why do temperatures in the stratosphere increase upward

METEOROLOGIST JEFF HABY The troposphere is the layer of the atmosphere closest to the earth's surface where the
weather takes place. The warmest temperatures in the troposphere are near the surface with the coldest temperatures being at the top of the troposphere. Although the sun light comes from the top to the bottom of the atmosphere, the troposphere is primarily heated from the bottom. This is because the surface is much better at absorbing a wide range of solar radiation as compared to the air. Much of the sunlight filters down through the air and to the surface. Certain gases do absorb radiation and warm the air such as carbon dioxide and ozone. However, a great amount of solar radiation makes it to the surface. The surface is warmed by the sun and then this energy is distributed upwards into the troposphere through a mixing of the air. Since the earth's surface is the primary heat source, temperatures will be warmest at the surface and decrease away from the surface. The average temperature profile of the troposphere will show a decrease in temperature with height. This does not mean that in certain weather situations that the temperature can not increase with height since it does in the case of inversions. Since on a typical day the temperature decreases with height rather than an inversion being in place, the average profile is a temperature decrease with height.


Since the temperature decreases with height in the troposphere it can be easy for air to rise vertically since cooler air over warmer air is an unstable situation. When air rises it cools adiabatically. This is another reason the temperatures are colder aloft than at the surface. For air to rise to a higher elevation it must cool. Air density and pressure is highest at the surface. When air rises it expands since the pressures are lower aloft. Expanding air cools. Because the sun primarily warms the troposphere from the surface and because air cools as it rises, the warmest temperatures in the troposphere on average are at the earth's surface. Thereвs a scientific reason why itвs smart to pack that extra sweater when youвre headed to the mountains. Temperatures drop steadily as altitude increases, at least in the first layer of atmosphere known as the troposphere. Temperature readings in the atmosphereвs other three layers, which are beyond the reach of any mountain peak, also change with increasing altitude, but they change at significantly different rates, and they don't always decrease.


Humans are most affected by changes in the troposphere. Of the four main atmospheric layers, the troposphere is closest to Earth. It extends approximately 12 km, or 7 miles, upward and is where all weather activity occurs. Because heat from the sun is retained in the ground, the air is warmest there, and it becomes gradually colder as you move upward. In the troposphere, temperatures decrease by an average of 6. 5 degrees Celsius per every rise of one thousand meters, which works out to about 3. 5 degrees Fahrenheit per thousand feet. Airplanes often fly in the stratosphere, which begins about 10 to 13 kilometers (33,000 to 43,00 feet) above the ground, to avoid the turbulent weather patterns in the troposphere. Temperature in the stratosphere layer increases with altitude, which is a phenomenon known as thermal inversion. There are two reasons for the inversion. First, the stratosphere has two layers, or strata: a colder, denser one on the bottom and a layer of warmer, lighter air on top. Second, an ozone layer in the upper stratosphere readily absorbs ultraviolet light from the sun. As this radiation increases molecular activity, molecular vibrations produce a spike in temperature. The pattern reverses yet again in the mesosphere; temperatures again decrease with increasing height as the ozone layer is left behind and the air thins out with increasing altitude.


The lowest portion of the low-pressure mesosphere is heated by the warm air of the upper stratosphere. This heat radiates upward, getting less intense as altitude increases. Over a distance of about 40 kilometers (25 miles), the mesospheric temperature decreases from an average of 0 degrees Celsius (32 degrees Fahrenheit ) to minus 90 degrees Celsius (minus 130 degrees Fahrenheit). It is hard to fathom the extremes of cold and heat that exist in the thermosphere. Temperatures in the 40-kilometer (25-mile) top atmospheric layer easily swing by hundreds of degrees in each direction, from minus 90 degrees to more than 1,500 degrees Celsius (minus 130 degrees to 2,700 degrees Fahrenheit). Oxygen molecules in the thermosphere absorb solar heat as they do in the stratosphere, but are much more affected by solar activity. Because few molecules are present in the thin air of the thermosphere, the existing molecules have much more room to move and can gain significantly more kinetic energy. They are so far apart, though, that temperature doesn't have the same meaning as in lower parts of the atmosphere.

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