TRAJECTORY PATHS A AND B (CYCLONIC). Paths A and B (fig. 4-l-5) are usually indicative of a strong outbreak of cold air and surface winds of 15 knots or more. This wind helps to decrease the stable conditions in the lower levels. If this modified air moves rapidly over rough terrain, the turbulence re-sults in low stratocumulus clouds and occa-sional snow flurries (see fig. 4-1-6).

A particularly troublesome situation often arises when the cold air flows from a cold, snow-covered surface to a water surface and then over a cold, snow-covered surface again.

Figure 4-1-5.Trajectories of cP and cA air in winter.


Figure 4-1-6.cP air moving southward.

This frequently happens with air crossing the Great Lakes. (See fig. 4-1-7.)

On the leeward side of the Great Lakes and on the windward side of the Appalachians, you can expect a rather low, broken to overcast sky condition with frequent and widespread snow squalls. Stratocumulus and cumulus clouds with bases at 500 to 1,000 feet and tops at 7,000 to 10,000 feet form on the leeward side of the Great Lakes. Over the mountains, their tops extend to about 14,000 feet.

Visibility ranges from 1 to 5 miles during rain or snow showers and occasionally lowers to zero in snow flurries.

Severe aircraft icing conditions may be ex-pected over the mountains and light to moderate aircraft icing on the leeward side of the lakes. Moderate to severe flying conditions are the rule as long as the outflow of cold air continues. East of the Appalachians, skies are relatively clear except for scattered stratocumulus clouds. Visibility is unrestricted and the surface temperature is relatively moderate because of tur-bulent mixing.

In the Middle West, clouds associated with this type of air mass continue for 24 to 48 hours after the arrival of the cold mass, while along the Atlantic Coast rapid passage of the leading edge of the air mass produces almost immediate clearing.

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