The speed of light and the speed of sound in our atmosphere is different, so that means it can be difficult to determine how close a lightning strike is.
Light travels much faster than sound, so that means you always see the flash prior to hearing thunder. Light travels over 186,000 miles per second.
Sound, on the other hand, travels approximately 1,100 feet per second, and this can change depending on air temperature. This means thunder travels at .2 of a mile per second.
Once you see a flash of lightning, you should start counting. If you count to five seconds, then lightning struck one mile away from you. If you count to ten seconds, then lightning struck two miles away from you.
When thunder roars, you should always go inside a safe structure. Lightning can strike up to ten miles away from the rainfall, so just because it is not raining does not mean you are safe. Be sure to have an action plan to keep safe during electrical thunderstorms.