350 MR. S. A. HILL ON THE WINDS OE NORTHERN INDIA, AND THEIR 
O 
Sea-level to 1000 feet.6'9 
1000 „ 2000 „.6-6 
2000 „ 3000 „.6-3 
3000 „ 4000 „.6-0 
In this region, therefore, we should expect the convective action to be even more 
energetic after the close of the rainy season than in the dry hot weather, though the 
rate of decrement falls off so rapidly that probably the action does not extend to so 
great a height. 
If, therefore, we find, as we do, that the wind directions in October conform more 
closely than in May to the normal directions inferred from the distribution of pressure 
at sea-level, it is probably because there is less difference in the distribution of pressure 
at low and high levels in October than in May.* This will be investigated in the 
next section. 
Part II.—The Velocity Anomaly. 
In no part of the world is the diurnal variation of the wind velocity better marked 
than on the plains of Northern India. In the rainy season there is more or less wind 
botli night and day, though the Velocity at night is, as a rule, considerably less than in 
the day time ; but during the dry season, from October to May, the nights are almost 
always perfectly calm, the only occasions when there is any wind at night occurring 
during the showery weather sometimes observed in January or February, and in 
occasional evening dust storms towards the end of the dry hot weather. At night, as 
the observations made at Alipore prove, the temperature increases for some distance 
with height above the ground, and under ordinary circumstances, therefore, there can 
be no convective interchange between upper and lower atmospheric strata. The 
diagrams on Plate 29 of the second volume of the ‘Indian Meteorological Memoirs’ 
show that from about 6.30 p.m. to 8.15 A.M., on the average of the year, the temperature 
increases with height. Between these hours, therefore, the wind movement is that due 
to the local pressure gradients at sea-level, whilst in the day-time, when convection 
takes place, the velocity is greater, owing to the descent of air from regions where the 
retardation by friction is much less than near the ground. This diurnal variation may 
be illustrated by the results of the Calcutta anemograms published at p. 23 of the 
first volume of the ‘ Indian Meteorological Memoirs,’ and by three-hourly readings of a 
common anemometer made at Agra on four days each month for eight years, both of 
which are given in Table VII. For the last few years, valuable anemographic traces 
* Another reason is that this rapid diminution of temperature in October obtains only during a short 
time each day. The temperature on the plains rises and falls so rapidly that, as will be shown in the 
next section, the mean rate of decrement during the six houi’S from 10 a.m. to 4 P.M. is only about half as 
great as that at the hottest time of the day. 
