SUMMARYThis paper deals with the general meteorology of Nigeria and adjacent territory. Part I describes the air masses and prevailing winds and the characteristic weather of four weather zones. Part I1 describes dust haze and dust storms which occur and discusses the question of forecasting the onset and clearance of dust haze. Part I11 deals with instability phenomena in the form of disturbance lines and local thunderstorms; a theory of their mechanism is put forward which agrees well with observed phenomena. PART I GENERAL Air Masses and Prevailing WindsWe begin by considering the air masses and pxvailing winds in the area at the height of the dry season-at the northem winter solstice. At this season there are cold anticyclones over the North American and Eurasian continents with a ridge of high pressure over North Africa and the Azores extending between them.
SUM MARYThe paper attempts to clarify certain points raised during the discussion on orographic rain held by the Society on February 2 1 , 1945 (Q.J., 71 p. 41). It is shown that rail1 in the warm sectors of depressions plays a large and ofttn dominant part in large falls of orographic rain. A brief description is given of the outstanding orographic rains since rgog, with a note on the synoptic situation. Some recent cases with upper air information bring out the importance of a deep damp layer (almost or quite saturated) in the warm air, and it is considered that convergrncc is the main factor in producing this, though other factors also are taken into account It is shown that potential instability in the warm air is not a neccssary factor for heavy orographic rainfall. A general discussion of this subject is included.
The main purpose of this note is t o point out the essentiat difference between the " banner cloud " and the local varieties of lenticular alto-cumulus included under terms such as " caps, " fohn CIOUCI," and " Contessa del vento." The t\vo forms are distinct, both in their appearance and in their method of development. Plate XII. shows a typical banner cloud on the Matterhorn" (14,6goft.), where it is very common. The cloud banner streams away on the lee side of a sharp prominent peak in a strong wind, and presents the appearance of a great mass of vapour pouring out of the mountain itself. An erroneous esplanation often offered iri the past was that the mountain itself cools the air flowing past it and so produces condensation. Another suggested esplanation is the cooling by direct rarefaction on the lee of the mountain in a strong wind. This reduction of pressure must, however, be small, and its chief effect is to cause, in favourable conditions, a rising current up the lee side of the mountain, which feeds the banner cloud. This is certainly the correct explanation, as may readily be verified by a n observer actually o n the mountain. In some cases clouds can be seen from a considerable distance rising up the lee side of the mountain and then streaming away as they near the *, f4, t-%v-l Y c \ A 3 2 7 FIG. I.-Probable wind system when D strong wind crosses 3 sharp prominent peak.summit. Fig. I is a rough diagram showing the probable wind system associated with the banners. On the other hand the " caps " are caused by the current rising up from the windward side of the mountain. They tend to appear almost simultaneously on numerous peaks, and there is usually also much alto-cumulus cloud, either at the time or soon after, at about their level or a little higher. In these circumstances it seems probable that the clouds form in a damp layer as the result of relatively small vertical displacements, most of rhe air flowing round the qountain. The upper photograph on Plate XIII. shows a typical " cap " on the Dent Blanche (14,320ft.) with a layer of altocumulus a little above it. The photograph was taken from t h e Obergabelhorn towards west-south-west, and the wind was strong * Photographs illustrating six somewhat different varieties of banner cloud on the Matterhorn and other Alpine cloud effects are included in " The Art and Sport of Alpine Photography," by Arthur Cardner (€1. F. and G . iyitherb!).
All times are G.M.T.)the most noicn.orthy feat (ires of this storm wcre its duration sarlt character oi thv lightning over a large area, but
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