The most densely populated desert in the world is in Rajasthan, and it exists because the mountain range faces the wrong way

One figure about the Thar Desert throws anyone reading it for the first time: 83 people per square kilometre. That is a higher density than Sweden, than Chile, than much of inland Spain. And we are talking about a desert: the most populated in the world.

The second thing that throws you is that, strictly speaking, this desert should not be here. It lies less than four hundred kilometres from the Arabian Sea and directly in the path of the monsoon. The explanation for why the water passes over it fits in one word: angle.

01 A desert with 83 people per square kilometre

The Thar covers 264,091 square kilometres, about two thirds of it in India and the rest in Pakistan. Most of the Indian portion falls inside Rajasthan and takes up its entire northwestern half. Average annual rainfall runs from 100 to 300 millimetres and falls in a short monsoon, July to September.

A false image needs undoing here. The Thar is not a continuous sea of dunes like the central Sahara. Shifting dunes cover a fraction of it; the rest is compacted sand plain, thorn scrub and dry grassland, with wells, villages and cultivated fields spread throughout.

That is the key to the density. A desert of pure sand supports nobody; a desert of scrub with scarce but predictable rain, and a very long tradition of water harvesting, supports a great many people at the limit. The figure of 83 per square kilometre does not describe abundance: it describes a finely tuned organisation of scarcity.

02 Eight hundred and fifty kilometres of mountain range

Crossing the state is a long, old mountain range: the Aravallis. It runs more than 850 kilometres from southwest to northeast, almost from one end of the state to the other, and its highest point, Guru Shikhar, stops at 1,722 metres. It is heavily eroded relief, of dark rounded hills and blunt crests, with none of the drama of a young range.

Anywhere else in the world, an 850-kilometre barrier between an ocean and a plain would produce abundant rain on one side. That is what the Western Ghats do, eight hundred kilometres to the south, intercepting the same monsoon and dumping thousands of millimetres on their windward slope.

The Aravallis do not. And the reason has nothing to do with their height, low though they are. It has to do with how they are set.

03 The problem is the angle

The southwest monsoon, in its Arabian Sea branch, advances from southwest to northeast. The Aravallis lie in exactly that same direction: southwest to northeast. Which is to say, the range does not stand across the wind's path; it runs parallel to it.

For a mountain to cause rain it has to force humid air upwards: as it rises it cools, the vapour condenses and falls. If the air travels along the range instead of into it, it is never forced to climb. It grazes the crest line and carries on without releasing anything.

A second factor compounds this. That same monsoon branch has already left most of its moisture on the Western Ghats before it gets here, so the air reaching northwestern Rajasthan arrives dried out as well. And the branch coming from the east, off the Bay of Bengal, arrives exhausted after crossing the whole northern plain, and the little it has left does meet the Aravallis, which this time do act as a stop.

The result is a desert in a place where the geography promised something else. Less than 400 millimetres a year across the northwest of the state, and between 100 and 300 in the heart of the Thar.

04 Three fifths one side, two the other

The range splits the state into two halves that do not resemble each other. Three fifths of Rajasthan lies northwest of the Aravallis: that is the arid part, the Thar. The remaining two fifths, east and south, sit higher — between 100 and 350 metres — receive considerably more water and are markedly more fertile.

That division explains how the cities are distributed. The ones almost everybody visits — Jaipur, Udaipur — sit on the range or on its eastern flank, where there is water and soil. The desert ones — Jaisalmer, Bikaner — began as staging posts on caravan routes, not as agricultural capitals: they lived off what crossed, not off what they grew.

05 How you live on two hundred and fifty millimetres

The answer to how so many people survive on so little rain is in the architecture, and it is visible everywhere. When water only arrives three months a year and the water table then falls for nine, two problems must be solved: storing it, and being able to reach it as it descends.

Out of that come the great tanks at the foot of the forts, the artificial lakes of the southern cities and, above all, the stepwells: structures dug into the ground with flights of steps descending tens of metres so that the water can still be reached as the level drops. It is an entire technology born of a single rainfall figure.

It also explains where settlements sit. In territory like this, a village is not placed where the view is good: it is placed where a hollow holds water after the monsoon, or where the water table is high enough to reach by digging.

06 What this changes for the traveller

The first consequence is that there is no single Rajasthan. The eastern and southern one is green for much of the year, with lakes, hills and cultivation; the northwestern one is arid and long-horizoned. Between Udaipur and Jaisalmer there are some six hundred kilometres and two landscapes with nothing in common.

The second is that the desert is not the emptiness it is advertised as. You cross villages continuously, there are ploughed fields between the dunes and traffic on the roads. Anyone coming for absolute solitude will find something else: a dry, densely inhabited territory, which is far rarer and far more interesting.

The third is that it pays to watch the water. In this state, almost everything impressive — the forts on high ground, the lakes with a palace in the middle, the wells that descend thirteen storeys — is a technical answer to the same question. Understood that way, Rajasthan's architecture stops being a catalogue of monuments and becomes a system.