Nepal's geography: eight climate zones across 150 kilometres

What the three belts are

Nepal runs along its Himalayan axis for about 880 kilometres and measures 150 to 250 kilometres across, covering 147,181 square kilometres.

Running south to north, the country divides into three belts, and every account of Nepal uses them because nothing else describes the place as economically.

The Terai is the lowland in the south, the northern rim of the Gangetic Plain, and it covers 17% of the country's area. It begins at 59 metres above sea level and it is tropical.

The Pahad, the hill belt, is the middle country: ridges, valleys and basins between the plain and the high mountains. The Kathmandu Valley sits in it, and so does most of the country's settled history.

The Himal is the high Himalaya in the north, rising past 8,000 metres. Mount Everest at 8,849 metres stands on the border.

Why the gradient is the whole story

A country 150 to 250 kilometres wide that climbs from 59 metres to 8,849 has a vertical range of nearly nine kilometres across a horizontal distance of under two hundred and fifty.

That produces eight climate zones, from tropical to perpetual snow, stacked in bands running east to west. Travelling a hundred kilometres northwards in Nepal crosses more ecological variation than travelling two thousand kilometres east or west across the Gangetic plain.

It also concentrates the population. The majority of Nepalese live in the Terai and the hills, which is to say in the two lower belts, because the third is largely uninhabitable. A country that is famous for its mountains is a country most of whose people live below them.

And it makes movement north to south difficult and movement east to west straightforward. Roads, settlement and administration all run along the grain of the terrain rather than across it.

How the rivers cut across everything

The drainage runs the other way, and that is what makes Nepal navigable at all.

The country is divided east to west into three major river systems: the Koshi, the Gandaki or Narayani, and the Karnali or Goghra, with the Mahakali on the western border. They rise beyond the main Himalayan range rather than within Nepal, cutting south through the mountains in gorges.

A second category of rivers rises in the Middle Hills and the Lower Himalayan range, and a third in the Terai itself. The three categories behave differently: the first carry snowmelt and run all year, the second depend more on the monsoon, and the third are seasonal.

The practical consequence is that the passes through the Himalaya in Nepal are river gorges rather than saddles between peaks. A trade route between the Gangetic plain and Tibet follows a river upstream until it runs out, which is why control of particular valleys mattered so much in the unification described in the history article.

What the Terai does for the country

The southern strip is 17% of the territory and it carries a disproportionate share of what the country runs on.

It is the northern rim of the Gangetic Plain, which means it is flat, alluvial, tropical and agriculturally productive in a way nothing else in Nepal is. It begins at 59 metres above sea level, the lowest ground in the country, and it is the only belt where large-scale mechanised farming is straightforward.

It is also where the country meets its open border. The Terai adjoins India across terrain that puts up no obstacle, so the road and rail connections, the trade, the goods traffic and the movement of people all concentrate there. A landlocked country's economic lifeline runs through its lowest and flattest 17%.

That concentration has a political dimension the geography creates rather than causes. The belt with the best farmland, the densest transport links and the closest ties to the neighbour is also the belt furthest from the historic centre of government in the Kathmandu Valley, and the distance is measured in ridges rather than kilometres.

What the hill country holds

The Pahad is where Nepal's history happened, and the reason is defensive rather than agricultural.

Ridges and valleys between the plain and the high mountains produce a series of separate basins, each with its own water, its own farmland and its own approaches. That is the terrain that supported the twenty-two kingdoms of the Baise Rajya, the twenty-four of the Chaubisi Rajya and the three kingdoms of the Kathmandu Valley described in the history article.

A basin in this belt is large enough to feed a small state and hard enough to reach that the small state survives. Unification, when it came, was a matter of taking such places one at a time over decades.

The Kathmandu Valley is one of them: level ground at altitude, well watered, on the route between the plain and the Tibetan passes. It has been the political centre of this territory for as long as there has been one.

What the mountains are made of

The Himalaya is the product of a continental collision that has not finished.

The Indian plate is moving into the Eurasian, and Nepal sits on the deformation zone. The range is still rising and the country is seismically active, which is a permanent condition rather than a periodic event.

The consequences were demonstrated on 25 April 2015, when an earthquake of moment magnitude 7.8 killed nearly 9,000 people and injured nearly 22,000, and triggered an avalanche.

A country whose terrain makes rescue slow and whose geology makes disasters likely has a compounding problem: the same ridges that isolate a village in normal times isolate it completely when a road fails.

Why the country is hard to reach

Nepal is landlocked, with two neighbours, and it remains isolated from the world's major land, air and sea transport routes.

The two neighbours are not equivalent. The southern border faces the Gangetic plain and is open terrain crossed by rivers and roads. The northern border runs along the crest of the Himalaya. A country with two neighbours effectively has one for most practical purposes, which is a geographic fact with permanent consequences for trade and for foreign policy.

Internal transport compensates where it can. Aviation is comparatively well developed for the country's income: 47 airports, 11 of them with paved runways, carrying frequent flights and sizeable traffic. In terrain like this an aircraft is the alternative to a two-day walk rather than a luxury, and the eleven paved runways are the country's real infrastructure bottleneck.

Between the tropical Terai and the permanent snow lie every intermediate zone, and each supports different agriculture.

The lowland grows what the Gangetic plain grows: rice, sugar, and the crops of a hot wet climate. The middle hills carry terraced fields on slopes that would be unusable flat, growing maize, millet and vegetables at altitudes that change what will ripen every few hundred metres. Above them the land supports grazing rather than tillage, and above that nothing.

Terracing is the visible signature of the country. Cutting a slope into level steps converts land that would shed both soil and water into land that holds them, and the hill belt has been reshaped this way over centuries of work. It reshapes whole hillsides, and the work is almost entirely manual.

The vertical arrangement also means a single village can farm several climates at once by working plots at different heights, which is a form of insurance against a bad season at any one altitude and a reason settlements sit where they do.

What the altitude grows and what lives in it

The altitude range that produces eight climate zones produces a correspondingly wide range of habitats, and the species list reflects it.

Nepal holds 107 species designated as threatened by the International Union for Conservation of Nature: 88 animals, 18 plants and one in the fungi or protist group. Among them are the Bengal tiger, the red panda, the Asiatic elephant and the Himalayan musk deer.

That list itself demonstrates the gradient. A tiger and an elephant are lowland tropical animals of the Terai. A red panda and a musk deer belong to the temperate and subalpine forests hundreds of kilometres to the north and thousands of metres higher. They are inside the same small country because the country is arranged vertically.

The conservation problem follows the same lines. Each belt is a narrow horizontal strip, so a species restricted to one of them has a range that is long and thin, easily severed by a road or a settlement running north to south across it.

How the monsoon arranges the year

The gradient that stacks the climate zones also decides when it rains, and the timing governs everything from harvests to road closures.

Moist air arriving from the south meets the first rising ground in the Terai and the hills, and gives up most of its water on the southern slopes. What crosses the main range is drier, which is why the country's northern valleys behave differently from the ones a hundred kilometres south of them.

The consequences compound with the terrain. Steep slopes, saturated soil and heavy seasonal rainfall produce landslides, and a landslide on a mountain road removes the only connection to whatever is above it. Rivers that run all year on snowmelt run very much harder in the monsoon, and the same gorges that make north-south travel possible make it dangerous for part of the year.

The high valleys carry a separate hazard that belongs to the same system. Glacial meltwater accumulates behind unstable moraine, and when the barrier fails the flood travels down exactly the gorges the roads and settlements follow.

None of this is unusual for high mountain country. What is unusual is the compression: the distance between the belt that receives the rain and the belt that receives the flood is short enough that both problems belong to the same district.

What the geography settles and what it does not

Three things about Nepal follow directly from the terrain and are worth stating as geography rather than as politics.

The population is where the land is workable. The Terai and the hills hold the majority, and the country's agricultural and demographic weight sits in the south, closer to India than to anything on the other side of the mountains.

The state's reach is a function of the ridges. Administering a country whose valleys are separated by walls is expensive, and the federal structure of seven provinces adopted in the constitution of 2015 is an attempt to organise around that rather than against it.

The exposure is permanent. Seismicity, monsoon landslides and glacial lakes are conditions of the territory. What varies is the state's capacity to respond, and the geography sets the difficulty of responding at a high level regardless of who is governing.

What the geography does not settle is the country's relationships. A landlocked state between two much larger neighbours has constraints rather than a script, and the terrain that makes the northern border almost impassable is the same terrain that has kept Nepal from being absorbed by anybody.