Tornados or whirlwinds are common sight in the desert. In flat areas a breeze can easily strengthen to a tornado, tearing everything apart in its way.
Such events will affect skirmish games (and castle builders) in a same manner. Tornados may randomly strike and destroy structures and people, which will also come with an unwelcome negative popularity. This event seems to be very destructive, although the damage caused by it would affect a small area.
The tornado can be either turned off or turned on in skirmish matches.