The Deflection of the Wind from the Gradient: Ferrel’s Law.—The law of the deflection of the wind prevailingly to the right of the gradient is known as Ferrel’s Law, after William Ferrel, a noted American meteorologist, who died in 1891. The operation of this law has already been seen in the spiral circulation of the winds around the cyclone and the anticyclone, as shown on the maps of our series. In the case of the cyclone the gradient is directed inward towards the center; in the case of the anticyclone the gradient is directed outward from the center. In both cases the right-handed deflection results in a spiral whirl, inward in the cyclone, outward in the anticyclone. The operation of this law is further seen in the case of the Northeast Trade Winds. These winds blow from about Lat. 30° N. towards the equator, with wonderful regularity, especially over the oceans. Instead of following the gradient and blowing as north winds, these trades turn to the right of the gradient and become northeast winds, whence their name. From about Lat. 30° N. towards the North Pole there is another great flow of winds over the earth’s surface. These winds do not flow due north, as south winds. They turn to the right, as do the trades, and become southwest or west-southwest winds, being known as the Prevailing Westerlies. Ferrel’s Law thus operates in the larger case of the general circulation of the earth’s atmosphere, as well as in the smaller case of the local winds on our weather maps.
Fig. 47.
CHAPTER X.
CORRELATION OF THE VELOCITY OF THE WIND AND
THE PRESSURE.
Prepare a scale of latitude degrees, as explained in Chapter V. Select some station on the weather map at which there is a wind arrow, and at which you wish to study the relation of wind velocity and pressure. Find the rate of pressure change per degree as explained in Chapter VII. Note also the velocity, in miles per hour, of the wind at the station. Repeat the
operation 100 or more times, selecting stations in different parts of the United States. It is well, however, to include in one investigation either interior stations alone (i.e., more than 100 miles from the coast) or coast stations alone, as the wind velocities are often considerably affected by proximity to the ocean. And, if coast stations are selected, either onshore or offshore winds should alone be included in one exercise. The investigation may, therefore, be carried out so as to embrace the following different sets of operations:—
A. Interior stations. B. Coast stations with onshore winds. C. Coast stations with offshore winds.
Enter your results in a table similar to the one here given:—