shower de\dopment in the lee of the mountain where we would expect to find a rain shadow! Thompson's map shows no obvious rain-shadow, a t least not o f a sizt. and intensity expected of such a massive mountain. Indeed, the isoliyets tend to bend away downstream between Nanyuki and Naro Moru perhaps slimving the presence of showers forming in the lee of the mountain.

We may therefore look upon the observed rainfall pattern as being com- prised of two parts. The first, due to convective storms and falling mostly during the inter-monsoon periods, would be an approximately radially sym- metrical pattern, perhaps with its centre displaced downwind 10 miles or so from the summit and drawn out towards the west over the neighbouring plains. The second, due to drizzly rain resulting from mechanical lifting and falling mostly during the monsoon periods, would be a band lying along the mid-slopes of the up-wind half of the mountain.

GUST AND CALM- SOME OBSERVATIONS O N A N UNSTABLE DAY 13y r \L. lN w.\-1l.s

I lay on my back in the side-garden and watched the clouds streaming towards the east in the cool blustery wind. No showers came along to disturb my siesta and so I had time to learn something about the patterns of convection when it is allowed through the first mile or so of the lower atmosphere.

I had the edge of tlie house as a reference point and could watch holes develop in cloudy areas. Often they developed in an almost stationary man- ner as Figs. 1-3 show. An area of broken cloud (Fig. I) evaporates into a hole (Fig. z ) in almost one place as is proved by Fig. 3. These photos were taken at 45-sec intervals. The cloud in the top left of Fig. I and the top right of Figs. z and 3 is the same denser cloud element which was not easily eroded and srrves as a reference together with the side of the. house on the extreme right.

Our side-garden is boxed by a bungalow to what on this day was windward, and by the sheer sidc of the house to leeward. Gusts chivvied me ;is they were deflected down off the reflector of the housc wall which is quite plain apart from a couple of sash-windows. When the wind had downward components, as in gusts, it battered me off the wall and I shivered and when these passed then all was calm and sunny-and warmer for sun-seekers.

So this afternoon our garden became a sort of gust analyser, selecting the downward components for tlclivery to the garden and allowing the rest to go by. Once again an observation of constant occurrence was confirmed-gusts mainly appeared under the leading edges of cloudy areas, but when

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two cloud elements followed close upon one another then the real gust appeared under the second edge. Sometimes gust fronts came under wisps of dying cloud but in general gusts arrived under some cloud feature-either a dense leading edge in the process of erosion by descending air or revealed by the last surviving fragments of a once proud cloud.

The death of such a one is shown in Figs. 4-7 where a dense element (Fig. 4) begins to diffuse away (Fig. 5 ) and breaks up (Figs. 6 and 7). These photos were taken at I-min intervals.

On this day the average time between major gust-fronts was about 2+3 min and as the wind was about 15 kt so gust cells of length 4.000 ft were the most prevalent.

Almost all the observations that I made that afternoon on my back in the garden were con- sistent with the circulatory gust-cell model sug- gested by M. A. Giblett, C. S. Durst and others (1924) in connection with their investigation into the structure of wind over level country (Fig. 8).

The most interesting single observation how- ever is how parts of cells grow against the wind so that cloud developments remain almost stationary in the sky for significant fractions of a minute or more. Such an effect does not seem to be entirely consistent with the Giblett model.

REFERENCE GIBLETT, M. A. and others 1924 The structure of wind

over level country. Met. Off. Geophysical Memoirs No. 54

Fig. 8. Idealized flow in convective gust cells (after Giblett). Note that the gust front, F, at the surface is depicted as lying under the leading edge of the following cloud. T denotes a gust tongue

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