Randomly generated on a random number of tiles for a random number of turns based on the actual biome of those tiles. Imagine, for example, a storm on a group of 5-6 grassland tiles, effecting tiles output and units. Rising the food output but dropping the production. It could create a marsh on tiles. Units are more vulnerable during snow...or stronger if they are Russians. Winds on sea tiles giving any naval units, from Ancient to Renaissance era, a temporary boost in movement. Desert storm helping civs like Egypt or Arabia but obstructin civs like Norway or Russia.