Column: Our Common Atmosphere – Where Do We Live?
We live at the bottom of an ocean of air called the atmosphere. It is a sea of life sustaining gases that surround the earth. As we move vertically we will find different layers. Each layer is characterized by abrupt changes in temperature resulting from differences in the absorption of incoming solar radiation.
The layer in which we live, the one closest to the earth, is called the troposphere. It contains 75-80% of the mass of the earth’s air. It extends only about 17 kilometres above sea level at the equator and about 8 kilometres over the poles. If the earth were the size of an apple, this lower layer, containing all of the air that we breathe, would be no thicker than the apple’s skin! The picture shows an image of the Earth with the atmosphere colour enhanced – thin and fragile.
This thin and turbulent layer of rising and falling air currents and winds is the planet’s weather breeder. About 99% of the volume of the air consists of two gases – nitrogen (78%) and oxygen (21%). The rest is water vapour varying from 0.01% at the frigid poles to 4% in the humid tropics. Calculations made by scientists tell us that, if oxygen was to climb above 25%, that the earth would be an inferno (all vegetation and other organic material would burn). On the other hand, if the oxygen level was to fall below 15%, not even the driest materials would burn.
Historically, as oxygen increased in our atmosphere, an ozone layer formed. Ozone is sort of super oxygen – O3 rather than O2. When oxygen (O2) is bombarded by ultraviolet light it forms ozone (O3). The creation of this layer allowed new and more complex forms of life to evolve on earth.
Ozone acts to absorb (soak up) ultraviolet light and prevent it from reaching the surface. This is the so-called “ozone layer”, which is essential for complex living organisms, like you and me, to exist. Without the protective ozone layer, the ultraviolet light (radiation) from the sun would just kill us.