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Weather plays main roles in earth’s atmosphere and for aviation and aircraft performance. The term “Weather” (WX) is the state of the atmosphere at a current time and location to variables in temperature, moisture, wind, visibility, and pressures. It can also be applied to adverse or destructive atmospheric conditions.

Earth’s atmosphere is made up of mixture of different gases that are allows in motion. The atmosphere is what supports and protects life on earth. It absorbs the suns energy, recycles water along with working with the electro and magnetic fields.

Composition of the Atmosphere:

The Composition of Earth’s atmosphere is made up of the following molecules:​​

  • Nitrogen — 78 percent

  • Oxygen — 21 percent

  • Other Gases — 1 percent


The first layer of the atmosphere is known as the Troposphere, is between 4 - 12 miles over the North and South poles and up the 48,000 feet over the Equator regions. (The atmosphere is thinner at the poles, and thicker at the equator.)

​​​The Troposphere is the layer that supports life and contains the most weather, clouds, storms and temperatures. For every 1,000 feet in altitude the temperature decreases 2 degrees Celsius, and pressure decreases one inch per 1,000 feet in altitude.

There is a boundary at the top of the Troposphere called the Tropopause. The altitude of the Tropopause varies with latitudes and seasons, which takes on an elliptical shape. The Tropopause is associated with jet streams and clear air turbulence.

Above the Troposphere lies the stratosphere which, extends from the Troposphere up to 160,000 feet. In this layer, there is little weather with stable air, and certain types of clouds.


Earth's atmosphere is a constant circulation; the major part is due to the unequal heating of the Earth's surface. The heating changes air movement and air pressure, this movement of air around the surface is called, atmospheric circulation.

The heating of the surface is caused by radiation from our Sun. This causes a circular motion, which causes warm air to rise thus replaced by cooler air.

  • Warm air rises: heat causes the air molecules to spread apart. As warm air expands, it becomes less dense.

  • When air-cools, the molecules become closer and tighter together becoming more dense and heavier than warm air. As cool air becomes heavy, it tends to sink and begins to replace warmer air.

Since the equator is closer to the Sun, it gets more heat, higher temperatures and less dense air. Unlike the Polar Regions, which are farther away from the Sun resulting in cooler and denser air, which begins to sink toward the surface.

Air has weight:

The unequal heating of the Earth’s surface also causes atmospheric pressure. Air molecules are invisible to the naked eye, but these air molecules have weight to them.

  • At sea level air weighs 14.7 pounds per 1 square inch.

  • At 18,000 feet, air weighs 7.4 pounds per 1 square inch.

The actual pressure at a given time and place will be different due to altitude, temperature, and density of the air. These atmospheric pressure conditions affect aircraft performance, especially takeoff, rate of climb, and landing performance.

Coriolis force

​The Coriolis force is the force created by the rotation of the earth. We as humans cannot feel this force, but large masses of air or bodies of water are affected.

In the Northern Hemisphere, the air is deflected to the right at a curved path, rather than in a straight line. As does the same goes for the Southern Hemisphere, with the exception that the air is defected to the left.


The Coriolis force causes the air to separate into three different cells in both hemispheres. In the Northern hemisphere the warm tropical air rises upward from the surface and begins to travel north, then is deflected to the right, due to the Coriolis force, which then the air is now moving eastward. This circulation pattern results in the prevailing upper level westerly winds in the conterminous United States.

Circulation Pattern Friction:

This circulation patterns of air become even more complicated caused by changes in seasons, continents, oceans and frictional forces caused by the topography of the Earth's surface such as mountains, canyons, valleys etc. Just 2,000-3,000 feet above the surface, the wind is beginning to slow down due to friction as well as change directions. The direction of the wind is different from that of the wind within a couple thousand feet above the surface.

Measurement of Atmosphere Pressure

The standard atmospheric conditions are used for most aircraft instruments and performance charts and data.

  • Standard atmosphere pressure at sea level is 29.92” Hg (1,013.2 millibars (mb).

  • Standard temperature at sea level is 15⁰C (59⁰F).

Weather stations convert barometric pressure to sea level pressure. This is accomplished by adding 1 inch Hg for every 1,000 feet in altitude.

For an example, if a weather station is located at 5,000 feet above sea level and is reading of 24.92”.

As altitude increases atmospheric pressure decreases.

Altitude and Atmosphere Pressure

Atmospheric pressure decreases 1 inch with every 1,000 feet increase in altitude. The air becomes less dense at higher altitudes.

As mentioned previously in the beginning of this lesson, temperature decreases 2⁰C with every 1,000 feet gained in altitude.


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