Theory Behind Tropical Relative Humidity
Water vapor, water that has evaporated into the atmosphere, is a natural amplifier of climate change. When the climate warms (e.g., from increasing carbon dioxide [CO2]), the warmer air “holds” more water vapor, so the water vapor concentration increases. Water vapor itself is a powerful greenhouse gas, so an increase in water vapor leads to more warming, which allows the air to hold more water vapor, and so on. Therefore, water vapor amplifies CO2’s warming effect. The assumption, which was also predicted by models, was that the relative humidity (RH), or amount of water vapor in the air divided by the amount the atmosphere can hold at a given temperature, does not change with temperature. Until now, however, no clear reasoning for this basic assumption had been provided. A recent study by a Department of Energy-funded researcher at Lawrence Berkeley National Laboratory has developed a theoretical explanation for explaining this assumption, as well as how relative humidity should vary with height in the tropical atmosphere. Given a few inputs, including the pressure, temperature, and amount of air that convective plumes take in as they rise (i.e., the entrainment) and release at the top of the plume (i.e., the detrainment), a set of equations was derived that predicts the RH as well as the change of temperature with height. The theory was used to show that the magnitude of RH is maximum at the surface, decreases with height to a minimum in the lower troposphere, then increases to a high value in the upper troposphere. The theory also confirms the assumption that RH does not vary with temperature as the atmosphere warms. This new study provides an important theoretical foundation for understanding how the atmosphere responds to temperature change.
Romps, D. M. 2014. “An Analytical Model for Tropical Relative Humidity,” Journal of Climate, DOI:10.1175/JCLI-D-14-00255.1.