106 REPORTS ON THE STATE OF SCIENCE. 
proves that it cannot be attributed to instrumental errors. For example, 
on April 1, 2, 3, 1908, the values of H,, T. v ere 
H Te 
Aprill . : : : : 10°5 15° Manchester 
oe ae : , : : 12 by Pyrton Hill 
eRES Aca 4 , : 3 7 24° Manchester 
The result is not surprising, since the English stations are subject to 
more frequent and rapid changes in the pressure distribution than the 
Continental stations. But it is nevertheless difficult to see how changes 
in the convective region can affect T, and the mean temperature found in 
the advective region.! 
The fact that the lowest temperatures occur over the equatorial region 
suggests that the general nature of the process may be as follows: The 
cool air in the upper equatorial regions moves Polewards, and in the 
natural course descends again to feed the trade winds. Owing to the 
irregularities of the earth’s surface, the change of seasons and the very 
considerable difference between the North and South Hemispheres, the 
regular process will be disturbed and even in general will not be sym- 
metrical. This will result in encroachments of the equatorial cold air on 
the advective region of temperate latitudes, and such encroachments will 
produce anticyclonic regions. The advective atmosphere would be reached 
there at a higher level and initially at a lower temperature than in the 
normal or average state ; but the temperature would be gradually raised 
by absorption of thermal radiation to the normal value for that latitude. 
The fact that H, has minimum values in March and September when 
equatorial temperatures are highest appears at first to be contrary to the 
idea. But the first effect of the increased equatorial temperature will be 
to increase the strength of the trade winds,’ and as at the same time 
there is in progress a transference of air across the equator to the Southern 
hemisphere, a transference which can be made only through the upper 
return current, there will be a deficiency of descending air and the equa- 
torial cold air will encroach less than usual on the Northern advective 
region. Naturally, if the earth were symmetrical, it would be expected 
that the process in September would be the reverse of this. But the 
autumnal transference of air to the Northern Hemisphere will be initially 
much more intense towards the great Asiatic continental region and, in a 
less degree, to North America, than to the Atlantic and European area, 
and the result may well be that the equatorial current again encroaches 
less than usual on that region. If such is the case it may be expected that 
the value of H, over the Asiatic area will not show the September 
minimum, and that if it occurs over America it will at least be less marked 
than over Europe. The high value of H, in October indicates that in 
that month the encroachment has become more general. 
The results of observations made with pilot balloons to heights greater 
than H, point to a decrease in wind velocity on entering the advective 
region. 
1 See, however, Shaw, Perturbations of the Stratosphere ; The Free Atmosphere in 
the Region of the British Isles, M.O., 202. 
? Over the Atlantic the N.E. trade wind is strongest in April, but has a secondary 
maximum in February. It is weakest in September. The S8.E. trade wind is 
strongest in February, and has a secondary maximum in April. It is lightest in 
May, and has a secondary minimum in September, See Hepworth, Brit. Assoc. 
Reports, Dublin, 1908, p. 625. 
