Alfred Ward Jr., age 11, of Brownsville, Texas, for his question:

WHAT IS AN ISOTHERM?

An isotherm is a line drawn on maps to connect places that have the same temperature. It is also called an isothermal line.

Weathermen make monthly isothermal maps by first averaging all the temperatures at any one place during the month to find the mean or average temperature. Then they draw isotherms connecting places which have the same average.

Places which have the same latitude on the map often vary in temperature, and so isothermal lines are irregular. They curve back and forth, up and down, over the map.

There is the greatest irregularity in the Northern Hemisphere where great land masses are separated by large water areas with varying temperatures on land and water in the same latitude. In the Southern Hemisphere, isotherms follow the parallels of latitude more closely.