145 
1916-17.] The Formation of Anticy clonic Stratus. 
below and spread out on reaching the reversed gradient. Ultimately the 
heating of the surface layers may result in the disappearance of the reversed 
gradient, as described in my earlier paper. It is not easy to distinguish the 
rise of temperature of hazy air due to absorption of heat from that due to 
convection from the surface ; at night in summer the hazy air only seems 
to fall about 1° F. in temperature, except within 1500 feet of the ground. 
In the early morning in summer the top of the haze is much better defined 
than later in the day, and the tops of stratus clouds are usually more level, 
with the temperature minimum coinciding exactly with that level ; these 
effects are probably partly due to radiation, partly to convection. In 
winter it is probable that the accumulated effect of radiation in cooling 
cloudy and hazy layers is of great importance. 
Ground fogs may be included under the title of anticyclonic stratus, as 
they have a great resemblance to ordinary layers of stratus, and every 
intermediate type of cloud may be found. I have only made a few 
observations of ground fogs, but these indicate that they persist only so 
long as the temperature gradient above them is below the adiabatic rate 
for saturated air. There may be a reversed gradient within the fog, 
and I have also observed this on a few occasions with stratus clouds. 
Normally the adiabatic gradient persists to the top of a layer of stratus, 
or of haze. 
On the average I have found that in fine and fairly calm weather in 
France, about sunrise in September, a reversed surface temperature gradient 
exists up to about 1000 or 1200 feet, and above 1500 feet the gradient is 
similar to that of the previous evening; the average rise up to 1000 feet 
appears to be about 10° F. On the morning of September 15, however, 
there was a rise of 8° F. for the first 500 feet, and above that an adiabatic 
gradient ; on that occasion the air was drier than usual, and part of the 
previous night had been cloudy. The diagram on p. 146 shows a typical 
reversed gradient, C I) being the temperature curve of the previous after- 
noon, A D the curve about sunrise, and BED the curve two or three 
hours afterwards. About this time there are often small clouds due to 
convection at the level of E, and sometimes also flat patches which are 
probably due to the adiabatic expansion of the layer between E and the 
surface. 
Over the North Sea, particularly in early summer, stratus often forms 
at the height of about 1000 or 1500 feet above the surface, and occasionally 
the same development takes place when mild air spreads over the land 
after a severe frost ; possibly a curve of type BED may be produced as a 
result of contact with the cold surface, an adiabatic gradient being main- 
VOL. XXXVII. 10 
