Humans think of themselves as living on the surface of the earth, which, of course, is true. However, what is less obvious and more difficult for people to visualize is that life exists at the bottom of an ocean of air (the atmosphere).
The atmosphere is held tightly against the planet by the pull of gravity from within the planet. Investigations have revealed, however, that the atmosphere is not a homogeneous and unchanging mixture of gases from the surface of the planet on out into space. Rather, the atmosphere exists in layers and the conditions of temperature and pressure within each layer are different from the layers below it or above it.
Snuggled next to the planet is the bottom layer of the atmosphere and the one in which humans live – the troposphere.
The Thickness of the Troposphere
The lowest layer, the troposphere, contains about 80% of the mass of the atmosphere and extends from ground level to an altitude that varies from 8 to 18 km (5 to 11 miles).
The thickness of the troposphere is not the same everywhere; it varies with latitude and the seasons. The troposphere is thickest near the equator and thinnest near the poles.
The thickness of the troposphere takes a sharp rise at mid-latitudes and this in turn exerts an influence on global weather patterns.
The Troposphere and Temperature
The Earth’s surface – rocks, water, and vegetation – is heated by solar energy. This energy radiates back into the atmosphere as heat causing the air to be warmest at the Earth’s surface.
The temperature of the air in the troposphere decreases with increasing distance from the Earth’s surface. The Earth’s surface acts like a heater in a cold room. The further you get from the heater, the more the air temperature drops.
The fact that air temperature decreases with an increase in altitude is most obvious in mountainous regions. On wet days it may be raining at the base of a mountain but snowing on top. This accounts for the fact that snow and ice persists on mountain peaks year round.
In fact, the air temperature decreases by an average of 6.5°C per km (3.6°F per 1,000 ft) above the Earth’s surface and temperatures as low as -60°C (-76°F) occur at the top of the troposphere.
The Troposphere and Weather
The term troposphere is derived from the Greek, tropos, meaning “to mix” or “turn.” This is an accurate term in that the troposphere is the layer of the atmosphere in which atmospheric circulation occurs.
As warm air rises, it is replaced by denser, cooler air which in turn is heated by the warmth of the Earth’s surface and the cycle continues. As a result, the lowermost portion of the troposphere is in constant motion and it is this air motion that is responsible for the changing patterns of weather.
The uppermost part of the troposphere, however, is very stable and free of clouds. This makes it the perfect place for large commercial aircraft to fly. Frequent flyers know that air turbulence is most commonly felt during takeoff and climb to cruising altitude or during descent from cruising altitude for landing.
The fact of decreasing temperature with altitude is fundamental not only to weather patterns but also ensures the maintenance of water on our planet. As warm air rises, it cools and condenses to form clouds. The condensing water droplets within clouds grow larger until they reach a critical size and fall back to Earth as precipitation.
The cycle of condensation and precipitation prevents evaporating water from escaping the planet altogether.
The Tropopause
The tropopause is the boundary at the top of the troposphere where the upper troposphere gradually blends into the overlying stratosphere. In the tropopause the air ceases to cool with increasing height and becomes almost completely dry.
Not only does temperature decrease with increasing altitude but the density of air decreases as well. At the height of tropopause the atmospheric pressure is one thousandth of the pressure at the Earth's surface. Furthermore, the atmospheric pressure within the tropopause never varies from the equator to the poles.
The height of this boundary decreases from 18 km (11 mi) near the equator to about 8 km (5 mi) near the poles. The decrease is not gradual, but instead occurs in a narrow zone at mid-latitudes where cold dense polar air meets warm tropical air in the troposphere below.
Held in place by the tug of gravity, the warm and breathable troposphere makes life on this planet possible.
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