The low-pressure region and the uplift just mentioned are associated with a well-defined boundary between (polar) cold air and (subtropical) warmer air, called a 'front', discussed in Chapter 13. A cold front involves cold air underrunning warmer air, just as with a small-scale density current, and results in the uplift of the warmer air. This creates long bands of cloud over the front and behind it. But sometimes the warm air advances over the cooler air, creating a warm front and then the cloud tends to be more widespread and uniform. Frontal ascent is generally about 10 km/day.
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