The Environment

Some Topics



  • Air Circulation

    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 circulations exist between the equator and poles due to earth's rotation.
    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 due to 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 poleward-moving mass to deviate to the East, while equatorward-moving 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 can cooler the North and South poles.

    (National Oceanic and Atmospheric Administration. Global Air Circulations, Accessed Online Feb. 2017. )

    • Discussions of Environmental Science and Human Ecology were developed by Environmed Research Inc. Sechelt, B.C. Canada. Online Topics were developed from the book, The Environment. You will find detailed information about the sun, weather, soils, forests, oceans, atmosphere, air pollution, climate change, water resources, air quality, energy sources, and preserving habitats. The Environment is available from Alpha Online as a Printed book or as an eBook Edition for Download. The 2018 edition is 286 pages.

      The Author, Stephen Gislason MD
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