Educational film explaining how weather works. Film begins with shots of different landscapes in different weather conditions. Narrator describes "clear skies" and "warm times", "cold times" (scene of snow), "dry times" (shot of dry hills), and "times of precipitation" (CU shot of branch in the rain). Shot of [California?] landscape as narrator asks, "What makes the weather change? The answer lies in air masses..." Shot of eskimos in "Polar Region." Shot of sun through rocks. Shot of tropical landscape. Shot of sun through palm fronds. Shot of map of North America (Mexico, US, Canada, etc.). Map demonstrates the polar and tropical regions. Map demonstrates through large white circles the six regions where air masses develop. Differentiates between continental and maritime air masses. Map focuses on the Polar Canadian air mass over Canada. Shot of church on water. CU shot of newspaper article with headline: "Rain Soaks City/ Long Heat Wave Broken." Shot of large home in snow. Another CU shot of newspaper article with headline: "Freeze Hardens - Worst to Come."