Dear Tom,

I was recently driving at night on I-65 south from Gary and saw lightning in clouds over Indianapolis, approximately 120+ miles away.  Given ideal conditions, what would be the farthest one could see lightning in the distance?

William Price Homer Glen

Dear William,

Because of the curvature of the Earth’s surface (and because light beams curve slightly as they pass through the atmosphere), when you look to the horizon your line of sight is higher and higher off the ground the farther out you look out. Thunder rarely carries through the air more than 15 miles, but lightning can be seen literally hundreds of miles if the flashes are sufficiently high, and your line of sight is not blocked. Chicago National Weather Service forecaster Mark Ratzer tells us that lightning at the top of a thunderstorm at a height of 60,000 feet could be seen from up to 265 miles away.