These occur before the first visible lightning flashes.
Lightning is hard to see at a distance in the daytime.
Giving you plenty time to clear the area before the storm arrives within 'striking distance'. Warning you before you hear the thunder. You are in strike danger before you hear thunder. Warning performance is not affected by cold, heat, haze, fog, pollution, smog, rain, etc. Automatic system test when first turned 'ON' assures you that the system is working.