Page images
PDF
EPUB

The lower line shows the averages for the three months, regard being had to the number of cases in each month. Comparing these results with those given in my last article for areas of low barometer (this Jour., vol. x, p. 5), we find that for each month the course of high barometer is more southerly than low barometer. These differences are shown in column fourth. Column fifth shows the differences in velocity for each month. These observations indicate that while the average track of storms east of the Rocky Mountains, across the United States, is nine degrees to the north of east, areas of high barometer advance toward a point several degrees south of east, and with a velocity somewhat less than the former.

Monthly minima of temperature.

In a former article (this Jour., vol. ix, p. 7) I gave a table showing the lowest temperature observed at New Haven for each month during a period of three years, together with the height of the barometer, direction of the wind, and degree of cloudiness for the corresponding dates, and I expressed the opinion that these low temperatures are due in part to the descent of cold air within an area of high barometer. Inasmuch as some persons ascribe these low temperatures to a flow of air from a higher latitude, it has appeared to me that it would be instructive to study the same phenomenon at a locality where a current of air from a colder latitude is impossible; and such a locality must be found at the point of minimum temperature for the northern hemisphere. Now according to Dove's charts; during the winter months Jakutsk, in Siberia, lat. 62° 2' N., is situated very near the center of greatest cold for the northern hemisphere. I have, therefore, sought for a complete meteorological journal at this station, and have found it in Middendorff's Sibirische Reise, Band I, pp. 28-49, extending from Sept., 1844, to June, 1846. The following table shows the results obtained for each month of this period. Column first shows the date of the lowest temperature for each month; column second shows the lowest temperature for each month in degrees of Reaumur; column third shows the direction of the wind, and column fourth shows the force of the wind; column fifth shows whether the sky was clear or overcast; column sixth shows the height of the barometer expressed in Russian half lines; column seventh shows the mean height of the barometer for each month; column eighth shows how much the observed height of the barometer differed from the mean height expressed in English inches (one-half line=0·05 inch English).

From this table it will be seen that the monthly minima of temperature are almost entirely independent of the direction of

the wind. A northerly direction occurs 10 times; southerly, 6 times; easterly 7 times; and westerly, 5 times. In every instance (except one) these winds were faint, and generally very faint. In two-thirds of the cases the sky was entirely clear, and in only two cases was the sky entirely overcast. three-fourths of the cases the barometer stood above its mean height.

In

Lowest temperature for each month at Jakutsk, Siberia, for the years 1844, 5 and 6.

[blocks in formation]

In all these particulars, except the direction of the wind, the phenomena at Jakutsk are quite similar to those observed at New Haven. So far as I have yet learned it is true universally that periods of unusual cold are generally accompanied by a barometer above the mean. Now it has been shown (this Jour., vol. ix, p. 2) that within areas of high barometer the motion of the air is outward from the center of this area, and therefore there must be a downward motion to supply the air flowing outward. In other words, it must be regarded as an observed fact that periods of unusual cold are generally accompanied by a descent of air from the upper regions of the atmosphere.

Influence of Winds on the temperature, moisture and pressure of the atmosphere.

In order to determine the influence of winds upon the temperature, etc., of the atmosphere, I selected the Meteorological observations made at Girard College, Philadelphia, from 1840

to 1845. A large sheet of paper was ruled with 16 vertical
columns, and at the head of these columns were placed the letters
N.; N. N.E.; N.E., etc., for the directions of the wind. I
first took the observations for the three winter months, and be-
ginning with Dec. 1, 1840, found that at the first observation
the wind was N.W. The temperature at this observation was
compared with the mean temperature of that month for the
same hour, and the difference (with its algebraic sign) was
placed in the column headed N.W. I proceeded in like man-
ner with each succeeding observation during the winter months
for the whole period of five years. The average of all the num-
bers in each column was taken and the results are shown in
column second of the following table. These numbers clearly
indicate that the coldest winds are from the neighborhood of
the N.W., and the warmest winds from the neighborhood of the
S.E. As, however, the progress of the numbers is somewhat
irregular, I have taken the average of each successive three
numbers in this column and placed the results in column third.
These numbers show a remarkable regularity, indicating a mini-
mum with a N.W. wind, and increasing thence uninterruptedly
to a maximum with a S.E. wind, and thence decreasing unin-
terruptedly to the minimum. The entire range of the numbers
in column third is 8°46 Fahr., while the extreme range in
column second is 10.°18.

Influence of winds on the temperature, moisture and pressure of
the atmosphere.

[merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][ocr errors][merged small][merged small]

N. N. E.-0.45-0.25-2·78-2·85+005+003-072072

N. E.

E. N. E.
E.

E. S. E.
S. E.

S. S. E.
S.

S. S. W.
S. W.

040 026 029

+1.28 +0.40-2·07-251 +012 +005 —·047-055
+0.36 +1.00 -2.68-221-003 +011 046
+1:37 +2.66 -1.87-1.82+023+025
+6.26 +4.25 −0·90-1·15+055+043-016-004

+5.11+5·94-0·66-0·24+051 +060+031 +023 +6:46 +569 +0.83 +0.55 +075 +059 +054+046 +5.50 +500 +1:48 +1.68+052 +054+052 +057 +3:05 +3.88 +2·72 +248 +036 +029 +066 +061 +3:09 +2:47 +3·24 +251 +029+029 +066 +056 W. S. W. +1·26 +1.08 +1.57 +242 +·023+007 +036 +038 W. -1.12-1.19 +244 +1.60 -032016 +012 +005 W. N. W.-3.72-2·03+0·80 +0·10-039 032-034 -034 N. W. -1.26-2.52 -2.94 -1.79 -025025 081067 N. N. W.-2.57-1.80 -3.22-3.28 012 015086-088

[blocks in formation]

I proceeded in a similar manner with the observations for the three summer months and the results are shown in column

fourth. The average of each successive three numbers in this column was taken and the results are shown in column fifth. In order to determine the influence of the winds on the moisture of the air I took the observations of the force of vapor (expressed in inches of mercury) and compared each observation with the mean force for that month, and the given hour. The results for the three winter months are shown in column sixth of the preceding table, and the averaged numbers are shown in column seventh. The results for the three summer months, obtained in like manner, are shown in columns eighth and ninth. In order to determine the influence of the wind on the pressure of the air, I placed each observation of the barometer in the column having the observed wind at the top, and took the averages of the numbers in each column. The result for the three winter months (subtracting 29 inches) are shown in column tenth and the averaged numbers in column eleventh. Similar results for the three summer months are shown in columns twelve and thirteen.

On comparing the numbers in this table we see that in winter at Philadelphia the lowest temperature occurs with a wind from the N.W., and in summer with a wind from a point about 15° west of north.

Now if we suppose a mass of air to be transferred from a higher latitude to a lower, we should expect that its relative temperature would be the lowest when it moved in a direction perpendicular to the isothermal lines. Observing this rule we should conclude that in winter the coldest wind at Philadelphia must come from a quarter about 15° west of north, provided it commenced its motion from any place within 600 miles from Philadelphia. But if it came from a distance of over 1000 miles from Philadelphia then the coldest wind would come from a point 30° west of north. But the coldest wind actually comes from a point 45° west of north; that is, at Philadelphia in winter the coldest wind blows from a point 15° more westerly than the coldest region about Philadelphia.

In summer, if we extend the comparison to a distance of 1000 miles from Philadelphia, we shall find the coldest region to lie in a N.E. direction; but if we confine ourselves to a radius not exceeding 600 miles, we shall find nearly the same temperature prevailing in all directions between the limits of N.E. and N. 25° W. But the coldest wind is observed to blow from a point N. 15° .W., which lies within the limits above determined. On the whole, we conclude that at Philadelphia the coldest wind comes very nearly from the coldest region within a distance of from 500 to 1000 miles from Philadelphia, with a suspicion, however, that the former is a few degrees more westerly than the latter.

In winter the warmest wind at Philadelphia is found to blow from a direction S. 40° E., while in summer it blows from the S.W. The former direction takes us to the Gulf Stream at about its nearest point, and at a distance of 250 miles. In summer the warmest region within 400 or 500 miles of Philadelphia lies in a direction S. 30° W., while the warmest wind blows from a point 15° more westerly. On the whole, the observations indicate that both the warmest and coldest winds at Philadelphia blow pretty nearly from the regions of greatest heat and cold, but there is reason to suspect that these directions are not quite coincident.

From the table of monthly minima of temperature at New Haven given in my.former article, (this Jour., vol. ix, p. 7) it will be seen that the average monthly minimum is 25° below the mean temperature of the corresponding month. The table last given shows that a small portion of this effect (viz. 5°) may be ascribed to the influence of the direction of the wind, but there remains unexplained four-fifths of the whole effect which is to be ascribed to the influence of other causes.

The preceding table shows that both in summer and winter the force of vapor at Philadelphia is greatest with the same wind which brings the highest temperature; and it is lowest with the wind which brings the lowest temperature. The deviations from this rule are so small as to render it probable that the discrepancies would entirely disappear in the means of a long series of observations.

Since cold air has a greater density than warm air, and dry air has a greater density than vapor of water, it might be expected that the wind which brings the lowest temperature and the least vapor, would bring the highest pressure. We see. however, from the preceding table that such is not the case. In winter the highest pressure comes with a wind from the N.E., or perhaps N. 55° E.; while in summer the highest pressure comes with an east wind, which directions are distant more than 90° from the coldest quarter of the horizon. So, also, in winter, the lowest pressure comes with a S. W. wind, and in summer with a west wind, both of which directions are quite distant from the warmest quarter of the horizon. It seems probable that the excess of pressure which accompanies an easterly or N.E, wind is but the result of the high barometer which usually precedes a N.E. storm.

Diurnal inequality in the rain-fall.

In my former article (this Jour., vol. x, p. 3) I noticed a diurnal inequality in the progress of storms and was hence led to infer that there must be a diurnal inequality in the fall of

« EelmineJätka »