Imagine that you are on the ground in the wintertime under a cold winter air mass with a
warm front approaching. The transition between the cold air and the warm air takes place
over a long distance. This means that the first signs of changing weather appear long before
the front is actually over you. In fact, weather changes may appear hundreds of kilometers
ahead of the front. Initially, the air is cold: the cold air mass is above you and the warm air
mass is above it. High cirrus clouds mark the transition from one air mass to the other.
Over time, cirrus clouds become thicker and cirrostratus clouds form. As the front ap-
proaches, altocumulus and altostratus clouds appear and the sky turns gray. Since it is
winter, snowflakes fall. Soon the clouds thicken and nimbostratus clouds form. Snowfall
increases. Winds grow stronger as the low pressure approaches. As the front gets closer, the
cold air mass is just above you but the warm air mass is not too far above that. The weather
worsens. As the warm air mass approaches, temperatures rise and snow turns to sleet and
freezing rain. Warm and cold air mix at the front, leading to the formation of stratus clouds
and fog (Figure16.19).
Figure 16.19: Cumulus clouds build at a warm front. ( 35 )As the front passes over you, the temperature and dew point rise and the rain likely ends.
Winds change direction. The transition is not nearly as dramatic as when a cold front passes
over, since there is more mixing of the two air masses occurring in a warm front.
The Pacific Ocean also plays a role in modifying the warm fronts that reach the west coast
of the United States. These storms are so broad that it is very difficult to spot exactly where
the warm front is!