can typically be heard from roughly 10 miles away, although atmospheric conditions and background noise can change that considerably. That means if you can hear thunder, lightning is already close enough to be a potential danger.

You also shouldn't assume a storm is safely distant just because the rain hasn't arrived. Lightning can strike outside the area where rain is falling and several miles from the main part of a thunderstorm. A thunderstorm's wind and other signs can also arrive before its rain; darkening skies, sudden cool air and gust fronts can all signal an approaching storm.

Does Counting Seconds Tell You Whether a Storm Is Getting Closer?

It can give you a rough indication. If successive lightning flashes are followed by thunder after progressively shorter intervals, the lightning activity may be getting closer to your location.

If the intervals are becoming longer, it may be moving farther away.

But thunderstorms aren't single points moving neatly across a map. New lightning can develop in different parts of a storm, so the method shouldn't be used to decide whether it's safe to remain outdoors.

Summer thunderstorms can develop and change surprisingly quickly, which is one reason conditions at your location can change within minutes.

When Is It Safe to Go Back Outside?

The National Weather Service recommends going indoors when you hear thunder: When thunder roars, go indoors.

A substantial building or an enclosed, hard-topped vehicle is safer than remaining outside.

After the last thunder, wait at least 30 minutes before resuming outdoor activities. Lightning can still occur after the main rain has passed or when the storm appears to be moving away.

When severe weather is possible, it's also useful to know the difference between a weather watch, warning and advisory rather than relying on what the storm looks or sounds like from your location.

Lightning Is Fast. Thunder Lets You Measure the Distance.

Lightning and thunder are really different ways of experiencing the same event.

The flash is the electrical discharge itself. Thunder is the sound produced as that discharge rapidly heats and expands the surrounding air.

Because light reaches us almost instantly while sound takes time to travel, those few seconds between flash and rumble provide a surprisingly simple way to judge distance.

The next time a summer thunderstorm rolls through, count the seconds between the flash and the thunder. Every five seconds is roughly another mile between you and the lightning.