The Atmosphere
Life on earth depends on the earth’s atmosphere, a layer of gases that envelop the planet. This blanket of gases reaches altitudes of over 100 kilometers and is held in place by earth’s gravity. The layer of the atmosphere at the earth’s surface is known as the troposphere. The troposphere is approximately 16 kilometers thick at the equator and approximately 8 kilometers thick at the poles, with an average thickness of 12 kilometers.
We live in the troposphere. Weather happens in the troposphere.
To fully appreciate the thinness of the troposphere, think of some landmark, i.e. a town, a bridge, a river that is located about 12 km from your location and imagine tipping that distance up vertical. Mount Everest, the world’s highest peak extends 8.8 kilometers into the troposphere.
Even though the troposphere represents a relatively small portion of earth’s atmosphere, it contains 99% of the atmosphere’s water, an important piece, when it comes to weather.
Solar radiation heats the earth’s surface and the temperature of the air at the earth’s surface is maintained by the atmosphere. The mean temperature on the surface of planet earth is 140 celsius. Generally speaking, if we go upward into the troposphere, the temperature drops. This temperature change with height is called the “lapse rate”.
Solar Heating
We have all felt solar radiation. While the ozone layer in the upper atmosphere absorbs most of the harmful ultra-violet radiation, we do receive enough solar energy at the surface to heat the land and in turn the air at the earth’s surface.
Most of the energy arriving to earth’s atmosphere is reflected back into space. “Albedo” latin for “whiteness” is a measure of the reflection of solar radiation, where a black surface (0) absorbs all incident radiation and a white surface (1) reflects all incident radiation. The surface of the earth will heat at different rates depending on it’s colour. Freshly fallen snow could have an albedo as high as 0.9, while forests range from 0.1 to 0.2.
Besides the earth’s reflectivity, there are other factors that impact the absorption of solar heat at the surface.
The earth’s rotation means that the earth’s surface heats up during the day, but it loses heat during the night time.
Land heats up more quickly than water and looses heat more quickly than water.
The area around the equator receives the most direct radiation, while the poles receive the most indirect. In our annual trip around the sun, the earth’s tilt of 23.5 degrees produces the seasons, which in turn heats and cools the hemispheres seasonally.
The uneven heating and cooling of the earth’s surface is a major contributor to local weather.
