In reply: The "30/30" rule is a simple and easy-to-remember rule designed to reduce the probability of lightning strikes. Two important studies over recent years have led to its development.

First, in 1993, Holle et al analysed the number of casualties relative to flash rates during thunderstorms and found that most casualties occur at the beginning and end of storms.1 They concluded that individuals typically wait too long to seek safe shelter and often resume too soon.

Secondly, in 1999, Lopez and Holle examined the distribution of successive flashes for large numbers of different types of storms, and found that, although most were separated by less than 8 km, a significant number of successive flashes occurred up to 13 km apart.2 This was noted to be more likely with larger, more complex storms.

Given that lightning can strike kilometres forwards or backwards from the storm front, being within 10 km of lightning activity (as estimated by a "flash-to-bang" count of 30 seconds) reflects a risk that the next flash might conceivably be at the observer's location, irrespective of whether there are blue skies overhead. This is why blue sky alone is not enough reason to break the 30/30 rule.