Page:Handbook of Meteorology.djvu/149

 The obscuration of outlines of objects at ground level may warn the airman in some cases that he is nearing a downdraught. The obscuration may be a dust storm; if under a cumulo-nimbus cloud it is pretty certain to be a dust-storm; in snow-clad regions it may be due to wind-blown snow.

In any case, although they do not materially impair visibility, such local convectional movements are more disconcerting than the cyclones and anticyclones. The forecasting of these and the charting of their tracks has become a science. Small local convectional movements cannot ordinarily be predicted along with general forecasts; but in various cases they may be predicted locally. Thus, during abnormally hot days in the valleys east of the Coast Ranges of California, the updraught is so great that strong sea winds prevail along the coast. The air is clear until after sunset; then, because of rapid cooling, fog billows roll in through the Golden Gate and cover much of the lowlands.

The Ceiling or “Lid.” —The paradoxical epigram, “air to be warmed must first be cooled” and vice versa is strictly true. If a body of air, having been thoroughly mixed, comes to rest, the temperature is not the same throughout its mass; it is cooler at the approximate rate of 1 degree Fahrenheit for every 183 feet of ascent (about 10 degrees centigrade per kilometer). It may be defined as being in convective equilibrium while in such a condition. Now, if a body of air at the top be cooled a few degrees, it contracts and becomes relatively heavier. Because it is heavier, it begins to drop. It is warmed by compression as it descends, but it is always surrounded by warmer air; so it drops until it reaches a plane which it cannot penetrate—sometimes a layer of colder air; sometimes the ground.

Similarly, if a mass of air at the ground be warmed ever so slightly above the surrounding air, immediately it begins to ascend, being floated upward because it is lighter. It is chilled by its own expansion as it ascends, but as its temperature remains higher than that of the surrounding air it continues to rise until it reaches a layer of air as warm as itself. At that level it ceases to ascend, and spreads out instead. This plane therefore becomes a “lid.” Perhaps it may reach the stratosphere,