[396] Taken during a violent N.W. dust-storm.
| MORNING | ||||
| Hour | Th. | Black Bulb | Diff. | Phot. |
| 9.30 a.m. 10 a.m. 10 a.m. 9 a.m. 9 a.m. 9 a.m. | 77·0 69·5 77·0 63·5 61·2 67·0 | 130 124 137 94 106 114 | 53·0 54·5 60·0 30·5 44·8 47·0 | -- 10·320 -- 10·230 -- 10·350 |
| Mean | 69·2 | 117·5 | 48·1 | 10·300 |
| AFTERNOON | ||||
| Hour | Th. | Black Bulb | Diff. | Phot. |
| 3.30 p.m. 3 p.m. 3 p.m. 3.30 p.m. 3 p.m. | 81·7 80·5 81·5 72·7 72·5 | 109 120 127 105 110 | 27·3 39·5 45·5 32·3 37·5 | -- 10·320 10·330 10·230 10·390 |
| Mean | 77·8 | 114·2 | 68·4 | 10·318 |
| SUNRISE | |||
| Exposed thermometer | On earth | On grass | |
| Temperature Mean diff. from air Max. diff. from air Number of observations | 51·1 4·0 9·0 6 | 48·3 2·5 3·7 3 | 46·6 6·2 9·0 5 |
| NINE P.M. | |||
| Exposed thermometer | On earth | On grass | |
| Temperature Mean diff. from air Max. diff. from air Number of observations | 56·4 5·3 7·5 7 | 53·8 4·9 5·5 6 | 54·4 7·2 10·0 7 |
On one occasion, and that at night, the dew-point was as low as 11·5°, with a temperature of 66°, 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° 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°, 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°, and on these hills 38°, 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° 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.