Cyclones, which are storms that rotate around a central low-pressure area, occur when high-pressure cold air fronts slide on top of low-pressure warm air fronts. When the warm air rises, cold air on all sides drops, and spinning occurs due to the Coriolis effect.
Usually, when a cold front and a warm front meet, the warm front slides over the cold front because it is lighter. However, the cold front with heavier air sometimes slides over the warm front. When this condition, which is called an inversion, occurs, the warm air tries to rise through the cold air. As the warm air rises, the decreased pressure allows the cold air around it to drop. Because the earth is spinning, the cold air drops in a spiral rather than a straight line, creating wind in the Coriolis effect. When enough cold air drops quickly enough, it results in a cyclone.
If the cyclone occurs in the tropics over a patch of warm ocean, it may develop into a hurricane, which is often called a tropical cyclone. Cyclones that develop over land, such as the ones common in the American Midwest, are called mesocyclones. These cyclones may develop into tornadoes. The third type of cyclone, the polar cyclone, is also called a polar vortex. These persistent cyclones exist high above the North and South Poles. When one weakens, the cold air caught in the vortex shifts toward the equator, often causing sudden cold snaps in non-polar latitudes.