for the stratosphere Is a planetary lid that envelopes the convectional air; but a lid may form anywhere between the ground and the stratosphere. Wherever a layer of warm air rests on one of still, cold air a lid is formed. Smoke, dust, and other fine foreign matter spread out to form stratus cloud when it reaches such a lid. If the two air layers are not turbulent there is little or no mixing.
Low stratus clouds indicate the height of a lid near the ground—half a mile or more; but the stratiform appearance is seen to best advantage when the clouds are not higher than 30 or 40 degrees above the horizon. Near the zenith they lose their stratiform shape, being then seen in “elevation” and not in “plan”; but frequently they indicate themselves to the practised eye of an observer. The strato-cumulus clouds that follow an anticyclone also indicate a lid. The high fog that completely covers the sky at heights varying from 7000 to 10,000 feet—practically a stratus or an alto-stratus cloud—is a lid. Cross-winds at a very considerable height likewise may indicate a lid.
The presence of a lid has much to do with the comfort of the airman. In penetrating a lid the plane is apt to get a sharp bump. If lightly ballasted, a free balloon may rebound after descending upon a lid and shoot upward several hundred feet. Sir Napier Shaw has called the attention of aeronauts to this possibility.
A low lid affects visibility to a marked degree. Under the lid, fine floating dust, smoke and the various gases of combustion spread out in stratus cloud and greatly impair visibility. The famous London fog is due to the persistence of a low-lying lid.
The impairment of visibility depends partly on the height of the lid and partly on the character of the content of the air underneath it. In open, swarded regions where the air is free from pollution, not much impairment of visibility is likely to exist. In regions where soft coal is used as power fuel, chimney products may accumulate to such an extent that impairment becomes a very serious matter; and the lower the lid, the greater becomes the impairment. Obscurity is apt to grow until increasing pressure breaks the lid and brings about a clearing of the air.
