Why Is Pokhara So Wet and Mustang So Dry?

You met the difference between weather and climate in Grade 10 Grade 10: Weather and climate: weather is what the air is like on one day; climate is the average weather of a place over many years, usually 30. To compare two climates, scientists compare such averages. Here are Pokhara’s and Jomsom’s, month by month.

Two things stand out. First, Pokhara’s rain is not spread through the year: it comes in one great season, from June to September. Second, Jomsom hardly has a rainy season at all. The temperatures differ too: Pokhara averages about 21.5 °C over the year, Jomsom about 11.7 °C.

To understand both towns, we have to start far away from them: with the whole Earth, and the Sun.

You learned in Grade 6 that sunlight falling straight down heats the ground more than sunlight falling at a slant, because a slanting beam spreads the same light over more ground. (Why Is It Cold in Winter? shows this with a beam of sunlight you can tilt.) Near the equator, the noon Sun is high in the sky all year. Near the poles it is always low, and for months it doesn’t rise at all. So, averaged over a year, each square meter at the equator receives about 2.4 times as much sunlight as a square meter at the poles (measured at the top of the air, before clouds and ice reflect any of it).

If nothing moved the heat around, the tropics would get hotter and hotter and the poles colder and colder. They don’t, because the Earth has two great fluids that carry heat from the warm places to the cold ones: the air and the oceans. Their movements, the winds and the ocean currents, make the patterns of climate on Earth. Everything in this topic comes from two causes: the Sun heats the Earth unequally, and the Earth turns.

You know the start of this from Grade 8 Grade 8: Sea breeze and land breeze: where the ground is hot, the air above it is warmed, expands, becomes less dense and rises; cooler air flows in along the ground to take its place. A sea breeze is a small loop of moving air, a convection current, a few kilometers across. The Earth makes the same kind of loop on a giant scale.

Near the equator, the ground and the sea are hottest. Warm, moist air rises there, all the way round the Earth. Rising air cools, and as it cools its water vapor condenses into clouds and falls as rain (Grade 8’s Convection currents and weather; more in Where Does the Rain Come From?). So the equator has a belt of towering clouds and heavy rain almost every afternoon, and the world’s great rainforests grow there: the Amazon, the Congo, the islands of Indonesia.

The rising air can’t go up for ever. About 15 km up it spreads out north and south, high above the ground, towards the poles. On the way it cools, becomes denser, and around 30° north and south of the equator it sinks back to the ground. Sinking air is squeezed as it comes down, and warms up, so no clouds can form in it. Under it, the sky is clear almost every day, and rain is rare. That is why most of the world’s great hot deserts lie near 30° N and 30° S: the Sahara, the Arabian Desert, the Thar, the deserts of Australia and the Kalahari. From there the air flows back along the ground towards the equator, and the loop is closed.

This loop is called a Hadley cell, and there is one on each side of the equator. Farther towards each pole there are two more loops: a Ferrel cell, between about 30° and 60°, and a polar cell, from about 60° to the pole. The air rises again near 60°, where cold polar air meets warmer air and storms form (the stormy belt that brings Britain and Norway so much rain), and sinks again at the poles, which are deserts of ice: Antarctica gets less snow in a year, as water, than Jomsom gets rain.

Now look at Nepal on Figure 26.2. It is at 28° N, right in the belt of sinking air, of deserts. By this part of the model, all of Nepal should be as dry as Rajasthan. Something else must be at work, and it is: but first, look at the arrows on the Earth’s face. The winds at the ground don’t blow straight north or south. They blow at a slant.

If the Earth didn’t turn, the air returning to the equator along the ground would blow straight from the north in the northern half of the Earth, and straight from the south in the southern half. But sailors have known for centuries that the steady winds of the tropics, the trade winds, blow from the north-east north of the equator, and from the south-east south of it. Something bends them. It is the Earth turning under them.

Try it with the widget. A pen is pulled straight along a ruler, held still above a disc, from the middle out to the edge. Turn the disc, and watch the line the pen leaves on it.

The pen goes straight, but the line on the disc curves. Someone riding the disc doesn’t feel it turn; to them, it looks as if something pushed the pen sideways. We all ride the turning Earth. So to us, anything that moves a long way over the Earth, a wind or an ocean current, seems to swerve: to the right in the northern half of the Earth, and to the left in the southern half. This swerve is called the Coriolis effect, after the French engineer Gaspard-Gustave de Coriolis, who worked out the mathematics of it in 1835.

There is a simple way to see why air from the north swerves to the west. The Earth turns once a day, so every place on it is carried round towards the east. A place on the equator goes all the way round, 40,075 km, in 24 hours. A place nearer the pole goes round a smaller circle in the same time, so it moves more slowly.

Now think of air flowing south, from 30° N towards the equator. It starts out moving east with the slower ground of 30° N. As it comes south, the ground beneath it is moving east faster and faster. The air falls behind: it drifts westwards over the ground. Moving south and drifting west, it blows from the north-east: the north-east trade winds. (Real winds don’t keep their speed perfectly, because they rub against the ground, but the direction of the swerve is right.) In the same way, air moving towards the poles drifts east, which is why the winds between 30° and 60° blow mostly from the west: the westerlies.

These steady wind belts are the prevailing winds: the winds that blow most often at a place. In Figure 26.2: the trade winds near the tropics, the westerlies in the middle latitudes, and the polar easterlies near the poles.

The winds don’t only move air. As they blow across the ocean, they drag its surface water along with them, and over thousands of kilometers they set whole oceans moving. These great flows of sea water are ocean currents. Like the winds, they are bent by the Earth’s turning; and unlike the winds, they can’t cross the land, so the continents turn them. In each great ocean, the currents go round in a huge loop, called a gyre: clockwise in the northern half of the Earth, anticlockwise in the southern half.

The currents carry heat the way the winds do. Warm water from the tropics flows towards the poles, usually along the eastern coasts of the continents, and cold water comes back towards the equator along the western coasts. The most famous warm current is the Gulf Stream, which flows from the Gulf of Mexico up the coast of North America and across the Atlantic towards Europe. Off Florida it carries about 25 times as much water as all the world’s rivers together, and farther north, where more water has joined it, over 100 times.

Cold currents do the opposite. Off the west coast of South America, the cold Humboldt Current flows north from near Antarctica. It cools the air above it from below, and cool air under warmer air can’t rise to make rain clouds. Together with the sinking air of the subtropics and the shelter of the Andes Mountains, it makes the Atacama Desert of Chile one of the driest places on Earth: some places in it get a few millimeters of rain a year at most, and have gone years without any. The Namib Desert, on the cold west coast of southern Africa, is a desert for the same reasons.

The slow conveyor belt in the deep

There is a second, slower kind of ocean current, deep down. In the far North Atlantic, near Greenland, and around Antarctica, the sea water gets very cold, and when sea ice freezes it leaves most of its salt behind in the water around it. Cold, salty water is dense, so it sinks to the ocean floor, and creeps along the bottom towards the equator and into the other oceans. Far away, it slowly rises again and flows back near the surface to replace the water that sank. This worldwide loop is called the global ocean conveyor belt. It is another convection current, like the air’s, driven by water that is cold and dense in one place and warmer in another. It is very slow: a drop of water takes about 1,000 years to go all the way round.

Now we can come back to Nepal, and the question of why it isn’t a desert.

Remember the sea breeze. By day, land heats up faster than the sea, because water has a much higher specific heat capacity; air rises over the warm land, and cooler, moist air flows in from the sea. By night it turns round. Now make the land the whole of South Asia, and the sea the Indian Ocean, and instead of day and night, think of summer and winter.

In April and May the land of South Asia heats up much faster than the ocean: the plains of northern India and Pakistan reach over 45 °C, and the high Tibetan Plateau, about 4,500 m up, heats the air above it too. Hot air rises over the land, leaving low pressure, and from June the moist air over the warm Indian Ocean flows in to replace it, across thousands of kilometers. As this sea air reaches the land and is pushed up, over hills and over the land’s own rising air, it drops its water as rain. In winter the land becomes colder than the sea, and the flow turns round: cool, dry air flows out from the land towards the sea, and the rains stop. This great seasonal wind is the monsoon. The word comes from the Arabic mawsim, “season”.

The sea breeze is not the whole story: the rainy belt of rising air near the equator, in Figure 26.2, moves north and south a little with the Sun through the year, and over South Asia in summer it is pulled far north, over the hot land. That is why the monsoon overpowers the dry, sinking air that makes deserts at the same latitude elsewhere.

The monsoon’s air reaches Nepal by an unexpected road. Part of it comes across the Arabian Sea from the south-west; part crosses the Bay of Bengal, and there, blocked by the Himalaya and pulled by the low pressure over north India, it turns and flows west along the mountains. So the monsoon reaches Nepal from the east and south-east, not the south-west. It arrives first in eastern Nepal, usually around 13 June, and leaves around 2 October, the average dates measured by the Department of Hydrology and Meteorology. In those four months, Nepal gets about four-fifths of its whole year’s rain.

So the monsoon brings the rain. Why does it fall on Pokhara, but not on Jomsom?

When moist monsoon air reaches the foothills, it can only go one way: up. Rising air cools, as you saw in Grade 8 (it has less air pressing on it, so it expands, and expanding air cools), and once it has cooled enough its water vapor condenses into cloud and falls as rain. The steeper and higher the slopes, the more rain is squeezed out. Pokhara sits at the foot of the Annapurna range, about 40 km from peaks over 7,000 m, and the hills just north of it are among the wettest places in Nepal: Lumle, at 1,738 m, about 20 km north-west of Pokhara, averages about 5,500 mm of rain a year.

By the time the air has climbed over the Himalaya, it has lost much of its water. And as it sinks down the far side, it is squeezed and warms up, so its humidity falls and no clouds can form. The land on the far side of a mountain range, sheltered from the wet winds, is in a rain shadow. Mustang lies just north of the main Himalayan range, in its rain shadow, and so does the whole Tibetan Plateau beyond it.

From space, the rain shadow is easy to see.

Mustang’s little rain doesn’t all come in the monsoon. A few times each winter and spring, storms traveling east from the Mediterranean and the Middle East, called western disturbances (lows, areas of low pressure: see Where Does the Rain Come From?), bring snow to Mustang and western Nepal. That is why, in Figure 26.1, Jomsom gets almost as much rain and snow in March as in June.

There is one more reason Jomsom differs from Pokhara: it is almost 2 km higher, and higher places are colder. You know this from any trip into the hills: Nagarkot is cooler than Bhaktapur below it, and snow lies on the mountains long after it has melted in the valleys.

The air is warmed mostly from below, by the ground, which absorbs sunlight. And air that rises has less air pressing on it, so it expands and cools. So, on average, the air gets colder the higher you go, by about 6.5 °C for every kilometer of height, though the real figure changes from place to place and season to season. (A bubble of air rising through it cools faster, about 10 °C per kilometer until clouds form in it, as Where Does the Rain Come From? shows; 6.5 °C per kilometer is how the air around it changes with height.)

You now have a model that can explain the climate of almost any place. It has two causes, unequal heating and the Earth’s turning, and these effects:

• a.Latitude: the nearer the equator, the more sunlight. The belts of rising air (rainy: near the equator and 60°) and sinking air (dry: near 30° and the poles).
• b.Prevailing winds, bent by the Earth’s turning: whether the wind comes off a warm ocean, a cold ocean or a dry continent.
• c.Ocean currents: warm currents warm and wet the coasts they pass; cold currents cool and dry them.
• d.Land and sea: land heats and cools much faster than the sea, which makes monsoons, and makes places far inland hotter in summer and colder in winter than places on the coast.
• e.Mountains: rain on the slopes facing the wind, a rain shadow behind.
• f.Altitude: about 6.5 °C colder for every kilometer up.

Place	Its climate	The main reasons
Pokhara	warm, very wet summers, dry winters	monsoon air from the Bay of Bengal, pushed up the Annapurna range
Jomsom	cool, dry all year	in the Himalaya’s rain shadow; nearly 2 km higher
The Sahara	hot, very dry	sinking air near 30° N
The Congo rainforest	hot, wet all year	rising air near the equator
London	mild, damp	westerly winds off an ocean warmed by the North Atlantic Current
The Atacama	cool for a desert, almost rainless	cold Humboldt Current, sinking air, the Andes’ rain shadow
Mawsynram, India	wettest place on Earth	monsoon air forced up steep hills

• Where Does the Rain Come From? How water evaporates, condenses into clouds and falls, and how air masses meeting change the weather from day to day.
• Why Is It Cold in Winter? The tilt of the Earth, the height of the Sun and the length of the day, which move the belts of this topic north and south through the year.
• Is the Earth Really Getting Warmer? The evidence that the climate this topic explains is changing, and why.