Points to Consider
- When thinking about the weather, what factors do you consider important in the air
that surrounds you? - How do air temperature, humidity, and pressure differences create different sorts of
weather? - Think about the types of weather described in this lesson. Imagine types of weather
that you have not experienced, look at photos, and ask friends and relatives who’ve
lived in other places what their weather is like.
16.2 Changing Weather
Lesson Objectives
- Describe the characteristics air masses have and how they get those characteristics.
- Discuss what happens when air masses meet.
- List the differences between stationary, cold, warm, and occluded fronts.
Introduction
The weather in a location often depends on what type of air mass is over it. Another key
factor revolves around whether or not the spot is beneath afront, the meeting place of
two air masses. The characteristics of the air masses and their interactions can determine
whether the weather is constant over an area, or whether there are rapid changes in air
temperature, wind, precipitation and even thunderstorms.
Air Masses
Anair massis a batch of air that has nearly the same temperature and humidity (Figure
16.14). An air mass is created above an area of land or water known as its source region. Air
masses come to have a distinct temperature and humidity when they remain over a region
for several days or longer. The heat and moisture leave the ground and move into the air
above it, until the overlying air takes on the temperature and humidity characteristics of
that particular region.
Air masses are created primarily in high pressure zones. They most commonly form in polar
and tropical regions, which have very distinctive temperature and humidity. The temperate
zones are ordinarily too unstable for air masses to form. Instead, air masses move across
them, making the middle latitudes the site of very interesting weather.
Air masses can be 1,600 km (1,000 miles) or more across and several kilometers thick.