I have no idea whether there is a formula, but I would suspect that they take into account factors like recent rainfall (to create puddles), humidity (to keep puddles from drying out) and temperature (to speed development of larvae).
I have no idea whether there is a formula, but I would suspect that they take into account factors like recent rainfall (to create puddles), humidity (to keep puddles from drying out) and temperature (to speed development of larvae).