November i r, 1909] 



NA TURE 



49 



are denoted by H,. and T^. The following table gives 

 the values of H^, T;., for certain places in Europe : — 



There is very little variation for places between lat. 45° 

 and lat. 55°, but at Pavlovslc H^ is about i km. below 

 the average. Observations made in the equatorial regions 

 show that the value of H^ there exceeds 15 km., so that 

 there must be a considerable increase in its value in cross- 

 ing the limit of the trade-wind region, and it appears 

 probable that the equatorial currents and the trade winds 

 form a closed system with little interchange of air with 

 higher latitudes. 



The annual variation in H^, T^ is shown by the follow- 

 ing table : — 



Annual Variation in H^. 



The remarkable feature is the relatively high temperature 

 and low value of H^ in March and September. This 

 peculiarity and the fact that T^ is least near the equator 

 suggest that the general nature of the process may be as 

 follows. The cool air above the equator 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 northern and southern hemispheres, the process 

 will be neither regular nor symmetrical. Consequently, 

 the equatorial cold air will encroach on the advective 

 region of temperate latitudes, and such encroachments will 

 produce anticyclonic regions. The advective atinosphere 

 would be reached there at a higher level, and initially at a 

 lower temperature than in the 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 this view ; but the first 

 effect of increased temperature will be to increase the 

 strength of the trade winds, and as at the same time 

 there is 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 equatorial cold air 

 will encroach less than usual on the northern advective 

 region. The reverse process would be expected to occur 

 in September, but the autumnal transference of air to the 

 northern hemisphere will be initially much more intense 

 towards the great continental regions than to the .Atlantic 

 and European area, and it may well be that the equatorial 

 current again encro.iches less than usual on that region. 

 1 1 may be expected that the value of H^ in Asia and 

 America will not show the September minimum. 



The explanation of the discontinuity in the temperature 



NO. 2089, VOL. 82] 



gradient appears to be this. The fall of temperature is 

 governed mainly by convection, and a necessary condition 

 for convection to persist is that the radiation shall exceed 

 the absorption in the upper layers of the convective system. 

 A limit is therefore set to the height to which convection 

 can extend, and at this limit the discontinuity in the fall 

 of temperature occurs. It has been shown that the 

 observed height is about the same as the limiting height 

 of the convective system found from theoretical considera- 

 tions based on the experimental knowledge of the radiating 

 power of the atmosphere. 



The results of the observations of wind velocity may be 

 briefly summarised as follows. In general, the velocity 

 increases with height, the greater part of the increase up 

 to 2000 m. taking place in the layers immediately above 

 the surface ; 75 per cent, of the total increase takes place 

 in the first 160 m. Above 500 m. numerous cases_ occur 

 where the velocity decreases with height. The velocity for 

 heights up to 10 km. is given approximately by the equa- 

 tion \'p = V„p„ (Egnell's law), where V is velocity and 

 p density, V„()„ being the values near the surface. The 

 law implies that the pressure gradient remains constant 

 and independent of the height. Now, owing to the fact 

 that the temperature is higher over regions of high pressure 

 than over regions of low pressure, the ratio of pressure 

 gradient to density increases with height. The condition 

 for a constant gradient up to 8 km. is approximately 



/„= ''* J^ degrees C, 



where J„ is the excess of the mean temperature of the air- 

 column at a place at pressure p + ip above that at a place 

 at pressure p. Observations show that for 5/> = 20 mm., 

 („ = 4° C. nearly, or double the amount necessary for 

 constant gradient. It is to be expected, therefore, that 

 Vp will increase up to S km., and the few pilot-balloon 

 observations available point to such an increase. 



The direction of the upper wind usually veers from that 

 at the surface. The following table shows the deviations 

 for winds from different quadrants in England and at 

 Berlin : — 



Deviation of the Upper Wind. 

 England. 



The deviation at Berlin is in nearly all cases greater 

 than in England, especially for north winds, which back 

 slightly in the upper air in England. 



There is no marked difference between anticyclonic and 

 cyclonic conditions in the change of wind velocity and 

 direction with height. The following table gives the values 

 deduced from observations at Berlin and Lindenberg in 

 1905 :— 



Height Surface i kin. 2 km. 



fDevialion ... — ... .^o 



Anticyclonic I Velocity 



(A) 



C) clonic 

 (C) 



Ratio lo sur- \ 

 l^ face velocity J 

 /"Deviation 



I Velnciiy 



I Ratio to sur- "\ 

 I, face velocity J 



4-1 



33 

 8'4 ni.p.s. 



2-05 



•78 



37 

 107 



I 82 



The deviation is slightly greater and the ratio slightly 

 less in C than in A. It would be natural to suppose that 

 surface friction and irregularities would produce a decrease 

 in velocity which increased at a greater rate than the- 

 velocity itself, and in that case the ratio in C would be 

 greater than in A, as was actually found by Berson from 

 the manned balloon observations. 



