Weather conditions should be monitored continuously. Lightning may strike several kilometres away from the parent cloud and precautions should be taken even if the thunderstorm is not directly overhead. Lightning can reach you even if the storm is 16 km away and there’s a clear sky above you.
To judge how close lightning is, count the seconds between the flash and the thunder clap. Each second represents about 300 metres. If you can count less than 30 seconds between the lightning strike and the thunder, the storm is less than 10 km away and there is an 80 percent chance the next strike will happen within that 10 km. If you count less than 30 seconds, take shelter, preferably in a building or all-metal automobile (not a convertible top) or in a low-lying area.