Occasionally the tropopause is difficult to discern because of extensive mixing between the upper troposphere and the lower stratosphere. This situation is common in portions of the mid-latitudes, usually defining the location of jet streams a narrow belt of high-velocity winds often in excess of km. Because the height of the tropopause is dependent on the average temperature of the troposphere, temperature changes in this layer can influence the location of extratropical storm tracks and cloud depth.
Embedded frequently within the troposphere are thin sublayers in which the temperature actually increases with height, known as temperature inversions. Radiation inversions result from nocturnal surface cooling. Under certain ambient conditions e. Conversely, subsidence inversions occur from mid-upper tropo-spheric processes that produce areas of sinking air that are being warmed by compression; hence, lower tro-pospheric temperatures are actually colder than those aloft.
This setting tends to stabilize the air, inhibiting vertical mixing and cloud growth. A semipermanent sublayer of the troposphere is the planetary boundary layer PBL , a section directly influenced by surface daily conditions. Comprising typically the lowest 1 km.
The depth of the PBL amplifies and diminishes with the daily solar cycle, such that the greatest thickness is during the day when the atmosphere is most turbulent. Evidence suggests that the troposphere has undergone a significant rate of warming during the past century. The tropospheric temperature trend in the latter half of the 20th century is estimated at a 0. Higher temperatures mean increased surface evaporation and tropospheric water vapor content. At the top of the troposphere is a thin layer in which the temperature does not change with height.
This means that the cooler, denser air of the troposphere is trapped beneath the warmer, less dense air of the stratosphere. Air from the troposphere and stratosphere rarely mix. Ash and gas from a large volcanic eruption may burst into the stratosphere , the layer above the troposphere.
Once in the stratosphere, it remains suspended there for many years because there is so little mixing between the two layers. Pilots like to fly in the lower portions of the stratosphere because there is little air turbulence. In the stratosphere, temperature increases with altitude. What is the heat source for the stratosphere? The direct heat source for the stratosphere is the Sun. Air in the stratosphere is stable because warmer, less dense air sits over cooler, denser air.
As a result, there is little mixing of air within the layer. The ozone layer is found within the stratosphere between 15 to 30 km 9 to 19 miles altitude. The thickness of the ozone layer varies by the season and also by latitude. Because of this, the ozone layer protects life on Earth. High-energy UV light penetrates cells and damages DNA, leading to cell death which we know as a bad sunburn. Organisms on Earth are not adapted to heavy UV exposure, which kills or damages them.
Temperatures in the mesosphere decrease with altitude. The air in the mesosphere has extremely low density: As a result, air pressure is very low Figure below. A person traveling through the mesosphere would experience severe burns from ultraviolet light since the ozone layer which provides UV protection is in the stratosphere below.
There would be almost no oxygen for breathing. Meteors burn in the mesosphere even though the gas is very thin; these burning meteors are shooting stars. The density of molecules is so low in the thermosphere that one gas molecule can go about 1 km before it collides with another molecule. Since so little energy is transferred, the air feels very cold Figure above. Within the thermosphere is the ionosphere.
The ionosphere gets its name from the solar radiation that ionizes gas molecules to create a positively charged ion and one or more negatively charged electrons. The freed electrons travel within the ionosphere as electric currents. Because of the free ions, the ionosphere has many interesting characteristics. 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. Right: Transport of ozone depicted by curved blue line. Third Edition by Dr. James Holton, Another important feature of the stratosphere is the cold pool of air that forms at high latitudes during the winter. This cold air is centered in the lower stratosphere at about 25 km. During the Southern Hemisphere winter air can reach temperatures colder than C near the South Pole.
In the Northern Hemisphere, the lowest temperatures reach about C. As a result, a zone of strong westerly winds or vortex forms and surrounds each pole. Because the temperature contrast is greatest in the vicinity of the South Pole, the vortex that forms there during the Southern Hemisphere winter is considerably stronger than the vortex that forms during the Northern Hemisphere winter.
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