There is a method to increase precipitation called “Cloud seeding”. Artificial rain by utilizing sunlight – Hadley microcell – is another interesting approach for creating artificial rain. Hadley cell is a natural phenomenon and part of Hadley cell close to the equator receives rich precipitation.
Why rain happens
There are several mechanisms of creating precipitations in nature. One of them is convectional precipitation. The stages of convectional precipitation are:
- Air is heated from solar energy
- Warm air is lighter than surrounding air and lifts up.
- At altitude the air expands adiabatically. Its temperature decreases due to adiabatic expansion.
- Moisture in the air condenses. This process releases latent heat and additionally decreases air density that gives additional lifting force.
- Condensed water falls to ground. When condensed water leaves air, the air becomes more lighter and receive additional lifting force.
Convectional precipitation in subtropical deserts
Convection needs to receive energy to work.
- Significant part of sunlight is reflected due to relatively high albedo.
- In order convection occurs heated air needs to be replaced by new volume of air. But airflow is restricted due to ground surface geometry. So, heated air gets stuck to the ground. Considering low heat conductivity of air it leads to overheating of ground surface and higher thermal radiation.
- Heat conductivity of air is much lower than one of ground. It leads ground absorb sun energy much readily compared to air.
As a result the air receives very small amount of sun energy (maybe 3% or less). Most of the sun’s energy is reflected, is re-emitted as thermal radiation or goes into the ground, and convection receive almost no energy. This implies convection almost doesn’t work. Additionally Hadley cell creates descending air current in subtropics that suppresses this weak convection.
For comparison let us consider the processes at equatorial forests. Albedo is relatively low which mean sun’s energy is better absorbed. Trees absorb sunlight and heat the air. Heated air is readily replaced by air from below. Ground receives and absorbs insignificant amount of sunlight. As a result the air receive high amount of solar energy and convection works well.
What is Hadley Microcell
Hadley microcell is artificial ground surface modification. It is several kilometers in diameter. A land is covered by special heating elements. Each heating element is elevated several meters above the surface. A heating element is permeable for air to allow free airflow. So Hadley microcell mimics equatorial forests. Building a Hadley microcell can probably increase precipitation.
P.S. Trees make convection. So trees and forests are important for precipitation. Deforestation reduces precipitation.
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