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The Troposphere

The troposphere is thinnest at the poles.

The troposphere is thinnest at the poles.

     The troposphere is the lowest layer of the atmosphere. It is where we live. The troposphere reaches from the surface of the Earth to between 7 and 20 km (kilometers) above the surface. The troposphere is not the same thickness all around the Earth. The troposphere is the thickest at the equator and the thinnest at the poles.

     There are several reasons that the troposphere is thicker at the equator than the poles. First, the Earth is not a perfect sphere. It is slightly ovoid (3-dimensional oval). This means that the poles are just a little bit closer to the center of the Earth than the equator. That means the gravity at the poles is stronger than at the equator, but just by a tiny bit. This stronger gravity holds the air molecules closer to the Earth which makes the troposphere smaller at the poles.

     Also, the Earth spins on an axis centered at the poles. This means that the equator, which is farther away from the axis, spins a little bit faster than the poles. This spinning pushes the air molecules farther away from the Earth, making the troposphere bigger at the equator.

The equator is slightly closer to the Sun, so the air at the equator is warmer than the air at the poles.

The equator is slightly closer to the Sun, so the air at the equator is warmer than the air at the poles.

     Finally, the equator is slightly closer to the Sun than the poles. This makes the equator warmer. Warmer air molecules expand, making them bigger. This means that the air molecules take up more space at the equator which makes the troposphere bigger there.

     The troposphere has the most water vapor and dust of any of the layers. In fact, it holds 99% of the water vapor in the entire atmosphere. Clouds are formed from water vapor, so clouds occur in the troposphere. Note that clouds are made up of both liquid water and water vapor. All weather happens in the troposphere. To learn more about weather, click the link below.

Airplanes fly in the jet stream just below the tropopause.

Airplanes fly in the jet stream just below the tropopause.

     The boundary between the troposphere and the next layer, the stratosphere, is called the tropopause. The jet streams are just below the tropopause. The jet streams are like rivers in the sky. They are fast moving winds that separate hot air and cold air. The jet streams have a big impact on weather. They are also important for airplanes. If an airplane can fly with the jet stream, the plane will travel faster and use less fuel. If the airplane has to fly in the opposite direction of the winds in the jet stream, it will fly more slowly. The air in and around the jet stream can be very bumpy. This causes turbulence for the airplane, and can be scary for passengers.

     Hot air rises and cool air falls, so the air is constantly moving around the troposphere. After rising, hot air from the equator makes its way to the colder poles. Cool air drops closer to the Earth and moves toward the equator. The movement of this air is called “convection currents”.  

     The Greek root “tropo” means turning or mixing. Scientists call the lowest layer of the atmosphere the troposphere because the air mixes in the troposphere. The air mixes because warm air rises and cool air falls back to Earth. As the air moves it also changes temperature, so the air molecules in the troposphere are constantly moving and mixing.

The air at the top of a mountain is colder than the air in a valley.

The air at the top of a mountain is colder than the air in a valley.

     The hottest part of the troposphere is the surface of the Earth. The air gets colder as you travel higher. This is why it is colder on the top of a mountain than it is in the valley below it.

    If you travel high enough, you will leave the troposphere and enter the stratosphere.