The energy that the Earth receives from the Sun is the basic cause of our changing weather. Solar heat warms the huge air masses that comprise large and small weather systems. However, at higher altitudes, the atmosphere reacts strongly to changes in solar activity. An air mass is a very large batch of air that has nearly the same temperature and humidity throughout. An air mass acquires its characteristics from the region over which it forms, called its source region. When the air mass sits over the source region for several days, it picks up the temperature and humidity of that region. Air... Show more

The energy that the Earth receives from the Sun is the basic cause of our changing weather. Solar heat warms the huge air masses that comprise large and small weather systems. However, at higher altitudes, the atmosphere reacts strongly to changes in solar activity.

An air mass is a very large batch of air that has nearly the same temperature and humidity throughout. An air mass acquires its characteristics from the region over which it forms, called its source region. When the air mass sits over the source region for several days, it picks up the temperature and humidity of that region. Air masses form in high pressure zones. They may form over continents, in which case they are dry, or over oceans, in which case they are moist. They most commonly form over polar or tropical regions. Polar air masses have cold temperatures, and tropical air masses have warm temperatures. Temperate zones are typically too unstable for air masses to form.    
After air masses form, they are slowly pushed along by high-level winds. Cold air masses tend to flow toward the equator, and warm air masses tend to flow toward the poles. This movement of air masses brings heat to cold areas and cools down warm areas. Movement of air masses is one several processes that help balance out the planet’s temperatures.    
When an air mass moves over a region, it shares its temperature and humidity with that region. Storms may arise if an air mass moves over a region with different characteristics. For example, when a cold air mass moves over warmer ground, the bottom layer of air is heated. The heated air rises, forming clouds, rain, and sometimes thunderstorms.    
When a warm air mass travels over colder ground, the bottom layer of air cools. The cool air is dense, so it stays near the ground below the warm air above it. This forms a temperature inversion.

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