Refer to figure.

COLD-AIR DROP/COLD POOL

A cold-air drop is a low pressure system, that only exists in the upper troposphere - it’s a cooler-than-normal pocket of air in the higher levels of the atmosphere.

They primarily form nearer to the poles in the colder air masses, and (in the Northern Hemisphere), extend South towards warmer air. At some point, they become detached from the cold Polar air and become a “cold pool” of air in the upper troposphere that has quite an effect on the weather in that region, due to the instability they bring.

CHARACTERISTICS

• Cold pools are a challenge for the weather forecast, because the numerical weather prediction models cannot predict the exact direction of movement.
• Would be indicated by the low contour (isohypses) height on the 300hPa and 500hPa charts, whereas the surface charts and 800 hPa charts may show no sign of any such low pressure region (it exists only above these altitudes, often around 10 000 to 30 000 ft). They cannot be detected on surface analysis charts.
• The weather associated with a cold pool will typically be convective especially over land in summer when thunderstorms can be expected.
• If associated with a static high pressure area such as the Siberian High, a cold pool could be present for weeks. But, in the area of the polar front, the weather systems usually move from West to East in a series of waves and, like the polar front depressions, their duration will be only a few days. => On average cold pools persists for approximately 3 days.

Learning Objective 050.08.03.04.03: Identify cold-air drops on weather charts.

Note: The 2016 syllabus referred to these are "cold pools", but the new syllabus refers to them as "cold-air drops". They are the same so any explanations are interchangeable.