141 
1916-17.] The Formation of Anticyclonic Stratus. 
atmosphere, as regards the quantity of condensed moisture. This, however, 
does not seem an unreasonable assumption ; I have often seen broken and 
shapeless clouds dissolve in the evening, and the last portions to dissolve 
are often flat patches all on one level, with a distinctly lower temperature 
gradient above them than below them. A layer of stratus formed in 
this way might have a somewhat undulating surface, but any protusions 
of cloud would be colder than the surrounding air, and would tend to be 
lowered by convection to the level of the other clouds ; so that finally 
there would be the level surface which is characteristic of all stratus clouds 
unless they are disturbed from below. This process might result in the 
formation of more than one layer of stratus ; these are in fact often found 
near the boundary of anticyclones, but the higher layers usually dissolve 
under the influence of a further increase of pressure, though a layer of haze 
with relatively low temperature may persist for some time afterwards. 
Though layers of cloud or haze with a reversed gradient above them 
may be produced as described above, further investigation is required to 
account for such pronounced reversed gradients as that shown in fig. 1, 
and for the formation of stratus in layers where there was originally no 
cloud. The effect of the motion of the air falls next to be considered. 
Mr. C. J. P. Cave has shown that in clear weather the wind round the 
northern and north-eastern sides of anticyclones normally increases with 
height.* The pilot balloon ascents with the British Expeditionary Force 
in France during the summer of 1916 showed on several occasions that the 
increase in velocity was greater for an interval containing a region of 
reversed gradient than for an interval in the adiabatic region underneath ; 
on some other occasions the increase in velocity in the region of reversed 
gradient was limited to the westerly component. Above the unbroken 
stratus of June 21, 1916, there was a very marked increase of the 
westerly wind. 
The formula for the increase of wind velocity, U, with height, h, is 
given by Sir Napier Shaw as 
1 d\J 
U dh 
\Jd6 
e\dh + 
3-42 x KB 4 
A 6 /Ap\ 
T/W 
> 
where p is the pressure, 0 the absolute temperature, and Ad and Ap the 
differences of temperature and pressure between two points on the same 
level, such that the line joining them is at right angles to the isobars. j- 
* The Structure of the Atmosphere in Clear Weather , by C. J. P. Cave, Cambridge, The 
University Press, 1912. 
t “Upper Air Calculus and the Britisli Soundings during the International Week (May 
5-10) 1913,” Journal of the Scottish Met. Soc., vol. xvi, No. 30, pp. 167-178. 
