Situated on Lake Erie, the city of Buffalo, New York, is one of the snowiest cities in the United States. Like other cities on large lakes, it is subject to a phenomenon called lake-effect snow. Water retains heat, so lake water is often warmer than the air. In the lake effect, cold air moves over warmer lake water, initiating a process that can produce large snowstorms. Moisture from the lake evaporates into the cold air, warming it. Because warm air is less dense than cold air, it begins to rise. As it rises and moves away from the lake, it cools down. As the air cools, moisture condenses and forms into clouds. If the temperature is cold enough, the moisture falls to the ground as snow. What is the main idea of the passage?