Why is the upper atmosphere cold despite convection?

Context

Convection is the process where warmer, less dense air rises and cooler, denser air sinks. This leads to the expectation that the highest parts of the Earth's atmosphere should be hot. However, observations show that temperatures decrease with altitude, as seen in the colder temperatures on mountaintops compared to lower elevations. The question explores the apparent contradiction between the expected effects of convection and the observed temperature profile of the atmosphere, considering the role of solar heating and air density.

Simple Answer

• Convection does happen in the atmosphere, but only up to a certain point.
• The sun mainly heats the Earth's surface, not the atmosphere directly.
• Air is heated by contact with the warm surface and then rises.
• As air rises, it expands and cools, because there is less pressure higher up.
• Above a certain altitude, other factors like radiation and the composition of the atmosphere become more important than convection in determining temperature.

Detailed Answer

The premise that convection doesn't affect the atmosphere is incorrect. Convection is a significant factor in atmospheric processes, particularly in the troposphere, the lowest layer of the atmosphere. Warmer air near the Earth's surface, heated by solar radiation absorbed by the land and oceans, does indeed rise. This rising air creates convection currents that influence weather patterns and distribute heat around the globe. However, the effect of convection is limited by altitude and other atmospheric factors.

The reason mountaintops are colder than surrounding areas is not a direct contradiction of convection, but rather a consequence of the way the atmosphere is heated. The sun's energy primarily heats the Earth's surface. This heated surface then warms the air in contact with it through conduction and radiation. As the warmed air rises due to convection, it expands and cools adiabatically (without heat exchange with the surroundings). This adiabatic cooling is a key process that governs the temperature profile of the troposphere.

The reduction in air pressure with altitude plays a crucial role in atmospheric cooling. As air rises, the pressure decreases. This allows the air to expand, and this expansion process consumes energy, causing a decrease in temperature. This is why even though convection initially transports warmer air upwards, that air cools as it rises due to this expansion and pressure drop. Therefore, it isn't the lack of convection entirely, but the adiabatic cooling associated with rising air, that leads to colder temperatures at higher altitudes.

Beyond the troposphere, other atmospheric layers exist, such as the stratosphere, mesosphere, and thermosphere. In these layers, the temperature profile becomes more complex and is influenced by factors other than just convection, such as absorption of solar radiation by ozone in the stratosphere. The higher temperatures in the thermosphere are due to the absorption of high-energy solar radiation by atmospheric gases, not primarily due to convection. So, the simple model of convection solely determining temperature breaks down at higher altitudes.

In summary, while convection plays a role in atmospheric heat distribution and affects temperature near the surface, it is not the sole determinant of temperature at all altitudes. Adiabatic cooling associated with rising air, decreased pressure at higher altitudes, and the absorption of solar radiation at different wavelengths in different atmospheric layers all contribute to the complex temperature profile of the atmosphere. Thus, the observation that mountaintops are cold and the upper atmosphere is not uniformly hot is not a refutation of convection, but rather a consequence of the interplay of several atmospheric processes.