Is Lightning Without Thunder Safe?

The ability to tell when you should avoid being outside sounds simple enough. Say, if it's pouring rain, your brain might give you a heads-up that you'll get wet. Maybe it's so windy that a tree has fallen across the sidewalk: The old noggin might kick in and tell you to stay inside to avoid getting beaned by a branch. And if you're hearing thunder, you'll probably find yourself thinking it's not the best time to vigorously practice calisthenics in the yard.

And the deal really is that simple: If you do hear thunder, you're absolutely at risk for a lightning strike. That is, you're as at risk for a lightning strike as one ever is.

The National Weather Service gives you pretty favorable odds, with a one in 12,000 chance of getting hit by a bolt from above over the course of your life [source: National Weather Service].

If there is thunder in the area, the only cause is lightning. But long story short, you're not going to hear thunder just because there are clouds in the area. And if lightning is present — even if you can't spot it right away — you're at risk for being struck.

A lightning bolt can travel a great distance. Strikes have even been known to occur 25 miles from the bolt's cloud of origin. This makes a lot more sense if you remember that lightning doesn't travel in a straight line down to the ground; it can reach horizontally across the sky before making contact below.

When Heat Lightning Strikes

Heat lightning is a term commonly used to describe lightning from a distant thunderstorm that is too far away for thunder to be heard. The lightning can often be seen illuminating the sky, but without the accompanying sound of thunder.

This phenomenon is often observed on warm summer nights, which might be why it's colloquially called "heat lightning." However, the term is somewhat of a misnomer as the lightning isn't caused by heat.

Here's a breakdown of the phenomenon:

Distance: Sound and light travel at different speeds. Light, which travels at about 299,792 kilometers per second (approximately 186,282 miles per second), is much faster than sound, which travels at about 343 meters per second (around 1,125 feet per second) in dry air at sea level. Due to this difference, lightning can be seen from a much greater distance than thunder can be heard. If a thunderstorm is more than 15 miles away, you might see the lightning but not hear the thunder.
Atmospheric Conditions: In certain conditions, sound can be refracted (bent) away from the Earth's surface due to temperature and wind variations. This can further reduce the distance over which thunder can be heard.
Name Origin: The name "heat lightning" might come from the fact that these distant flashes are often observed on hot, humid summer nights. However, again, the heat itself is not responsible for the lightning.

Not to be confused with dry lightning, which is a variation of cloud to ground lightning that occurs from a storm where very little precipitation reaches the ground. Heat lightning is just regular lightning from a faraway storm, and the term refers more to the conditions under which it's observed rather than any distinct meteorological phenomenon [source: National Weather Service].

Assessing The Risk of a Lightning Strike

So if you are hearing thunder, keep your eyes peeled for a lightning flash, even if you're not seeing any light or (more eerily) a single storm cloud. It's totally possible for strikes to occur on a clear day if a distant storm is lurking.

Another thing to keep in mind is that a lightning strike can travel through ground current, so you can totally be electrocuted without even seeing the strike or being struck directly [source: National Geographic].

Now, let's be clear: If you're seeing lightning at all, you're at risk for getting struck. You might not hear thunder if the storm is far away, but that doesn't mean you can be sure you're safe from a remote hit.

The general rule is that if you don't see lightning or hear thunder, you're probably safe to go about your day and assume that you're not about to join the ranks of the roughly 270–330 people who are injured or killed by lightning strikes each year.

Lots More Information

Sources

National Geographic News. "Flash Facts About Lightning." June 24, 2005. (Dec. 30, 2014) http://news.nationalgeographic.com/news/2004/06/0623_040623_lightningfacts.html
National Oceanic and Atmospheric Administration. "Lightning Safety." (Dec. 30, 2014) http://www.crh.noaa.gov/Image/gid/WCM/safety/lightning.pdf
National Severe Storms Laboratory. "Severe Weather 101." (Dec. 30, 2014) http://www.nssl.noaa.gov/education/svrwx101/lightning/faq/
National Weather Service. "How Dangerous Is Lightning?" (Dec. 30, 2014) http://www.lightningsafety.noaa.gov/odds.shtml
Sohn, Emily. "When Lightning Strikes Out of a Blue Sky." Discovery News. July 29, 2011. (Dec. 30, 2014) http://news.discovery.com/earth/weather-extreme-events/lightning-on-clear-day-110729.htm
USA Today. "Answers archive: Lightning safety and survival." (Dec. 30, 2014) http://usatoday30.usatoday.com/weather/resources/askjack/archives-lightning-safety.htm