The tropopause separates the lower troposphere and upper stratosphere, and is notable as the ceiling under which most of Earth’s weather occurs. It is affected by the thermal gradient, which changes beyond the tropopause. Air travel is affected by the tropopause, and it is almost always sunny when commercial airliners reach cruising altitude. Thunderclouds that flatten into an anvil shape can sometimes be observed visually. Tropical storms can sometimes break through into the stratosphere, contributing to the greenhouse effect.
The tropopause is a thin layer of the Earth’s atmosphere that separates the lower troposphere and the upper stratosphere. At the poles, it is about 36,000 feet (1,100 meters) above sea level and rises to 58,000 feet (1,700 m) around the equator. The tropopause is notable as the ceiling under which most of Earth’s weather occurs.
Five main layers make up the Earth’s atmosphere, and from lowest to highest they are the troposphere, stratosphere, mesosphere, thermosphere, and exosphere. Between each level is a boundary, named using the prefix of the lower level and the -pause suffix. These layers and boundaries are defined primarily by properties of temperature, pressure, and density, as well as a phenomenon known as the rate of temperature change.
The thermal gradient is the rate at which temperature decreases as altitude increases. In the troposphere, the temperature decreases by an average of 3°F for every 1,000 feet of altitude (6.5°C per 1,000m). The tropopause marks the level beyond which this tendency changes. Once in the stratosphere, the gradient reverses and the temperature begins to increase with altitude.
Air travel is affected by the tropopause, as jet propulsion is more efficient in colder temperatures. The lapse rate of the troposphere provides a reliable basis for calculating fuel consumption and pilots aim to fly just below the tropopause boundary to maximize performance. Most water vapor is held closer to Earth, so there are few clouds near the tropopause and generally little turbulence compared to those lower in the troposphere. This is why it is almost always sunny when commercial airliners reach cruising altitude.
From the ground, the tropopause can sometimes be observed visually by thunderclouds that flatten into an anvil shape. This occurs when the warm air from the storm pushes up until it reaches the top of the troposphere, where it is no longer less dense than the surrounding air. Then it spreads outwards rather upwards.
While the tropopause functions as a fairly substantial ceiling for the troposphere, it is not impregnable. Tropical storms that make their way over land, for example, can sometimes break through into the stratosphere. When this happens, the water vapor carried by the storm clouds can freeze and be carried thousands of kilometers through the stratosphere by air currents. Some scientists theorize that this phenomenon is a natural factor contributing to the greenhouse effect because the water vapor overhead serves to trap heat near the Earth.
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