Air Masses and Fronts

Now that you have completed the windy noteguide assignments, you know how and why air moves over Earth's surface. This task helps explain what happens to the air as it moves over the surface. As you work on this assignment, you may want to refer to the pages in your text assigned for tonight's homework, and to the weather map you cut out of the newspaper.

Above is a map of a newly discovered continent ruled by the benevolent King Mitchell. Air is spiraling outward from the high-pressure centers marked.

1. Draw arrows to show the air spiraling out from the highs (clockwise or counterclockwise?). Why does the air spiral?

As air travels over the surface, it will acquire the moisture characteristics of the surface. Air travelling over land stays dry; air travelling over the ocean will become moist.

2. Label the air near each high as moist or dry. Why does the air from a high start out dry?

As air travels over the surface, it will acquire the temperature characteristics of the surface. Air travelling over cold northern areas cools off, air travelling over warm southern areas will become warm.

3. Label the air near each high as cold or warm. What heat transfer processes are at work to change the air temperature?

An air mass is a large "blob" of air with similar characteristics throughout. Air masses are named according to their moisture and temperature characteristics.

4. Label the moist air "maritime", the dry air "continental".

5. Label the cold air "Polar", the warm air "Tropical".

You should now have four different air masses: continental Polar, continental Tropical, maritime Polar, maritime Tropical. On weather maps, the abbreviations cP, cT, mP, mT are used. The first letter of each abbreviation is not capitalized to avoid confusion with other abbreviations.

6. Label the map with the air mass abbreviations.

Fronts are the boundaries between air masses, marking the edges of individual air masses.

7. Draw solid lines where the air from different air masses meet.

Prevailing winds cause air masses and the fronts that form between them to move. If warm air replaces cold air, it is called a warm front. If cold air replaces warm air, it is called a cold front. Refer to the Windy Noteguide for prevailing wind information.

8. What causes the prevailing winds?

9. Label the cold fronts with little triangles sticking out of the front in the direction the front is travelling. Label warm fronts with little half circles on the side of the line. The symbols indicate front direction on weather maps.

When fronts move, warm air always rides up over colder denser air. As the warm air rises, it temperature drops, so relative humidity increases.

10. Why does relative humidity increase when temperature decreases?

11. What will form along the fronts if the temperature of the warm air drops enough to reach its dewpoint? Draw this on the map.

In the temperate middle climates, air that rises in fronts contributes to the global circulation of air back toward the poles and equator along the top of the troposphere. Adding to this same circulation is the air that rises in low-pressure centers. These lows usually form where fronts join together.

10. Draw a low on the map and the wind spiraling into it (clockwise or counterclockwise?).