Why do temps increase in stratosphere

Papers held up by rising air currents above a radiator demonstrate the important principle that warm air rises. Why does warm air rise Figure above? Gas molecules are able to move freely and if they are uncontained, as they are in the atmosphere, they can take up more or less space. Warmer, lighter air is more buoyant than the cooler air above it, so it rises.

The cooler air then sinks down, because it is denser than the air beneath it. This is convection, which was described in the Plate Tectonics chapter. The property that changes most strikingly with altitude is air temperature.

Unlike the change in pressure and density, which decrease with altitude, changes in air temperature are not regular. A change in temperature with distance is called a temperature gradient. The temperature gradient of each layer is different.

In some layers, temperature increases with altitude and in others it decreases. The temperature gradient in each layer is determined by the heat source of the layer Figure below. The four main layers of the atmosphere have different temperature gradients, creating the thermal structure of the atmosphere.

The layers of the atmosphere appear as different colors in this image from the International Space Station. Most of the important processes of the atmosphere take place in the lowest two layers: the troposphere and the stratosphere. The temperature of the troposphere is highest near the surface of the Earth and decreases with altitude. On average, the temperature gradient of the troposphere is 6.

What is the source of heat for the troposphere? The temperature is also higher near the surface because of the greater density of gases. The higher gravity causes the temperature to rise.

Notice that in the troposphere warmer air is beneath cooler air. This layer extends from around 31 miles 50 km above the Earth's surface to 53 miles 85 km. The gases, including the oxygen molecules, continue to become denser as one descends. The gases in the mesosphere are now thick enough to slow down meteors hurtling into the atmosphere, where they burn up, leaving fiery trails in the night sky. Both the stratosphere next layer down and the mesosphere are considered the middle atmosphere.

The transition boundary which separates the mesosphere from the stratosphere is called the stratopause. The Stratosphere extends around 31 miles 50 km down to anywhere from 4 to 12 miles 6 to 20 km above the Earth's surface. This layer holds 19 percent of the atmosphere's gases but very little water vapor. In this region the temperature increases with height. This increase in temperature with height means warmer air is located above cooler air.

This prevents "convection" as there is no upward vertical movement of the gases. Once it is absorbed, it is reradiated at different wavelengths, thereby warming the stratosphere. At the top of the stratosphere about 50 km, the stratopause , the temperature begins to decrease again as the altitude increases. Above the stratopause, in the mesosphere, thermosphere, and exosphere harmful gamma rays and X-rays are absorbed. The circulation of the atmosphere is very complex and has many causative factors.

The amount of solar radiation reaching the ground varies due to latitude, time of year, and cloud cover. Specific heat capacities for different surface materials vary greatly. Additionally, the Coriolis force , which results from the rotation of Earth, influences the movement of air. The net effect of these factors is the transporting of ozone from the tropics, where most ozone is formed, to the mid and higher latitudes. Of course, because of variations around Earth, the ozone movement is not uniform, and at a given latitude, there will be variations in concentrations.

Since ozone is produced and transported in the stratosphere, some understanding of the structure and circulation of the stratosphere is needed. The meridional circulation, or the circulation along longitude lines, shows rising stratospheric air in the tropics, which descends at middle and higher latitudes. Ozone is transported by this flow. Commercial jet aircraft fly in the lower stratosphere to avoid the turbulence which is common in the troposphere below.

The stratosphere is very dry; air there contains little water vapor. Because of this, few clouds are found in this layer; almost all clouds occur in the lower, more humid troposphere. Polar stratospheric clouds PSCs are the exception. PSCs appear in the lower stratosphere near the poles in winter.

They are found at altitudes of 15 to 25 km 9. They appear to help cause the formation of the infamous holes in the ozone layer by "encouraging" certain chemical reactions that destroy ozone. PSCs are also called nacreous clouds. Air is roughly a thousand times thinner at the top of the stratosphere than it is at sea level.