The weather of the United States east of the Rocky Mountains is dominated by large masses of air that travel south from the wide expanses of land in Canada, and north from the warm waters of the Gulf of Mexico. Not surprisingly, the term air mass means a large body of air (sometimes as large as one million square miles!) that can be characterized by their temperature and humidity. Such large bodies of air form over large portions of the surface of the Earth, and we use the term source region to describe the areas of the Earth over which large air masses form. The diagram below shows the major source regions and air masses for the United States: