Tropical cyclones form when warm ocean waters heat the air above, causing it to rise and create a low pressure zone. As more air flows in from outside this region, it warms, flows upward and deposits moisture in the form of clouds before cooling and falling. Eventually, this current creates a circular rotation in the clouds, drawing more energy into the cyclone and strengthening it.
Tropical cyclones require extremely calm conditions, as wind shear can disrupt the rotation cycle before it starts. As long as a cyclone travels over tropical ocean waters, it grows in strength and increases its wind speed, potentially turning into a hurricane. A tropical cyclone lasts until it no longer has access to warm water and begins to die as soon as it passes over land or into colder ocean areas.
Smaller cyclones called mesocyclones form over land and are precursors to tornadoes. In this case, strong wind shear created by overlapping fronts creates a zone of horizontal spinning air inside the cloud layer. Strong updrafts draw moisture into the storm, creating a thick wall cloud extending down to the surface. The updraft eventually draws the mesocyclone upward, changing its orientation to vertical. At that point, the cyclone wraps itself in the wall cloud and descends, becoming a tornado.