[Pyros]

Stratos,

when you wish to define the visibility you must take into consideration variables like illumination (brightness), air humidity and weather conditions.

For instance, you mentioned that you wish to calculate the visibility during a rain. But you didn't mention variables like the daytime hour, climatological zone and intensity of that rain.

For example, a mid-day (12:00 hour) tropical rain (Rain forest) has lower visibility than a heavy rain at 19:00 (summer-time) at the Mediterranean region.

cheers,