An air mass is a large volume of air in the atmosphere that is mostly uniform in temperature and moisture. Air masses can extend thousands of kilometers across the surface of the Earth, and can reach from ground level to the stratosphere—16 kilometers (10 miles) into the atmosphere.
The climates of most regions worldwide are affected by air masses. For example, maritime-tropical air sourced over warm waters of the Atlantic Ocean, Caribbean Sea and Gulf of Mexico, primarily between 10 and 30 degrees north of latitude, is the main contributor of precipitation for much of North America east of the Rocky Mountains. It’s also the cause of the persistent humidity typical of that big region’s summer season. In the Pacific Northwest, maritime-polar air tracking inland from the Aleutian Low in the winter supplies the heavy mountain rain and snowfall that nourish vast temperate rainforests and extensive alpine glaciers. Such maritime air masses also contribute to a moderating climatic influence on coastal temperatures, as oceans heat up and cool down more slowly and less dramatically than landmasses.