Brian Davis, age 14, of Ripon, Wisconsin, for his question:

Why is there no lightning in freezing weather?

To produce lightning, the weather needs a steep temperature gradient. There must be a very sharp drop in temperature between the surface level and the freezing air about seven miles above the ground. The stormy turbulence also needs conflicting masses of dry and damp air. Lightning and thunder occur when these opposing ingredients engage in weathery warfare. The turbulence builds up electrical charges in different parts of the storm cloud. These are discharged in flashes of lightning, which cause thunder.

Over land areas, steep temperature gradients are more likely to build up in summer. The surface gets much hotter than the freezing air aloft. In winter, there is not enough contrast between the surface and upper air to trigger lightning conditions. Over the ocean, conditions are different. There, most lightning storms occur during winter nights.