416 MR JOHN AITKEN ON THE 



A country fog is nothing more than a cloud at low level, but in the case of town fogs 

 the case is complicated by the condensation taking place in impure air. The conditions 

 giving rise to clouds are, however, more numerous than those causing fogs, the latter 

 being principally caused by radiation, and in some cases by hot moist air rising from the 

 ground. After a fog has been formed, there are at least four influences which affect its 

 density and duration or persistence : — 1st, the rate and constancy of direction of the air 

 circulation ; 2nd, the rise or fall of temperature ; 3rd, the rate at which the condensation 

 is taking place ; 4th, the affinity of the condensing nuclei for water vapour. It is to the 

 last of these influences that I wish at present to call attention, as there are some points 

 connected with it which have not been previously considered, and which are of importance, 

 as they determine the persistence or duration of the life of the fog. 



The affinity of the nuclei for water vapour acts in two ways — 1st, by causing a 

 thickening of the air before it is cooled to the dew-point, and, 2nd, by preventing the 

 differentiation which would take place if there were no affinity. The first of these points 

 has been considered in a previous paper : * we shall therefore confine our remarks to the 

 second. When condensation takes place in air containing ordinary dust, as in a cloud, 

 the thickness of clouding is affected by the rate at which the condensation takes place ; 

 the quicker the rate the denser the clouding. After the rate of condensation becomes 

 slower or ceases, the smaller drops tend to evaporate, while the larger ones tend to con- 

 dense more vapour, but where there is an affinity between the nucleus and water vapour 

 this differentiation is checked. 



As much of what is to be said here depends on the tendency of cloud-drops to 

 differentiate, it may be as well that this part of the subject be considered in detail. The 

 first step in the proof was laid by Lord Kelvin in a communication to this Society in 1870. 

 He showed that the vapour pressure at the concave surface of water in a capillary tube 

 must be less than at a plane surface ; and Clerk-Maxwell, in his Theory of Heat, 

 extended Lord Kelvin's reasoning to convex surfaces, and showed that the vapour pressure 

 at a convex surface is greater than at a flat one, and that small drops will evaporate in 

 air containing so much moisture that condensation will take place at a flat surface ; and 

 as the vapour pressure at a convex surface will be the greater the smaller the drop, it 

 follows that small drops suspended in air will tend to differ in size unless they are all 

 exactly the same diameter, which is an impossibility. 



From the above it will be seen that, after a country fog has been formed, some of the 

 particles will tend to dry up and others to increase in size. The result of this is that the 

 density or thickness of the fog tends to diminish — 1st, by the reduction in the number 

 of the particles, and, 2nd, by the increase in the size of the others, causing them to fall 

 to the ground quicker than smaller ones. Several fogs have been seen which seemed to 

 clear away in this manner. The fog-drops were seen by the fog-particle counter to be of 

 some size and to be falling rapidly, and the upper limit of the fog was also seen to get 



* Trans. Hoy. Hoc. Edin., vol. xxx. part i. 



