Frontal or cyclonic activity primarily occurs in the middle latitudes, especially in the winter but also commonly in the summer. Tropical air masses originate in the lower latitudes and are therefore generally warm. Polar air masses originate in the higher latitudes and are therefore generally cold. Continental air masses originate over land and are therefore generally dry. Maritime air masses originate over water and are therefore generally wet.
Tropical cyclones are low pressure storms in the tropics.
For most of the year they are relatively mild and weak in their effects. In late summer and early fall these tropical cyclones can develop into tropical storms and possibly hurricanes. Mid-latitude cyclones are the major weather force in the middle latitudes. For most of the time, they are large, mild, slow moving storms. Especially in the late spring and early summer they can develop into tornadoes.