Question

At 30 degrees North, is the evaporation rate positive or negative? Meaning is there more evaporation than precipitation, and if so why?

Answer 1

Percipitation is at a peak near the equator, probably at the tropical convergence zone. Away from the equator percipitation drops off to a minimum in the region of 20 to 30 degree band. From there percipitation rises and then falls of to near zero at the poles.

As the percipitation falls near to zero, evaporation rate is higher than percipitation rate.

There are three processes involving the transfer of water from Earth's surface into the atmosphere. There is evaporation per se which js the conversion of liquid water into water vapour. There is also sublimation the conversion of solid water as ice and snow into water vapour.

In understanding evaporation it is the potential evaporation which is crucial. The actual evaporation may ne limited by the surface availability of water. Two processes could be at work in evaporation. In the layer of air closest to the surface, the layer known as the laminar viscous layer, the transfer process is by molecular diffusion. In this layer Fick's law applies ie, the rate of evaporation per unit area E is proportional to the vertical gradient of specific humidity q.

This diffusion process is less significant than the transfer by turbulence. This transfer by turbulence could be proportional to the product of the fluctuations in specific humidity and vertical wind velocity. This would be eddy diffusion and process of this type are often assumed to follow a law identical in form to the one for diffusion but with the coefficient, called the eddy diffusivity coefficient being greater in magnitude.