2 (h.g.) Fronts and Air Masses
As jet streams blow from west to east, air masses are carried along their tracks. Fronts: As huge masses of air move across the land and the oceans, they collide with each other. But the air masses do not easily mix. Think about a bottle of oil and water. The less dense oil floats on top of the denser water. Something similar happens when two air masses with a different temperature and humidity collide. The air masses do not easily mix. The boundary where the air masses meet becomes a front.
As you have learned, cold air is dense and tends to sink. Warm air is less dense and tends to rise. When a rapidly moving cold air mass runs into a slowly moving warm air mass, the denser cold air slides under the lighter warm air. As the warm air rises, it expands and cools. Remember that warm air can hold more water vapor than cool air. The rising air soon reaches the dew point, the temperature at which the water vapor in the air condenses into droplets of liquid water or forms tiny ice crystals. Clouds form. If there is a lot of water vapor in the warm air, heavy rain or snow may fall.
As you have learned, cold air is dense and tends to sink. Warm air is less dense and tends to rise. When a rapidly moving cold air mass runs into a slowly moving warm air mass, the denser cold air slides under the lighter warm air. As the warm air rises, it expands and cools. Remember that warm air can hold more water vapor than cool air. The rising air soon reaches the dew point, the temperature at which the water vapor in the air condenses into droplets of liquid water or forms tiny ice crystals. Clouds form. If there is a lot of water vapor in the warm air, heavy rain or snow may fall.
