Air and storm systems travel over the Earth's surface. This global
circulation is determined by several factors: Earth’s rotation, the Earth’s tilt
relative to the sun, and the Earth’s water which is in constant motion. The sun heats the entire
surface - more when it is directly overhead. The equator becomes
very hot with the hot air rising into the upper atmosphere. This air moves
toward the poles where it becomes cold and sinks, returning to the
equator. The earth’s rotation, tilt, and the greater land mass in the northern
hemisphere complicate the circulation patterns.
According to NOAA, three
circulations are apparent:
Hadley cell - Low latitude air movement toward the equator that with
heating, rises vertically, with poleward movement in the upper atmosphere. This
forms a convection cell that dominates tropical and sub-tropical climates.
Ferrel cell - Mid-latitude mean atmospheric circulation cell for weather. In
this cell the air flows poleward and eastward near the surface and equatorward
and westward at higher levels.
Polar cell - Air rises, diverges, and travels toward the poles. Once over the
poles, the air sinks, forming the polar highs. At the surface air diverges
outward from the polar highs.
Surface winds in the polar cell are easterly (polar easterlies).
Three main air circulations exist between the equator and poles due to earth's
Between each of these circulation cells are bands of high and low
pressure at the surface. The high pressure band is located about 30° N/S
latitude and at each pol. Low pressure bands are found at the equator and
50°-60° N/S. Usually, fair and dry/hot . Deserts are located along the 30°N/S
latitude around the world. Between 50°-60° N/S latitude there is more
precipitation from more storms moving around the earth.
Air movements are both vertical (hydrostatic) and horizontal. Beyond the
tropics, the dominant forces act in the horizontal direction, and the primary
struggle is between the Coriolis force and the pressure gradient force. Balance
between these two forces is referred to as geostrophic. Given both hydrostatic
and geostrophic balance, one can derive the thermal wind relation: the vertical
gradient of the horizontal wind is proportional to the horizontal temperature
gradient. If two air masses, one cold and dense to the North and the other hot
and less dense to the South, are separated by a vertical boundary and that
boundary should be removed, the difference in densities will result in the cold
air mass slipping under the hotter and less dense air mass. The Coriolis effect
will then cause a poleward-moving air mass to deviate to the East, while
mass will deviate toward the west. The general trend in the atmosphere is for
temperatures to decrease in the poleward direction. As a result, winds develop
an eastward component and that component grows with altitude. Therefore, the
strong eastward moving jet streams are the result of a warm Equator and
cooler North and South poles.
(National Oceanic and Atmospheric Administration.
Global Air Circulations, Accessed Online Feb. 2017.