Weather is a phenomenon that was introduced in the Tempest expansion, with the addition of another type of weather in the Inferno expansion. When introduced in Tempest, weather was a number of temporary terrain effects that would be present on sea and ocean tiles that could help or hinder units on that tile both on the world map and in battle. The additional type of weather that was introduced in Inferno - the Dust Blizzard - can occur on land as well, but will only occur during a Dust Eclipse.
A given weather phenomenon typically occurs in clusters of a few tiles, sometimes a dozen or more, and will either briefly appear then disappear almost at random, or can move across sections of the map almost like a weather front, but generally the behavior of weather is unpredictable, and should be constantly monitored for effective navigation and combat on the seas. One thing that can make this easier is by controlling the Sky-Shaker Facility that may appear attached to a Fortress in Era V. This facility allows the owner to select a weather type, and for the next ten turns, the vast majority of the ocean will have that type of weather occupy it. This effect can be cut short by a seasonal change though.
The weather effects that can be encountered are as follows: