Arid climates

A hot arid environment is a true hot desert that receives very little precipitation — under about 250 mm a year — which is far less than the potential evapotranspiration, so there is a permanent water deficit and vegetation is sparse or absent.

A hot semi-arid environment receives more rain — roughly 250 to 500 mm a year — but still has a marked water deficit. It is the transitional belt around the desert margins, able to support scrub, grassland or seasonal grazing, such as the Sahel along the southern edge of the Sahara.

The key difference is therefore the amount and reliability of rainfall: hot arid land is drier (under ~250 mm) with bare surfaces, while hot semi-arid land is less dry (~250–500 mm) and can sustain some vegetation.

The graph shows high daytime temperatures, with maxima reaching about 40°C in the hottest months — typical of intense insolation on a bare desert surface.

Night-time minima are much lower, falling to about 8°C, so nights are cool or cold compared with the day.

The most striking feature is the very large diurnal (day–night) temperature range: with maxima near 40°C and minima near 8°C, the day–night difference is roughly 32°C, far larger than in a humid climate.

There is also a seasonal pattern, with summer months hotter than winter months, but the day-to-night range is large throughout the year.

The rain-shadow effect produces aridity through the forced rise and subsequent descent of air over a mountain barrier.

First, moist air moving inland from the ocean is forced to rise as it meets the mountains. This is orographic uplift. As the air rises it cools at the adiabatic lapse rate, water vapour condenses, cloud forms and heavy rain falls on the windward (upwind) slope. This is the wet side of the range, and the air loses most of its moisture here.

Next, having been largely stripped of moisture, the air crosses the summit and begins to descend the leeward (downwind) slope. As it sinks it is compressed and warms adiabatically. Because warmer air can hold more water vapour, its relative humidity falls and it moves further from saturation, so any remaining cloud evaporates.

The air therefore arrives on the leeward side warm and dry, and little or no rain falls. This sheltered, dry zone behind the mountains is the rain shadow, an area of aridity created by the relief rather than by latitude. The descending, warming, drying air works in the same way as the subsidence in the subtropical highs.

The very large diurnal temperature range in hot deserts is caused mainly by clear skies and very low humidity, which affect both daytime heating and night-time cooling.

By day, the skies are usually cloudless, so insolation passes almost unobstructed to the surface. The bare rock and sand have little vegetation or moisture, so almost all of the absorbed energy heats the ground and the air above it rather than being used to evaporate water. Surface temperatures therefore rise quickly to high daytime maxima, often over 40°C.

By night, the same clear, dry air provides no insulating blanket. There is little cloud and very little water vapour, so the atmosphere is poor at absorbing and re-emitting the outgoing longwave radiation from the ground. Heat escapes rapidly to space, and surface temperatures fall steeply, sometimes to near freezing.

In a humid climate, cloud and water vapour trap heat at night and keep it mild, so the diurnal range is small. Hot deserts lack this, so the day-to-night swing is large — the diurnal range can rival or exceed the seasonal range.

Aridity is a permanent water deficit in which precipitation is far lower than potential evapotranspiration; hot arid environments receive under about 250 mm of rain a year and hot semi-arid environments about 250–500 mm. Four main causes are usually identified: the subtropical high-pressure belts (the descending limb of the Hadley cell), distance from the sea (continentality), the influence of cold ocean currents, and the rain-shadow effect of mountains. This essay assesses their relative importance and argues that the subtropical high pressure is the most important cause at the global scale, because it explains the basic latitudinal belt of deserts, but that the other three causes are decisive in particular places and that most real deserts are produced by several causes acting together.

The subtropical high pressure is the primary, large-scale control. Air heated at the Equator rises and gives equatorial rainfall, flows poleward at altitude, and sinks at around 20°–30° to form permanent subtropical anticyclones. This subsiding air is compressed and warms, so its relative humidity falls and it becomes stable, suppressing cloud and rainfall; the trade winds it feeds are dry and divergent. Because this descending, drying air recurs at the same latitudes in both hemispheres, the great deserts — the Sahara, the Arabian Desert and the Australian interior — form a roughly symmetrical belt around 30°. No other cause explains this near-global, repeating pattern, which is why the subtropical high is the most important single cause overall.

Continentality is important in a different way: it explains why the deep interiors of large continents are intensely arid. Air that has travelled far over land has had its moisture rained out before it reaches the centre, so little is left to fall. This is why central Australia and the heart of the Sahara are so dry, and continentality also widens the temperature ranges that characterise these environments. However, continentality only operates where landmasses are large, so it is a regional reinforcement rather than a universal cause.

Cold ocean currents are decisive for a distinctive group of coastal deserts. The cold Benguela Current off the Namib and the cold Humboldt Current off the Atacama chill the air above them and reduce evaporation, so the air carries little moisture. Crucially, cooling the air from below creates a temperature inversion that traps the stable surface air and prevents it rising to form rain clouds, so moisture forms coastal fog rather than rain. This produces narrow but extreme deserts — the Atacama is among the driest places on Earth — but only along certain western coasts, so cold currents are locally dominant rather than globally important.

The rain-shadow effect explains aridity behind mountain barriers. Moist air is forced to rise, cools and rains on the windward slope, then descends the leeward side, warming and drying so that little rain falls — leaving an arid rain shadow. This is significant for many interior dry zones and reinforces existing deserts, but it depends on the presence of a mountain barrier, so again it is a local control.

The most accurate assessment is that these causes rarely act alone — aridity is multi-causal, and the relative importance of each depends on the place and the scale. The Atacama is the clearest example: it is dry because of the subtropical high, the cold Humboldt Current AND the rain shadow of the Andes all reinforcing one another. The Namib combines the subtropical high with the Benguela Current. Even the Sahara, dominated by the subtropical high, has its deepest aridity reinforced by continentality. Where several causes stack, they push in the same direction and produce the most extreme deserts.

In conclusion, the subtropical high pressure is the most important cause of aridity at the global scale, because it alone explains the symmetrical belt of deserts at around 30° in both hemispheres. But continentality, cold ocean currents and the rain-shadow effect are decisive in specific locations, and most deserts owe their dryness to a combination of causes rather than any single one. Relative importance is therefore best judged scale-by-scale and place-by-place: one cause sets the global pattern, but several combine to produce the aridity of any particular desert.

Aridity is a permanent water deficit where precipitation is far below potential evapotranspiration. The statement claims that pressure and wind systems — chiefly the subtropical high-pressure belts formed by the descending limb of the Hadley cell — are the most important cause. This essay weighs that claim against the other causes (continentality, cold ocean currents and the rain-shadow effect) and argues that pressure and wind systems are the most important cause at the global scale, because they explain the basic distribution of deserts, but that they are not a sufficient explanation for every desert, so the statement is only partly correct.

There is a strong case for the statement. The subtropical highs are the descending limb of the Hadley cell: air rises and rains at the Equator, flows poleward aloft, and sinks at around 30°, where it is compressed, warms, dries and becomes stable, suppressing rainfall. Because this circulation operates at the same latitudes worldwide, deserts form a near-symmetrical belt around 30° in both hemispheres — the Sahara, the Arabian Desert and the Australian interior all lie under the subtropical high. The trade winds the highs feed are dry and divergent, adding no moisture. No other cause explains this repeating, near-global pattern, so at the largest scale pressure and wind systems are indeed the most important cause.

However, pressure and wind systems alone cannot explain the precise location and intensity of every desert, which is where the other causes become decisive. Continentality explains why the deep interiors of large continents — central Australia, the heart of the Sahara — are the most intensely arid, because air loses its moisture crossing the land. Cold ocean currents explain the narrow, extreme coastal deserts: the Benguela Current off the Namib and the Humboldt Current off the Atacama cool and stabilise the coastal air, cut evaporation and create a temperature inversion that prevents air rising, so moisture forms fog rather than rain. The rain-shadow effect explains leeward aridity, where air rises and rains on the windward slope of a mountain range then descends warm and dry on the leeward side. Each of these is locally dominant in places where the subtropical high is not the whole story.

The decisive point is that these causes usually combine, so ranking pressure and wind systems as 'most important' depends on the scale considered. At the Atacama, the subtropical high, the cold Humboldt Current and the rain shadow of the Andes all reinforce one another to produce one of the driest places on Earth; at the Namib, the high combines with the Benguela Current. In these places the subtropical high provides the underlying aridity, but it is the additional causes that make the desert as extreme as it is. So while pressure and wind systems are necessary, they are rarely sufficient on their own.

In conclusion, I agree to a large extent that pressure and wind systems are the most important cause of aridity, but only at the global scale: they explain the symmetrical belt of deserts at around 30° that no other cause can. At the local scale, continentality, cold ocean currents and the rain-shadow effect are decisive, and most deserts are produced by several causes acting together. Pressure and wind systems are therefore the most important single cause, but they are best seen as the foundation on which the other causes build, rather than the complete explanation.