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
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.
Global circulation summary
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
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.