DISCUSSIONS 47 1

551.551 -25

The measurement of gustiness in the first few thousand feet of the atmosphere

By J. I. P. JONFS and H. E. BUTLER

(Read 18 June 1958. See Q.J.. 84, p. 17)

MR. B. C. V. ODDIE : I would like to remark on the exceptional interest of the records displayed by Mr. Jones. One in particular includes a cross-section of a small cumulus, and shows a wide region of descending air with very little turbulence ahead of the cloud and ascent and considerable turbulence over the whole width of the cloud, both within and below it. Much of this detail has previously been inferred, but never so clearly shown : more records of this kind are very desirable.

MR.J. I . P. JONES (in reply on behalf of the authors) : I would like to say that we are continuing to make records of vertical gustiness from time to time in connection with our programme on vertical diffusion, and it is hoped that the accumulated data will appear in due course.

551.507.321.2 : 551.510.42 : 551.551.8

The use of balloons in atmospheric pollution research

By D. H. LUCAS. G. SPURR and F. WILLIAMS

(Read I R June 195R. See Q . J . , 83. p. 508)

Mr. B. C. V. ODDIE : I think it remarkable that the balloon stayed in the plume, which on visual evidence mixed with the surrounding air (and therefore cooled) very rapidly indeed. The gas in the balloon could not cool so fast, so that the balloon should surely have some buoyancy to carry it out of the smoke.

Dr. R. S. SCORER : Measurements made on thermals at Imperial College show that particles cannot escape from the top unless their free ascent velocity is comparable with the rate of rise of the thermal because the fluid in the path of a thermal is mixed into it.

At the base some of the exterior air is entrained up into the centre in such a way that particles with a free fall speed of less than 0.4 w,, where w, is the rate of rise of the cap, cannot escape. Only particles with a fall speed in excess of 1.6 w, necessarily escape. Particles with intermediate fall speeds may or may not fall out (this work is soon to be reported upon by Miss Woodward).

The corresponding figures for a bent-over plume are likely to be rather similar ; consequently Mr. Spurr's balloons are likely to remain in the plume for a considerable distance even if they have a speed of ascent or sink relative to the plume air.

Mr. G. SPURR (in reply on behalf of the authors) : Dr. Scorer has to a large extent answered Mr. Oddie's query. As could be seen clearly in the demonstration film, the balloons remain in the plume almost as though they were held by the plume. I agree that the balloon will lose heat less quickly than the plume on emerging from the chimney, but the balloon is unlikely to reach the temperature of the gas during its passage up the chimney. These two factors appear to cancel each other out.