The cacti from the west coast of South America grow in the relatively narrow strip of land which lies between the Andes and the Pacific Ocean. Natural climatic controls in this region produce a barren landscape from the northern border of Peru to central Chile, where cacti are the predominant natural vegetation.

The arid nature of this coastland is a result of the combined effects of the high, unbroken, backbone of the Andes; the cold Humboldt current which runs northwards off the full length of the coast of Chile and Peru; and the very stable high-pressure anti-cyclone centred over the south-eastern Pacific. These natural controls combine to produce an arid land stretching along almost 2,000 miles of coast. Nowhere else in the world do we find a coastal desert of such aridity spanning so many degrees of latitude.

In Europe, we have a weather pattern of frequent cyclonic disturbances bringing continual changes of cloud and temperature, with varying amounts of rainfall. In contrast, the anti-cyclone off the Pacific coast of South America is undisturbed throughout the year, bringing a monotonous succession of bright, sunny, cloudless days to the western slopes of the Andes in Peru and northern Chile.

The natural air flow round the Pacific anti-cyclone produces northerly warm, dry winds which are more or less parallel to the coast and so bring no rain to the lands over which they pass. This general northward air flow is modified by the effects of the rapid rise in the surface temperature of the earth each day due to the combination of tropical sun, clear skies and absence of vegetation. By day, the air immediately above the hot rocks or sand also becomes very warm and tries to rise. On the other hand, the constant motion of the ocean waters swallows up the sun's heat so that the air over the sea shows little change in temperature between day and night.

During the day the air over the sea will be relatively cool compared with that over the land, so that it flows inshore whilst the heated overland air rises uphill. This produces a typical daytime onshore breeze which can be quite strong at high noon.

The cool Humboldt current, which flows northwards opposite the full length of the coast of Chile and Peru, also strongly influences the climate of the arid zone. Because of the Coriolis force, the water tends to flow at a small angle to the left of the air flow. The surface sea current thus sets slightly away from the coast, being replaced by upwelling water from even colder depths of the ocean. The coldest water is thus often found nearest to the coast and will be 10 F or more cooler than the average for the latitude.

The vast quantities of marine organisms brought up from the depth by the ocean currents support great shoals of fish: these in turn are harvested both by hordes of sea birds nesting on the cliffs and by the occupants of the isolated fishing villages scattered along the coast.

This cool offshore water exaggerates the typical difference between land and sea daytime air temperature, so strengthening the typical onshore day breeze. ·This effect can become so pronounced, especially in summer, that the onshore wind or "virazon" reaches gale force in the afternoon, preventing the handling of cargoes at the unsheltered coastal ports. During the hours of darkness, the reverse effect is depressed by the cool sea current and offshore night winds are mild or non-existent.

Flowing north with the Humboldt current is a stream of cool, moist air. The warm air typical of the latitude is chilled where it meets this cooler air current and where the two air masses mix, stratus cloud is formed. Where this cloud comes into contact with land or sea, it appears as mist or fog. This fog bank generally occupies an altitude between 1,000 and 2,000 feet above sea level, but varies in both depth and extent with locality and season.

The upwelling of the coldest water near the coast is more marked in certain places. The strongest upwellings are at about latitude 7^o / 8^oS (North of Trujillo) and at 15^o / l6^o S (around Lomas) with less strong upwellings at 22^o /23^oS (Tocopilla) and 30^oS (Coquimbo).

Water from the open ocean, at a temperature more typical of the latitude overflows the cold Humboldt current and approaches the coast at 9^o 30' S (Huarney), 13^o / 14^oS (Pisco) and a very marked one at 17^o /19^o S (between Mollendo and Arica).

Where the water immediately adjacent to the coast is rather cooler, the coastal fog is generally more marked and persistent.

The height and density of the fog also varies with the seasons. In summer the cloud base will gradually life until it reaches about 2,000 ft (extending there from up towards 4,000 ft), lowering again with the approach of winter until the base drops to about 1,000 feet. The fog is also most strongly developed in winter when the re-evaporative effect of the sun on the cloud top is least.

At Lima, the clear skies of March and April begin to be clouded in May, the cloudiness grows until, from late June until September, the sun is invisible for weeks at a time. At Mollendo, the months of February, March and April are least cloudy, whilst July, August and September are completely overcast for many days in the month.

Owing to the regular daytime onshore breeze, the fog bank drifts landwards on to the coastal hills. In Peru, the foothills rise more or less gradually from the shore; in places, sandy plains of varying width and extent separate the foothills from the shore. The fogbank thus lies on the hills and up the valleys at a very variable distance from the shore.

For most of the coast of northern Chile the foothills rise steeply from the shore to upwards of 3,000 feet: consequently, the fog only penetrates inland through the lower passes and the infrequent river valleys. For almost 500 miles of coast in northern Chile there is but one river - the Loa - breaching the cliff-like coastal barrier. When Charles Darwin, the famous naturalist, visited Iquique in 1832, he noted how the fog bank seldom rose above the cliffs of the coast range which rose almost abruptly from the sea.

In central Chile, the coastal range gradually reduces in elevation and changes slowly to a series of peaks and ridges. It is breached by an increasing number of rivers as the less arid climate of the Santiago region is approached. As a result, arms of fog penetrate into more inland country. Additionally, the effect of night-time radiation of heat to clear skies and the consequent formation of early morning mists - especially where cold air drains into valleys and basins - extends these foggy fingers inland at dawn. As far inland as Santiago, low lying areas are occasionally covered by fog banks.

In other parts of the world, onshore breezes bring mists to barren coastlands (Baja California, Kalahari, Somalia) but nowhere can they compare in extent and persistence with the fog bank - the "Garua" - running from Ecuador to Central Chile. This fog bank is the source of moisture for a band of vegetation on the coastal hills - a band perhaps a few miles wide in places, barely a mile wide in others - in an otherwise rainless, barren desert which is roughly between 50 and 100 miles wide. At its thickest and wettest and most persistent, this fog will support lush, semi-tropical vegetation. In the transition zone seaward and landward of this band and in the region of thinner or less persistent fog, only cacti, lichens, and Tillandsia grow. Inland or shoreward of these plants, there is only bare rock or shifting sand.

The central part of Chile is also visited by the northward shift of the Westerlies in winter (July), bringing an annual rainfall to Santiago which averages about 16": this rainfall decreases northwards so that at Coquimbo there is commonly only one rain shower each year - in some good years, two. This additional moisture allows the Neoporteria and Tephrocactus to spread from the coast, inland to the main Andean chain.

Inland from Los Villos, we find Eulychnia growing both on the coast and for about fifteen miles along quebradas and basins. Near Coquimbo, the Copiapoa appear and with various allies of Neoporteria continue nearly to the northern borders of Chile. From northern-most Chile to Punta Chala in Peru we find the genus lslaya.

The continuity of the climatic characteristics of this zone are exemplified by comparing John Akers' description of "Peruvocereus" clavatus found near Lima, which had spines and bristles grey and shaggy as a plant became covered with lichens and Tillandsia (Jnl. Cactus & Succulent Society of America XX, 4, April 1948) with Hans Lembcke's description and illustration of Eulychnia observed near La Serena, overgrown with great cushions of moss (Kakteen 9, 3, March 1958), although these two observations were made about 1,500 miles apart.