The Layers of Earth’s Atmosphere Science 10

Troposphere (~10 deg to -60 deg C) From the earth's surface to km above, temperature decreases with height. This fact results from the sun's radiation striking the earth and the earth then warming the air above it. So the closer the air is to the ground, the warmer it becomes. The temperature can increase with height in the lower troposphere. When this happens, it is called an "inversion". If the temperature remains the same with height, it is called "isothermal".

Because warm air tends to rise and cool air tends to sink, the troposphere is a location of much movement of air, or "turbulence". Hence, the troposphere is described by meteorologists as being "well-mixed". If pollutants are injected into the troposphere, they are mixed throughout its depth in a few days and, usually within a week or so, will fall back to the ground with the rain (e.g., acid rain). Thus, the troposphere has a self-cleaning mechanism.

Stratosphere (~0 deg to -60 deg Stratosphere (~0 deg to -60 deg The stratosphere is marked by a temperature inversion from about km to 50 km above sea level. Because warmer air lies above cooler air in this region, there are few overturning air currents and, thus, the stratosphere is a region of little mixing. Particles that travel from the troposphere into the stratosphere can stay aloft for many years without returning to the ground. For example, large volcanic eruptions force ash to be projected into the stratosphere, where it may remain for years and causing slight global cooling in the process.

It is relatively easy to see where the stratosphere begins when there are large convective storms around. The tops of these storms reach only into the lowest level of the stratosphere. Because of the temperature inversion, air rising in the thunderstorm eventually becomes cooler than its environment in the stratosphere and stops rising.

Why does the temperature increase with height in the stratosphere? Because the ozone (O3) layer mostly resides at this level in the atmosphere. Ozone absorbs UV radiation from the sun which, in turn, increases the motion of the ozone molecules. The ozone molecules then collide with other molecules in the air, increasing its temperature. UV radiation

Mesosphere (0 deg to -90 deg C) The mesosphere resides from about 50 km to km above the earth's surface. Because 99.9% of the mass of the atmosphere lies below the stratosphere, the air pressure and density in the mesosphere are extremely low (about 1/1000th of the surface). There is not enough oxygen to breathe here, although the percentage of oxygen in the atmosphere is about the same.

There is not a layer of ozone to cause heating, so temperatures are colder as height increases. The stratosphere warms the lowest levels of the mesosphere and the heat is slowly circulated throughout the mesosphere.

Thermosphere ( deg to -90 deg C) The thermosphere lies above about 90 km. Oxygen absorbs UV radiation and gains significant kinetic energy (i.e., few molecules are around to bump into). Unlike in the troposphere and stratosphere, temperatures in the thermosphere can change by hundreds of degrees depending on the amount of solar activity. This region marks where the percentage of atmospheric constituents change. There becomes far more atomic oxygen than molecular oxygen or even nitrogen.