[Physics] How to calculate precipitation chance with basic weather data


I would like to know if there is any algorithm which allows us to calulate precipation chance with following data: temperature, humidity, illuminance (in lux) and pressure. I've searched it in google, but no results satisfied me.

Best Answer

Simple climate models use the fact that the total amount of water is conserved, and assume that there are three processes that affect it: evaporation, precipitation, and moisture flux (i.e. how the moisture already in the atmosphere moves around). We can write the moisture flux vector simply as the product of the velocity vector and specific humidity: $\mathbf{F_M}=q\mathbf{v}$, and vertically integrated gives us total column water transport: $$ \mathbf{F}=\int_0^\infty \rho \mathbf{F_M}dz $$ where $\rho\equiv\rho(z)$ is the density of the air at a given height, $z$. So, the rate of change of precipitable water in the atmosphere can be written as the sum of the contributions from evaporation, precipitation, and column moisture flux convergence, hence: $$ \frac{\partial}{\partial t}\int^\infty_0 \rho qdz = \text{E}-\text{P} -\nabla\cdot \mathbf{F} $$ So an estimate for precipitation can be derived from surface evaporation rates (ignoring complicated transpiration terms from vegetation, this is essentially a function of sea surface temperature) and functions of specific humidity.

Insmaller scale models, things get significantly more complicated - depending on the scale, one must consider implicit convection terms and/or cloud microphysics as well.

Related Question