Practical Exercises in Elementary Meteorology

CHAPTER X.

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:—

Table II.—Correlation of Wind Velocity and Barometric
Gradient.

For interior (or coast) stations, with onshore (or offshore) winds, in the United States during the month (or months) of

Rates of Pressure Change per Latitude Degree	8-20	20-10	10-5	5- 3¹/₂	3¹/₂- 2¹/₂	2¹/₂ -2	etc.
Distances between Isobars in Latitude Degrees	0¹/₂	¹/₂-1	1-2	2-3	3-4	4-5	etc.
Wind Velocities (miles per hour)
Sums
Cases
Means

The wind velocity for each station is to be entered in the column at whose top is the rate of pressure change found for that station. Thus, if for any station the rate of pressure change is 3¹/₂ (i.e., .03 inch in one latitude degree), and the wind velocity at that station is 17 miles an hour, enter the 17 in the fourth and fifth columns of the table. When you find that the rate of pressure change for any station falls into two columns of the table, as, e.g., 10, or 5, or 3¹/₃, then enter the corresponding wind velocity in both those columns.

In the space marked Sums write the sum-total of all the wind velocities in each column. The Cases are the number of separate observations you have in each column. The Means denote the average or mean wind velocities found in each column, and are obtained by dividing the sums by the cases.

Study the results of your table carefully. Deduce from your own results a general rule for wind velocities as related to barometric gradients.

The dependence of wind velocities on the pressure gradient is a fact of great importance in meteorology. The ship captain at sea knows that a rapid fall of his barometer means a rapid rate of pressure change, and foretells high winds. He therefore makes his preparations accordingly, by shortening sail and by making everything fast. The isobaric charts of the globe for January and July show that the pressure gradients are stronger (i.e., the rate of pressure change is more rapid) over the Northern Hemisphere in January than in July. This fact would lead us to expect that the velocities of the general winds over the Northern Hemisphere should be higher in winter than in summer, and so they are. Observations of the movements of clouds made at Blue Hill Observatory, Hyde Park, Mass., show that the whole atmosphere, up to the highest cloud level, moves almost twice as fast in winter as in summer. In the higher latitudes of the Southern Hemisphere, where the barometric gradients are prevailingly much stronger than in the Northern, the wind velocities are also prevailingly higher than they are north of the equator. The prevailing westerly winds of the Southern Hemisphere, south of latitude of 30° S., blow with high velocities nearly all the time, especially during the winter months (June, July, August). These winds are so strong from the westward that vessels trying to round Cape Horn from the east often occupy weeks beating against head gales, which continually blow them back on their course.

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