Excerpt from The Aeroplane: A Concise Scientific Study

On account of surface irregularities, ground winds are of an entirely different character from the winds of the upper atmo spheric regions. The speed and steadiness of a surface wind increase with the height above the earth's surface, until the wind velocity reaches a more or less limiting value. There is evidence to Show that the gustiness of surface winds is due to the effect of the ground, and it is convenient to assume that gustiness disappears, and the limiting velocity is reached, in the air strata which are removed from ground influences. The upper layers probably have a regular periodic motion.

A surface wind blowing in from the sea has its velocity continuously diminished as it passes inland, on account of the retarding influences of surface friction and obstacles.

The experience of balloonists and pilots has proved the existence of vertical atmospheric currents. The formation of clouds, as is well known, depends upon the rising of warm moist air currents into the colder upper regions, although the absence of clouds does not necessarily imply the absence of ascending air currents. Occasionally, the velocity of these ascending currents is sufficiently great to support large hail stones. A cumulus cloud is generally the cap of an ascending air current, and a cumulus sky may be regarded as a danger signal indicating the prevalence of ascending and descending air currents. Rising currents, chiefly formed by the local heating of the earth's surface by the sun's rays, are always accompanied by descending air currents. The low-pressure centre of a cyclonic disturbance is also partial evidence of the existence of an ascending current, and, conversely, the high pressure core of an anticyclone indicates the presence of a descending air current.

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