414 
NATURE 
[March 1; 1883 
did not exceed 30 kilometres per hour and on the days 
when this rate was exceeded, and finds the diurnal 
periodicity well marked with light and moderate winds, but 
irregularly and only slightly marked with strong winds 
and stormy weather. 
In inquiring into the remarkable facts regarding the 
variation in the diurnal velocity of the wind observed in 
all climates, attention is first drawn to the two curves of 
Fig. 1, showing the observations of wind-force made on 
board the Challenger during the cruise. As regards the 
open sea, the diurnal curve shows practically no variation. 
The whole of the observations of the surface temperature 
of the North Atlantic made by the Challenger have been 
discussed, with the result that the daily range is only 
o°’7. Hence the statement may be regarded as substan- 
tially correct, that over the ocean the atmosphere rests on 
a floor the temperature of which is all but constant day 
and night ; and, so far as concerns the generation of 
ascending aérial currents from a heated surface, practi- 
cally constant. 
On approaching the land, however, the daily range of 
the temperature of the air over tte sea becomes mate- 
rially augmented, the daily range being 4°°3, and, as all 
observation shows, the temperature over land still more 
so. Now, bearing in mind that the temperature has 
risen above its daily mean at Io a.m., and fallen below it 
at 10 p.m., an examination of the curve of velocity near 
land in Fig. 1 reveals the fact that the increase in the 
diurnal velocity of the wind is entirely restricted to those 
hours of the day when the temperature is above ‘the daily 
mean, and the maximum velocity is reached at the hour 
when insolation, or the sun’s heating power, is strongest. 
The phenomenon of the diurnal variation in the wind’s 
velocity is thus associated in the closest manner with the 
temperature of the surface on which the air rests. Where 
there is practically no variation, as in the temperature of 
the surface of the sea, there is no variation in the velo- 
city ; but where, as on land, the temperature of the air 
has a strongly-marked daily period, the wind-force also 
is strongly marked, and the increase rises and falls with 
the degree of insulation on the surface. Further, the 
velocity increases, not with the increase in the tempera- 
ture of the air, but with the heating of the surface; in 
other words, with the conditions on which ascending 
aérial currents depend. 
It is also to be observed, as regards the curves of the 
five oceans, that they show in each ease and at all hours 
of the day a greater velocity of the wind on the open sea 
than near land. 
The following are the mean and extreme hourly veloci- 
ties, in miles per hour, for the five oceans :— 
bed | of) 8 | gé | 8s 
? az aa Ax WA | So 
| 
| Miles. | Miles. | Miles. Miles. | Miles 
Mean hourly velocity on 
OPEIseA yess Gas. -e2u|'TS:Ol4| UOsTs | ArAeS ns Moyea s+ s 
Mean hourly velocity | 
near land a 1570 | 14°7 96 | rro | 17°6 
Difference coe BF) NEO) 34 4°9 52 59 
Highest mean hourly velo- 
city near land ... ...| 170 | 16°4 | 1n‘6 | 13:7 | 20°8 
Lowest mean hourly velo- 
city near land . || ¥3°kr | 1350' "|| 10‘0 9°3 | 14°3 
Diurnal variation hear 
land 3'9' | 374 1°6 4°4 6°5 
Thus the winds are-lightest on the North Pacific, and 
strongest on the Southern Ocean, and these oceans show 
respectively the least and the greatest diurnal variation 
n the force of the wind on nearing land. 
From the number and character of the two sets of 
observations, it may be assumed, without risk of error, 
that the open-sea and the near-land winds, summarised 
and represented in Fig. 1, were atmospheric movements 
resulting from mean barometric gradients substantially 
equal to each other. From the above table it is seen that 
in each of the oceans the mean velocity near land is less 
than that on the open sea, the two extremes being the 
North Atlantic, with a difference of 30 miles, and the 
Southern Ocean with a difference of 5°9 miles; and that 
even the maximum velocity during the day is always less 
than the velocity on the open sea. The slight rise in the 
near-land curve during night is probably wholly caused 
by the land-breezes felt on board the Chad/enger when 
near land. In strictly inland places, tolerably well 
situated for making observations of the wind, this feature 
does not appear in the curve, and there the velocity falls 
to the diurnal minimum during the period of lowest tem- 
perature, or when the effects of terrestrial radiation are 
most felt on the surface of the ground. 
From these results it follows that, so far as concerns 
any direct influence on the air itself, considered apart 
from the floor or surface on which it rests, solar and 
terrestrial radiation do not exercise any influence in 
causing the diurnal increase of the velocity of the wind 
with the increase of the temperature of the air; or if 
there be any influence at all, such influence is altogether 
insignificant, as the observations of the Chad/enger on the 
five great oceans of the globe conclusively prove. The 
same observations show that on nearing land the wind is 
everywhere greatly reduced in force, the retardation being 
due chiefly to friction, and to the viscosity and inertia 
of the air in relation to the obstructions offered by the 
land to the onward course of the wind. The retarda- 
tion is greatest when the daily temperature is at the 
minimum, and it is particularly to be noted that though 
the temperature rises considerably, yet no marked in- 
crease in the velocity sets in till about 9 a.m., when the 
temperature has begun to rise above the daily mean. 
From this time the increase is rapid. The maximum 
velocity is reached immediately after the time of strongest 
insolation, and falls a little, but only a little, during the 
next three to five hours, according to season, latitude, 
and position. The velocity is low during the hours when 
the temperature is lower than the daily mean, and the 
least velocity occurs early in the morning. Even the 
maximum near land falls considerably short of the velo- 
city which is steadily maintained over the open sea by 
night as well as by day. 
The period of the day when the wind’s velocity is in- 
creased is thus practically limited to the time when the 
temperature is above the daily mean, and the surface 
superheated, and the influence of this higher temperature 
is to counteract to some extent the retardation of the 
wind’s velocity resulting from the causes already stated. 
The results show that the increase in the diurnal velocity 
of the wind is due to the superheating of the surface of 
the ground, and to the ascensional movement of the air 
consequent thereon, which tend to reduce the effects of 
friction and viscosity of the air. It is of importance in this 
connection to keep in view the fact, shown by hourly 
observations made at the instance of the Marquis of 
Tweeddale in 1867 on the temperature of the soil and 
air, that in cloudy weather a temperature much higher 
than that of the air near the ground was radiated from the 
clouds down upon the earth’s surface (Fournal Scottish 
Meteorological Society, vol. ii. p. 280). Hence in cloudy 
weather the superheating of the surface-layer of the 
ground will often take place, the greatest degree of 
heating being under an overcast sky, where the cloud- 
covering is of no great thickness, ard the temperature 
of the clouds themselves is much higher than that of 
the surface of the earth. On the other hand, little or 
rather no heating will take place, when the cloud-screen 
which overspreads the sky is of great thickness, and the 
