Spekboom could cool the ground in dry areas and enhance rainfall. Sand has air between the grains and this causes sand to act as an insulator concentrating the solar energy in the top few cm of dry sandy soil. So sand gets very hot and heats the air above it by contact and infrared radiation. While sand could reach a temperatures of 60 deg C or so I think Spekboom would be a lot cooler. Leaves of trees and Spekboom, etc, act like "convection machines" because there is a large leaf area at the top of trees, etc, (where leaves are heated in the sun) in contact with the air. So the tree is air-cooled.
If you cool the ground you cool the air above it and the lifted condensation level (LCL) is reduced making rain more likely when the air is lifted by blowing up mountains, etc. The equation is LCL=125(Tair-Tdew) where LCL is in m, Tair is the air temperature in deg C and Tdew is the dew point temperature in deg C.
If you just heat or cool air the dew point temperature remains the same, so cooling air reduces the LCL (height to which air must be lifted for clouds to form). The dew point temperature depends only on the water vapour pressure in the air. Since the atmospheric pressure remains the same and the mole fraction of water vapour remains the same when air is heated or cooled and vapour pressure in the air is (mole fraction of water vapour)x(atmospheric pressure), the dew point remains the same.
Cooling air reduces vapour pressure deficit.
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