Within Earth's atmosphere, variations in average atmospheric pressure and temperature are related to altitude. Gases are compressible, so Earth's gravity causes atmospheric pressure to increase as elevation decreases. However, the temperature pattern is not so simple. As altitude increases, the temperature of the air decreases at some levels and increases at others. These temperature variations are a result of the different chemical and physical properties of the atmospheric layers, along with the solar radiation that filters through.

The outermost layer of the atmosphere, the thermosphere, is the first to receive the Sun's radiation and is strongly affected by solar energy. Shortwave ultraviolet (UV) light from the Sun is absorbed by atomic and molecular oxygen in the thermosphere, releasing energy that creates higher temperatures at higher altitudes. Below the thermosphere is the mesosphere. The air pressure is so low in this layer that there are too few molecules to significantly absorb any radiation that gets through the thermosphere. Consequently, the temperature decreases with increasing height within this layer. The density of the mesosphere is too low to support aircraft, but it is typically just dense enough for meteors to become visible at this level. Within both the thermosphere and the mesosphere there is a layer of gases that are ionized by high-energy solar radiation. This ionosphere interacts with the solar wind to produce auroras and can also aid in long-range radio communication.

The lower two layers of the atmosphere contain 99% of the atmosphere's mass. The troposphere, where all weather occurs, is the lowest and densest layer in the atmosphere. As Earth's surface absorbs radiation from the Sun, it warms the lower level of the troposphere, causing the air to become less dense and rise through the cooler air above it in the process of convection. Because temperature and pressure drop with increasing altitude in this layer, convection creates a lot of turbulence -- agitated or irregular atmospheric motions. However, the stratosphere -- the atmospheric layer above the troposphere -- is very stable because its temperature increases with altitude, preventing convection. This is because when this layer is struck by UV radiation, its molecular oxygen bonds with atomic oxygen to create ozone, which in turn absorbs more UV light and splits back into regular oxygen. This ozone-oxygen cycle continually converts UV radiation into heat, heating the upper stratosphere and protecting Earth's surface from damaging UV radiation.

What does this illustration show?

  that air has no weight
  that air particles become farther apart in the upper atmosphere
 
that air pressure is greater, farther away from Earth

The illustration shows that air pressure is greatest at?

  Leve 1
  Level 3
 
Leve 2

The illustration shows that air pressure is less at?

  Leve 1
  Level 3
 
Leve 2

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