Located about 10 miles from the equator, most of the clouds are found in the Troposphere. The layer starts at the earth’s surface and goes up to a height of about 7–20 km above sea level. At the bottom of the Troposphere, near the ground level, air is warmest. The higher it goes, the colder it gets. Hence, at high altitudes, the pressure as well as the density of air is comparatively less. There is a boundary between the top of the Troposphere and Stratosphere, which is the layer above it. The height of this boundary depends on the latitude, season and the time of the day (whether it is day or night). It is called Tropopause. The height of the Tropopause at the poles is about 5 miles.
The word ‘Troposphere’ comes from the Greek word ‘’tropos’ which means ‘change’. Its derivation, however, reflects the fact that turbulent mixing plays an important role in the atmosphere’s structure and behaviour. The phenomena of weather, which is daily associated, occur in the Troposphere.
Being the most closest to the earth, the Troposphere sources elements that provide the Planet Earth with a favourable atmosphere – not overly hot like Venus or overly cold like Mars. This function of balance is modulated by the atmosphere’s natural trace gases. Referred to as ‘greenhouse gases’, they are quite different from nitrogen and oxygen. They control the speed at which heat can escape earth by absorbing and emitting infrared radiation. This category of trace gases consist of water vapour (95%), carbon dioxide (3.62%), methane (0.36%), nitrous oxide (0.95%) and CFCs as well as other miscellaneous gases (0.07%). With a notable exception of water vapour, the chemical composition of the Troposphere is essentially uniform. The proportion of water vapour is normally greatest near the surface and decreases with height. Troposphere directly interacts with biosphere (the land and vegetation), the hydrosphere (the oceans), the cryosphere (the ice caps), the lithosphere (the topography) and the human world.
Generally, the Troposphere is thought to be wider at the equator due to temperature, convection and radiative physics. However, there are other possible factors that contribute to this discussion:
- The Earth’s Rotation: The diameter of the Earth is about 43.5 km bigger at the equator than that at its poles. Hence, one can say that it is because of the rotation of the earth, that is, the centrifugal effect of the spinning mass. But, for it to be effective there must be a coupling between the atmosphere and Earth. However, at the surface the atmosphere goes round at the same speed the earth does.
- Gravity: If there would be no gravity, the atmosphere would be pulled towards sphericity. It is the gravitational pull of the moon and the sun that creates tides on the earth. Despite the tides being commonly associated with oceans and large bodies of water, they have been created in the atmosphere by the gravity and even in the lithosphere. One important fact is that the atmospheric tidal bulge extends far into space. However, on the contrary, the bulge of the lithosphere is limited to approximately 12 inches twice a day.
Tropopause is at the very top of the Troposphere where the temperature reaches a minimum. Some scientists refer to it as a ‘thermal layer’ or ‘cold trap’ as it is a point where rising water vapour cannot go higher because it changes into ice and is trapped. In the absence of this trap, the earth would lose all its water. Convection currents and winds are caused due to the uneven heating of the regions by the sun. When warm air from the earth’s surface rises and reaches the tropopause, it cannot go higher as the air above it is warmer and comparatively lighter. This prevents much air convection beyond the tropopause. It acts as an invisible barrier and hence the weather phenomena occur within the Troposphere.
The atmospheric pressure decreases with higher altitude and is maximum at sea level. This happens due to the atmosphere to be nearly in hydrostatic equilibrium so that the pressure is equal to the weight of the air above a given point. As the altitude increases, the temperature of the Troposphere decreases. This is because of the Environmental Lapse Rate (ELR) which is nothing more than the difference between the surface and the tropopause divided by the height. The reason behind this difference in the temperature is that most absorption of the sun’s energy occurs at the ground, which then heats the lower levels of the atmosphere. In addition, the radiation of heat occurs at the top of the atmosphere and cools the earth by maintaining the overall heat balance of the earth.