Practical Exercises in Elementary Meteorology

CHAPTER XIII.

CORRELATION OF THE DIRECTION OF THE WIND AND THE
TEMPERATURE.

It is evident, from even the most general observation of the weather elements, that the temperature experienced at any place is very largely dependent upon the direction of the wind. Thus, for instance, in the United States, a wind from some northerly point is likely to bring a lower temperature than a southerly wind. To investigate this matter more closely, and to discover how the winds at any station during any month are related to the temperatures noted at that station, we proceed as follows:—

Select the Weather Bureau station at which you wish to study these conditions. Note the direction of the wind and the temperature at that station on the first day of any month. Prepare a table similar to the following one.

Table III.—Correlation of the Direction of the Wind
and the Temperature.

At ..................... during the Month of ........
Wind Directions	N.	NE.	E.	SE.	S.	SW.	W.	NW.
Temperatures






Sums									Total
Cases									Total
Means									Mean

Enter the temperature at 8 A.M. on the first day of the month in a column of the table under the proper wind direction. Thus, if the wind is NE., and the temperature 42°, enter 42 in the second column of the table. Repeat the observation for the same station, and for all the other days of the month, recording the temperatures in each case in their appropriate columns in the table. Omit all cases in which the wind is light, because winds of low velocities are apt to be considerably affected by local influences. When the observations for the whole month have been entered in the table, add up all the temperatures in each column (sums). Find the mean temperature (means) observed with each wind direction by dividing the sums by the number of observations in each column (cases). Add all the sums together; divide by the total number of cases, and the result will be the mean temperature[5] for the month at the station. The general effect of the different wind directions upon the temperature is shown by a comparison of the means derived from each column with the mean for the month.

[5] Derived from the 8 A.M. observations. This does not give the true mean temperature.

Fig. 49.

A graphic representation of the results of this investigation will help to emphasize the lesson. Draw, as in the accompanying figure (Fig. 49), eight lines from a central point, each line to represent one of the eight wind directions. About the central point describe a circle, the length of whose radius shall correspond to the mean temperature of the month, measured on some convenient scale. Thus, if the mean temperature of the month is 55° and a scale of half an inch is taken to correspond to 10° of temperature, the radius of the circle must be five and a half times half an inch, or 2³/₄ inches. Next lay off on the eight wind lines the mean temperatures corresponding to the eight different wind directions, using the same scale (¹/₂ in. = 10°) as in the previous case. Join the points thus laid off by a heavy line, as shown in Fig. 49. The figure, when completed, gives at a glance a general idea of the control exercised by the winds over the temperatures at the station selected. Where the heavy line crosses a wind line inside the circle it shows that the average temperature accompanying the corresponding wind direction is below the mean. When the heavy line crosses any wind line outside the circle, it shows that the average temperature accompanying the corresponding wind direction is above the mean. Such a figure is known as a wind rose.

The cold wave and the sirocco are two winds which exercise marked controls over the temperature at stations in the central and eastern United States. The cold wave has already been described in Chapter V. It is a characteristic feature of our winter weather. It blows down from our Northwestern States or from the Canadian Northwest, on the western side of a cyclone. It usually causes sudden and marked falls in temperature, sometimes amounting to as much as 50° in 24 hours. The sirocco is a southerly or southwesterly wind. It also blows into a cyclone, but on its southern or southeastern side. Coming from warmer latitudes, and from over warm ocean waters, the sirocco is usually a warm wind, in marked contrast to the cold wave. In winter, in the Mississippi Valley and on the Atlantic Coast, the sirocco is usually accompanied by warm, damp, cloudy, and snowy or rainy weather. The high temperatures accompanying it (they may be as high as 50° or 60° even in midwinter) are very disagreeable. Our warm houses and our winter clothing become oppressive and we long for the bright, crisp, cold weather brought by the cold wave. In summer when a sirocco blows we have our hottest spells. Then sunstrokes and prostrations by the heat are most common, and our highest temperatures are recorded. The word sirocco (from Syriacus=Syrian) was first used as the name of a warm southerly wind in Italy. The cause and the characteristics of the Italian sirocco and of the American sirocco are similar, and the name may therefore be applied to our wind as well as to the Italian one. In the Southern Hemisphere, at Buenos Ayres, Argentine Republic, there is a similar contrast between two different winds. The pampero is similar in many respects to our cold wave. It is a dry, cool, and refreshing wind, blowing over the vast level stretches of the Argentine pampas from the southwest. The norte is a warm, damp, depressing northerly wind corresponding to our sirocco.

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