In the spring and early summer, frontal cyclones are common weather events that move from west to east in the mid-latitudes. At the same time, the ground surface in the mid-latitudes is receiving elevated levels of insolation which creates ideal conditions for air mass thunderstorm formation. When the cold front or dry line of a frontal cyclone comes in contact with this warm air it pushes it like a bulldozer both horizontally and vertically. If this air has a high humidity and extends some distance to the east, the movement of the mid-latitude cyclone enhances vertical uplift in storm and keeps the thunderstorms supplied with moisture and energy. Thus, the mid-latitude cyclone converts air mass thunderstorms into severe thunderstorms that last for many hours.
Severe thunderstorms dissipate only when no more warm moist air is encountered.
This condition occurs several hours after nightfall when the atmosphere begins to cool off. Figure 5t-6 illustrates the features associated with a severe thunderstorm. This storm would be moving from left to right because of the motion associated with a mid-latitude cyclone. It causes the tilting of vertical air currents within the storm so that the updrafts move up and over the downdrafts. The green arrows represent the updrafts which are created as warm moist air is forced into the front of the storm.