28 THE VOYAGE OF H.M.S. CHALLENGER. 



mean, and the influence of the higher temperature is, in some degree, to counteract 

 the retardation of the wind's velocity resulting from friction and from the viscosity of 

 the air encountered near land. 



An explanation not unfrequently adduced is that the variation is due to the 

 ascending currents with their reduced velocities, and the descending currents with their 

 increased velocities, which set in as the necessary result of the unequal heating of the 

 surface at different hours of the day. Now if this were so, the increased velocity during 

 the hottest hours of the day would he closely congruent with the diurnal curve of 

 atmospheric pressure, commencing with the time when pressure begins to fall from the 

 morning maximum, in other words, from the time the ascending current sets in, and 

 would reach the maximum at the hour of the afternoon minimum of pressure, that is, 

 the time when the ascensional current is strongest. Observation does not bear this out, 

 since the increase in the diurnal velocity sets in before pressure begins to fall from the 

 morning maximum ; and the maximum, in the summer months when the whole 

 phenomena are most pronounced, occurs from two to four hours before the time of the 

 afternoon minimum of pressure. The time of occurrence of the maximum velocity 

 is from 1 to 2 p.m., or when the diurnal insolation is strongest. Observations thus 

 point to the conclusion that, while ascensional and descensional currents play a part in 

 bringing about the diurnal variation, by far the more important part is due to the 

 difference between the temperature of the earth's surface and that of the wind blowing 

 over it at the moment. It is evident that when the surface of the ground is super- 

 heated, and an ascensional movement of the air has set in from the heated surface, the 

 retardation of the wind's velocity, resulting from friction and from the viscosity of the 

 air, is more or less counteracted, and the velocity of the wind is thereby increased. On 

 the other hand, during the night, when terrestrial radiation is proceeding, the tempera- 

 ture of the surface rapidly falls, all ascensional movement ceases and gives way to a 

 descensional movement of the lowermost stratum of the air down the slopes of the 

 country, with the result that during these hours the retardation of the wind's velocity 

 from friction is greatest. 



Variation in the Amount of Cloud. — The diurnal variation in the amount of 

 cloud in the sky over the open sea is very small. The following are the means of 

 277 days' observations on board the Challenger, stated in percentages of sky covered 

 with clouds : — 



