Imagine that you are standing in one spot as a cold front approaches. Along the cold front,
the denser, cold air pushes up the warm air, causing the air pressure to decrease. If the
humidity is high enough, some types of cumulus clouds will grow. High in the atmosphere,
winds blow ice crystals from the tops of these clouds to create cirrostratus and cirrus clouds.
At the front, there will be a line of rain or snow showers or thunderstorms with blustery
winds. Behind the front is the cold air mass. This mass is drier and so precipitation stops.
The weather may be cold and clear or only partly cloudy. Winds may continue to blow into
the low pressure zone at the front.
The weather at a cold front varies with the season. Thunderstorms or tornadoes may form
in spring and summer, when the air is unstable. In the spring, the temperature gradient
can be very high, causing strong winds to blow at the front. In the summer, thunderstorms
may be severe and may also include hailstorms. In the autumn, strong rains fall over a large
area. If the front moves slowly, enough rain may fall to cause flooding. Cold fronts in winter
may bring frigid temperatures and heavy snows. The cold air mass is likely to have formed
in the frigid arctic.
When the temperature gradient across a cold front is low, a cold front has little effect on
the weather. This may occur at some locations in the summer. Along the western United
States, the Pacific Ocean warms and moistens cold air masses so that the temperature
gradient across a cold front is small.
Warm Fronts
Awarm front( 2 ) is found where warm air mass slides over a cold air mass (Figure16.18).
Since the warmer, less dense air is moving over the colder, denser air, the atmosphere is
relatively stable. Warm fronts travel much more slowly than cold fronts because the leading
cold air mass is dense and sluggish.
Figure 16.18: A warm front. Warm air moves forward to take over the position of colder air.
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