292 DISCUSSIONS

Mr. Bonacina will see from Figs. 4 and 6 of my paper that much of the August blocking occurred well to the east of the British Isles. The rCgime upwind of a region of blocking is usually a disturbed westerly or cyclonic type, in keepifig with his remarks.

The course of the differences between northern and southern hemisphere circulation patterns is fundamentally topographical and geographical (in the broadest sense) as Dr. Sutcliffe remarks. The most evident and most immediate manifestation of this lies in the much greater strength of the westerlies in the south. Presumably wave-like distortions and instabilities do occur there but, as in the northern-hemisphere winter, owing to the strong zonal flow (as much as the absence of obstacles) fail to stabilize into slow-moving large-amplitude patterns.

In connection with Prof. Sheppard’s comments, I have little to add to Section 3 of my paper. If a more precise delineation is required for any specific purpose, I have no doubt that some measurement or combination of measurements (such as amplitude, wavelength, rate of diminution of zonal flow with longitude, etc.) could be devised.

Mr. Whiten is not quite accurate in his opening remarks. Only 21 out of the 53 cases were retrogressive at some time with progression or stagnation a t the beginning and end. His ideas are reminiscent of the self-development and self-slowing principles to be found elsewhere in the literature.

551.543 : 551.553.532.59

Short-period variations in surface pressure and wind

By I. J. W. POTHECARY

(Read 15 December 1954. See QJ., 80, p. 395)

MR. J. S. SAWYER : Such periodic variations of wind and pressure as uere described by Mr. Pothecary are relatively rare events. I should like to ask whether the conditions in the air mass which transmitted the disturbance were unusual, or whether the source of the disturbance itself was exceptional. If the source covered only a small area, it must have released considerable energy; if it covered a larger area then it is difficult to explain how the linear wave front was formed.

MR. C. K. M. DOUGLAS : The initiating disturbance described in the paper was on an unusual scale. Extensive and severe outbreaks of thunderstorms associated with really hot continental air do not occur every summer over the English Channel or Brittany. Marked cooling by precipi- tation at night requires a high cloud base and a dry layer above the surface inversion, which exists over both land and sea in these conditions. Land-breeze convergence might have added to the severity of the storms in the Channel (the nocturnal maximum of rainfall in the Channel due to this cause is recognized by forecasters). Banded structure is a common feature of rain distribution and of clouds up to a high level, but the precise relation between this feature and the type of wave described in this paper is not known.

Has Mr. Pothecary been able to obtain any data from stations in northern France ? If it were possible to extend the isochrones over that area it would help to determine the position and, perhaps, the rough shape of the originating disturbance.

I suggest that the example of a short-period variation in surface pressure and wind quoted, is simply a more extensive form of a fairly common ‘ wave ’ formation, liable to occur whenever a cold front becomes slow-moving or semi-defunct .

Has Mr. Pothecary looked into the instrumental evidence on this, or other similar occasions, to determine the distance over which these wave disturbances may be propagated and remain indentifiable ? If the distance is large, several hundred miles, before damping becomes considerable, then the apparent rarity of the phenomenon seems strange, since an air mass capable of vibrating in the observed manner is not unusual, nor is it unusual for a contiguous air mass to possess considerable instability.

The air is, more often than not, suitable for the transmission of gravity waves and so I do not think that the importance of the inversion lies here, but rather in the fact that the existence of a sharp dividing surface between two air masses made it possible for momen- tum to be quickly transferred from one mass to the other. In this case the mechanism seems

MR. J. R. BIBBY :

MR. E. N. VELLACOTT :

PROF. P. A. SHEPPARD :

DR. R. S. SCORER :

DISCUSSIONS 293

to have been the penetration into the lower layer of down draughts generated by thunderstorms in the upper layer, causing a blocking of the motion of the lower layer and the setting up of a gravity wave in consequence.

The mechanism for the setting-off of the thunderstorms themselves, over the Channel at night, is of considerable interest and I have elaborated on the remarks of Mr. C. G. F. Bewsey (' Some features of the diurnal variation of convection,' Weather , 9, p. 203, 1954) in a forthcoming paper, ' The growth of cumulus over mountains Arch. Met. Geophys. Biokl. (in press).

With regard to Professor Sheppard's question on how far gravity waves of this kind can travel I would say that since they travel with a speed comparable with that of the wind they would be expected to travel distances comparable with the distance downwind of an obstacle in which lee waves are found. I have seen these on the Lancashire coast, in a north-westerly wind, set up by the mountains of Ireland, and so the distances which Mr. Pothecary supposes them to have travelled seem very reasonable. I do not think any comparison can be made with pressure-jump lines observed in the United States because they are maintained in existence by precipitation as they progress, and the mechanism is rather different.

I should like to ask whether Mr. Pothecary has any observations of vertical currents from radiosonde ascents made during the occurrence of the waves for comparison with the phases of the pressure oscillations.

MR. I. J. W. POTHECARY (in reply) : The questions of Mr. Sawyer and Prof. Sheppard have already been adequately answered in the contributions from Mr. Douglas and Dr. Scorer respectively.

I would inform Mr. Bibby that no data were obtained from stations in northern France, which were all to the south of the front on which the storms broke out. Although a change of wind with height and an inversion were features of the air mass over northern France neither effect was as well developed as over southern England. Any wave motion induced in the lower levels would have been dispersed fairly quickly.

Answering Mr. Vellacott I would say that gravity waves similar to those described are known to occur when cold fronts accelerate or decelerate and this occasion might be regarded as a special case of this phenomenon. The blocking effect of the outflow of air from the area of intense thunderstorms over the western English Channel could be assumed similar in effect to the acceleration of a cold front.

The air ahead of a cold front must contain similar discontinuities to the air over southern England on this occasion for the effect to be well developed.

In reply to Mr. Bull I would like to say that the only radiosonde ascent which would have given useful information about vertical currents was that from Larkhill, but the last of the oscilla- tions had passed shortly before the start of the ascent.

MR. G. A. BULL :

551.577.34 (94) : 551.557.3

Secular changes in the rainfall regime of SE. Australia

By E. B. KRAUS

(Presented by Prof. G. Manley 15 December 1954. See Q.J., 80. p. 591)

DR. S. K. PRAMANIK : No details have been given about the records of rainfall. Did the sites, the exposures, and the types of raingauge remain the same throughout the periods, and, if any change occurred, were suitable corrections applied to the observed data ? If there have been such changes they might in themselves explain the apparent variation in rainfall unless suitable corrections have been applied. For example the summer rainfall curves of Figs. 2 and 3 could he explained by a deterioration and then an improvement of exposure or by a change to a site where the rainfall is different; the curves also show an increase for thirty years and then a sort of flattening, which is to be expected as thirty-year running averages were used.

We found when examining the climatic changes in India, that the period of thirty years was rather too long and ten-year running averages were more suitable, particularly when the