General ciculation of the atmosphere is important to know to be aware of the large scale patterns of the atmosphere. For flying, it is particular to know about airmasses and fronts. Some summary information on these topics is presented below.

An air mass is an extremely large body of air whose properties of temperature and moisture content (humidity) are
similar in any horizontal direction. Air masses can cover hundreds of thousands of square miles. Air masses are formed when air stagnates for long periods over a uniform surface. The characteristic weather features (temperature and moisture) of air masses are therefore determined by the surface over which they form. An air mass acquires these attributes through heat and moisture exchanges with the surface. As a result, an air mass can be warm or cold and moist or dry.

More on airmasses

[back to top]

Although an individual air mass can change over time, different air masses do not mixreadily. Therefore, when two air masses come in close contact they retain their separate identities for several days. The transition zone between two different air masses is called a front. Fronts can be hundreds of miles long and exist as long as the air masses they separate remain distinct.

Norwegian meteorologists around the time of World War I laid the foundation for our concepts of fronts and their movements. They observed large air masses with different temperature and moisture properties that advanced and retreated versus one another. Clashing air masses often led to disruptive weather conditions. The boundary between air masses was called a “front,” analogous to the boundaries used on military maps to separate battling armies.

More on Fronts.

[back to top]

Upward vertical motions near the equator explain the Intertropical Convergence Zone (ITCZ), which can be identified in global satellite imagery and precipitation maps. When rising air reaches the tropopause and moves poleward, it comes closer to the Earth’s axis of rotation and increases speed to conserve angular momentum. This produces the subtropical jet stream.

The descending air of the Hadley cell is compressed and warms, lowering the relative humidity. This sinking air explains the large deserts of Africa, Saudi Arabia, and Australia. The sinking air of the Hadley cell results in calm winds at the surface, producing the “horse latitudes” dreaded by ancient mariners who used wind power to travel. Some descending air that reaches the surface moves toward the equator to supply moisture to the ITCZ. As air
moves toward the equator, the Coriolis force acts to produce the steady northeast and southwest trade winds sought by sailors.

Some of the air associated with the descending branch of the Hadley cell moves poleward and clashes with cold polar air masses that are moving toward the equator. Fronts exist where the cold air meets the warm subtropical air, producing mid-latitude storms that tend to move from west to the east. These storms are embedded in the mid-latitude westerlies, causing them to move eastward. The polar jet stream exists in the vicinity of fronts.

In our simple conceptual model the globe is covered with water—a good approximation, because water covers 70% of the Earth’s surface. However, the differences between land and water are important in weather and climate.

[back to top]