The National Geographic Magazine, Vol. I., No. 2, April, 1889Various
History
The National Geographic Magazine, Vol. I., No. 2, April, 1889
Various
Geography -- Periodicals
S. A. Hill has treated of the annual oscillation of pressure, so
noticeable in India, and in so doing has investigated the changes of
pressure for three levels, up to a height of 4500 meters. The reduction
of monthly barometric means at high levels, having regard to the
vertical distribution of temperature, shows a double oscillation in the
annual curve at the level of Leh, which becomes a single one at the
height of 4500 meters, while this is substantially the reverse of the
oscillation observed below.
The subject is also treated in another way by Mr. Hill, through
analysis of normal monthly means for all India, whereby he succeeds in
presenting a formula, the first periodic terms of which represent the
two principal factors of the oscillation.
Mr. Hill has also discussed elaborately the anomalies in the winds of
northern India in their relation to the distribution of barometric
pressure. The anomalies are:--(1) in the hot season the wind direction
frequently shows no relation to the barometric gradient; (2) the winds
over the plains show little or no relation to pressure gradients, but
an obvious one to temperature, being greatest where the temperature is
highest.
It is pointed out as highly probable that the copious snowfalls of the
late winter in the northwest Himalayas not only produce low
temperatures on the Himalayan ranges, but subsequently cause dry
northwesterly winds over northern and western India, and on this
supposition, reliable forecasts of the character of the coming rainy
monsoons have been made for a number of years. Convection currents
between upper and lower air strata, it is suggested by Koppen, explain
diurnal variations in wind velocity and direction. At low stations the
maximum velocity occurs at the time of the highest temperature, while
at high stations the reverse obtains. Hill has examined into an
important point connected with this subject, that is, the great local
differences in the vertical variation of temperature. Hill concludes by
saying that high pressures at low levels are the result of low
temperatures, and in connection with the fact that wind directions are
largely influenced by the irregular distribution of pressure at high
levels, it is more important to know the abnormal variations of
pressure at the highest hill stations in India than those in the
plains.
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