SUNRISE
Exposed Th. On Earth On Grass
Temperature 51.1 48.3 46.6
Mean Diff. from Air 4.0 2.5 6.2
Max. Diff. from Air 9.0 3.7 9.0
Number of Observations 6 3 5
NINE P.M.
Exposed Th. On Earth On Grass
Temperature 56.4 53.8 54.4
Mean Diff. from Air 5.3 4.9 7.2
Max. Diff. from Air 7.5 5.5 10.0
Number of Observations 7 6 7
On one occasion, and that at night, the dew-point was as low as 11.5 degrees, with a temperature of 66 degrees, a depression rarely equalled at so low a temperature: this phenomenon was transient, and caused by the passage of a current of air loaded with dust, whose particles possibly absorbed the atmospheric humidity. From a comparison of the night and morning observations of thermometers laid on grass, the earth, and freely exposed, it appears that the grass parts with its heat much more rapidly than the earth, but that still the effect of radiation is slight, lowering its temperature but 2 degrees below that of the freely exposed thermometer.
As compared with the climate of Calcutta, these hills present a remarkable contrast, considering their proximity in position and moderate elevation.
The difference of temperature between Calcutta and Birbhoom, deduced from the sunrise, morning and afternoon observations, amounts to 4 degrees, which, if the mean height of the hills where crossed by the road, be called 1,135 feet, will be equal to a fall of one degree for every 288 feet.
In the dampness of its atmosphere, Calcutta contrasts very remarkably with these hills; the dew-point on the Hoogly averaging 51.3 degrees, and on these hills 38 degrees, the corresponding saturation-points being 0.559 and 0.380.
The difference between sunrise, forenoon and afternoon dew-points at Calcutta and on the hills, is 13.6 degrees at each observation; but the atmosphere at Calcutta is relatively drier in the afternoon than that of the hills; the difference between the Calcutta sunrise and afternoon saturation-point being 0.449, and that between the hill sunrise and afternoon, 0.190. The march of the dew-point is thus the same in both instances, but owing to the much higher temperature of Calcutta, and the greatly increased tension of the vapour there, the relative humidity varies greatly during the day.
In other words, the atmosphere of Calcutta is loaded with moisture in the early morning of this season, and is relatively dry in the afternoon: in the hills again, it is scarcely more humid at sunrise than at 3 p.m. That this dryness of the hills is partly due to elevation, appears from the disproportionately moister state of the atmosphere below the Dunwah pass.
II. Abstract of the Meteorological observations taken in the Soane
Valley (mean elevation 422 feet).
The difference in mean temperature (partly owing to the sun's more northerly declination) amounts to 2.5 degrees of increase in the Soane valley, above that of the hills. The range of the thermometer from day to day was considerably greater on the hills (though fewer observations were there recorded): it amounted to 17.2 degrees on the hills, and only 12.8 degrees in the valley. The range from the maximum to the minimum of each day amounts to the same in both, above 20 degrees. The extreme variations in temperature too coincide within 1.4 degrees.