Weather

It’s the world’s largest weather system. Affecting almost half of the world’s population, its seasonal changes in winds and rain determine the cycles of nature and humankind, year after year. It is the story of the world’s largest continent, fighting with two oceans, played out in the atmosphere above. From the blistering heat of India, to the frigid north of Siberia, this is the Asian Monsoon.

No discussion of Earth’s climate can go without some mention of the largest single pattern of weather on the globe. Because the Asian Monsoon covers so many individual climate zones, there was no room to give it a good treatment in any of the various chapters that covered Asia in my Secrets of World Climate series. And so it gets an chapter of its own in my Climate Casebook.

Etymology

To begin our understanding of this enormous weather system, we should start at the beginning, and the word itself. Monsoon comes through Portuguese monção from the original Arabic word mawsim, which means “season / change in wind direction”. You see, the Asian Monsoon even affects Arabia, even though that change in wind direction results in nothing yielding rain. A monsoon is, in its simplest form, always a change in wind direction, but in the case of the Asian Monsoon, this change in wind direction occurs around the whole of one side of the world’s largest continent, producing a wet summer and dry winter across a huge area of Earth.

Geographic Extent

Now those of you familiar with tropical climates will know something of wet and dry seasons, and how the trade winds of the tropics determine these. I covered this in my chapter on Tropical Wet and Dry Climates in my other series. They always occur within the tropics of Cancer and Capricorn, from 23°N to 23°C south. So wet and dry seasons related to changing wind direction are not unique to Asia. What makes the Asian Monsoon stand out are two things – the great intensity of the wet season in the Indian Subcontinent and the extension of the wet and dry season pattern in Eastern Asia way beyond the tropics into the temperate, continental and subarctic latitudes.

So the Asian Monsoon has these two notable aspects, and each defines the two broad halves of the monsoon.

The intense monsoon that affects India and Bangladesh is the most well known, and it determines the lives of the one and a half billion people that live there, more heavily than any other weather system in the world. The economic fortunes, even the survival itself, of these populations depends upon the timing and severity of the wet season that washes over these lands between June and September.

Less well known is the East Asian Monsoon. Though less intense, its range is much more extensive, and as many people experience its annual rhythm, with Eastern Russia, Mongolia, Korea, Japan, Taiwan and most of China affected.

Dynamics – The Annual Movement of the ITCZ

Before we look at these areas in more detail, we should try to understand what dynamics are special to the Asian Monsoon. This weather system and its causes are complex and even to this day are not fully understood. However it is generally accepted that the two strongest influences are the seasonal march of the doldrums above and below the equator that affects all tropical wet and dry climates, magnified by the seasonal heating and cooling of the giant Asian landmass.

The doldrums, also called the Intertropical Convergence Zone, or ITCZ, is the band of low pressure around the tropics where heating from the sun causes upward convection of hot air leading to a drawing in of air from the surrounding land or sea in the form of trade winds. As the global seasons alternate summer between the northern and southern hemispheres, the ITCZ follows, with it tracking north in the northern summer, and then south in the southern summer. This band is almost always accompanied by heavy thunderstorms and consequential rainfall as the hot and moist air is unstably thrust into the cold upper atmosphere.

Summer Monsoon

In the northern summer the ITCZ moves up into India and China, bringing with it southerly trade winds which blow onto the continent from the surrounding seas. This is what brings the rain with the monsoon, as it does with any tropical wet season. But this effect is strengthened further by the presence of the large Tibetan plateau, the heating of which leads to a deepening of the low pressure, strengthening the monsoon over India far beyond a standard tropical wet season.

Additional summer heating of the central Asian land mass north of Tibet, far from the moderating ocean, is also strong, and extends as far as Siberia, leading to winds blowing in from the Pacific across all of Eastern Asia from Hong Kong to the Russian Pacific coast. This is the only part of the world where summer peaks in rainfall occur in the mid to high latitudes and special climate zones, the Continental and Subarctic Monsoon types, were designated by Vladimir Koppen to account for this extension into the far north of what is essentially a tropical weather system.

Koppen Climate Zones, Winter Monsoon, Effect of Australia and Ocean Pressure Zones

While we’re on the subject of Koppen, we shouldn’t confuse the Koppen Climate type Am – the Tropical Monsoon, with the Asian Monsoon, as this zone can occur all over the tropics, from Miami and Rio in the Americas, to Central Africa, Indonesia and the Philippines. The Asian Monsoon encompasses a multitude of Koppen Climate zones, all of which have peak rainfall during the summer. These are Am – Tropical Monsoon, Aw – Tropical Savannah, Cwa – Subtropical Monsoon, Cwb – Subtropical Highland, Dwa/Dwb – Continental Monsoon, and Dwc/Dwd – Subarctic Monsoon.

So that is summer. But what about winter? Well, the opposite occurs. The ITCZ retreats back to below the equator while temperatures over Central Asia plummet below zero, leading to sinking, dense air and high pressure. This high pressure is the strongest on Earth and the continental winds that blow out from it to the oceans lead to dry winters across most of Asia. Only a few places, where such seasonal monsoon winds blow over water in both directions, receive rain or snow year round.

So this, in essence, is the main mechanism driving the Asian Monsoon. There is more complexity to it of course. Australia is believed to have a strong influence on Asia, even though it is much smaller, because it acts as a pressure “counter-pole” – with Australian high pressure in the northern summer against the Asian low, and vice versa six months later. The Indian and Western Pacific oceans are the other “counter-poles” that either provide moist winds in the form of high pressure, or sink dry continental winds in the form of low pressure.

The monsoon’s growth in summer and retreat in winter can be seen in this composite of monthly average rainfall across the continent. It’s quite a mesmerizing animation, isn’t it? I could watch it for hours… Ok, moving on. Let’s now journey through the lands affected by the monsoon and examine how local topography and other factors leads to particular effects from one region to another.

Arabia

The journey starts in the Arabian peninsula, with the origin of the word itself, but it’s also the westernmost point under the direct monsoon influence. In summer, winds blow towards the intense low pressure over Central Asia, but because these winds have travelled over the continent of Africa, they bring little to no rain. In winter, the winds switch direction, blowing out from the central Asian high, and once again are dry, continental winds. This is one reason why the Arabian peninsula is a desert.

Indian Subcontinent

Moving east into India, and the same wind directions, SW in summer, NE in winter, produces a dramatically different result. Now the SW summer winds blow over the warm Arabian Sea throughout the summer months, bringing in storm after storm that lashes the western coast of India from Kerala in the south to Gujarat in the north. All along this coast sits one edge of the Deccan plateau, and these moist winds are pushed upward and cooled, releasing their moisture in spectacular fashion as this graph of Mumbai shows.

The monsoon progresses across the Indian Subcontinent in stages, with the south receiving rains as early as late May, and the north as late as mid-July. Protected somewhat by the Deccan, much of the country experiences less severe monsoon rain. But where these strong, moisture-laden winds hit the Himalayas, they are thrust upward in a similar fashion to the west coast and dump almost all their moisture on the windward slopes. And with this mechanism, known as orographic lift, we come to a global superlative. The Indian Monsoon produces the wettest places in the world.

Mawsynram and Cherrapunji, both in the Indian state of Meghalaya lie in the foothills of the Eastern Himalayas, and the world’s wettest title alternates from year to year between these two towns. Here’s the graph of Cherrapunji, with the scale kept at the standard used for all my charts, just so you can get your head around how much rain falls here. In one month alone at the peak of the summer monsoon, more rain falls here than in an entire year in Singapore, in the heart of the wet Tropical Rainforest, three times more rain than the annual total in rainy Vancouver, and six times more than the annual amount in supposedly rainy London. Such is the power of the monsoon.

This graph of Lhasa in Tibet shows the effect of the Himalayas. Being sheltered from the full brunt of the monsoon behind the highest peaks of those mountains, including Everest, comparatively little rain gets through, although the mark of the monsoon is still clearly visible in the difference between winter and summer.

In the Indian winter, the dry NE wind blows from Central Asia out into the Indian Ocean, leading to completely dry conditions for months at a time, first with cool temperatures, and then very hot temperatures, which leads to the unique three seasons pattern of much of that country. A hot and wet rainy season, warm and dry “winter”, then very hot and dry “summer”.

Such is the strength and consistency of the seasonal monsoon winds, that they are even able to change the direction of an ocean current. A current’s direction is primarily determined by the winds blowing over it, so in summer, the around India flows east from Arabia to Myanmar, and in winter it flows west. So the name of this current alternates between the SW Monsoon Drift and the NE Monsoon Drift. This is the only major ocean current where such a reversal occurs every year. Such is the power of the monsoon.

China & Korea

Continuing further east, and we pass over South-East Asia. This area does feel the influence of the monsoon, but is within the normal latitudes where tropical wet and dry seasons occur, with Koppen types Am and Aw predominating, so does not require special mention in this article.

As we continue north and east, however, we enter Southern China, and the beginning of the Eastern Asian Monsoon. The interplay between continent and ocean now switches to the Pacific, and so we get a different character to that of the Indian – less intense, but spread out over a much greater area. The monsoon pushes inland hundreds of miles into the heart of China, subtropical in the south, continental in the north, and as one travels further north, the amount of rain, and the number of months in which it falls reduces, as the Pacific moisture is lost over the land carried by southerly winds.

Korea, like Northern China experiences the monsoon as a continental climate, with dry and cold winters blowing in from the Siberian High. But, being on the coast, the monsoon brings very wet summers, as this graph of Seoul shows.

Japan

Because Japan is an island archipelago, the change in wind direction produces rain or snow across all seasons, blowing in from the Pacific in summer and in winter, across the Sea of Japan that lies between this country and Russia. So one gets a more even distribution of precipitation across the year throughout the islands, although along this mountainous set of islands, the main population centres along the east and southern coasts experience summer rainfall peaks, while the north and west facing coasts get noticeable winter peaks, usually in the form of snow. In fact Sapporo, the largest city on the northern island of Hokkaido, is the snowiest city in the world, thanks to the winter monsoon winds.

Typhoons in East Asia are another distinction between this part of the monsoon and that of the Indian part. These often ferocious storms account for a considerable proportion of total rainfall in the season as they lash cities from Taiwan and Hong Kong in the South to Japan in the north.

Siberia & Conclusion

The East Asian Monsoon’s influence reaches its end in the far north of Eastern Siberia, where the influence of the winter Siberian High leads to very dry winters. In summer, southerly monsoon winds still penetrate this vast area, bringing rain, but by this stage, are weakened considerably and the amounts are little compared to those further south. Still it is a remarkable fact that a singly connected weather system can affect such a large area, across so many lines of latitude, from 20°N in India, through to 70°N in Siberia. Such is the power of the monsoon.

The Asian Monsoon varies in intensity from one year to another, affecting both India and China, so these average rainfall graphs, as always, can be deceptive in hiding the true pattern of annual variability. Too little rain, and effective drought can occur, since the rest of the year in many places is very dry already. Too much rain, and flooding occurs across large areas, in addition to landslides along the southern and eastern edges of the Himalayas. Too little and too much rain can lead to many thousands of deaths in this way. The monsoon is a taker as well as a bringer of life.

Searing heat. Freezing ice. Monsoon rains and dusty droughts. Spring. Summer. Autumn. Winter.

For hundreds of millions of years they have they have defined the rhythm of nature on our planet. They underpin the daily existence of billions of us today. A product of the clockwork of the heavens. These are the seasons of Planet Earth.

Astronomical Origins

When talking about climate, one cannot ignore the concept of the seasons. It’s the variation in temperature and precipitation throughout the year that defines each climate zone on our planet. Each zone has a unique combination of seasonal patterns. And these patterns determine the biome – the characteristic of vegetation – of the land in that zone, what agriculture can be sustained there, and ultimately whether it can support human population.

But what are the seasons? What is their origin? Why do they always repeat year after year? Why are they different in the tropics than in the higher latitudes? Why do they reverse in the southern hemisphere?

All these questions are answered in the relationship between our planet and its parent star – the sun.

When a planet orbits a star, it traces an elliptical path that is always in the same horizontal plane – the orbital plane. But the planet itself will rotate on its own axis at some other angle – what is known as axial tilt. The result of this is that as the planet moves around the star, the angle at which the star’s light strikes any particular point on that planet will change. As surface and atmospheric heating is directly related to the angle of the star’s light, the position of the orbit will therefore influence temperature for that region of the planet. So every planet in the galaxy with an atmosphere will have some combination of seasons, and so the phenomenon shouldn’t be thought of as unique to our planet. In fact the planets Mars, Saturn, Uranus, and Neptune and Saturn’s moon Titan all have distinct seasons that have been observed.

There are four significant points in any planet’s orbit in this regard – the first two being when each of the planet’s poles is pointing closest toward the star, and the second two being when the poles are exactly at right angles to the star.

In the case of our planet, our axial tilt is just over 23°. So on the day when the North Pole is facing closest toward the sun, June 21st to be precise, every point on a line encircling the earth 23° north of the equator will experience a midday sun that is exactly overhead. This is midsummer in the northern hemisphere, when heating from sunlight is at its greatest.

Exactly 6 months later, on December 21st, when the South Pole is facing closest toward the sun, every point on Earth 23° south of the equator will have a noonday sun overhead. This is midsummer in the southern hemisphere.

These two lines are respectively the Tropics of Cancer and Capricorn, called such because the sun is in those zodiac constellations on those special days, which are known as the summer and winter solstices. The word solstice comes from Latin [sol + stit], meaning “sun stationary”, and literally describes the stop and reversal of the sun’s highest or lowest daily point in its seasonal progression.

Exactly in between the solstices, on March 21 and September 21, the Earth’s poles are at an exact right angle to the sun, which means that on these days, the sun at noon at the equator is directly overhead. These days are called equinoxes – from the Latin [aequus + nox], meaning “equal night”. The hours of day and night across every point on earth are the same at 12 hours each on these special days.

In practice, midsummer temperature peaks lag behind midsummers day by a month or so – most will experience that July and August are the hottest months in the Northern Hemisphere, and not June. And the same goes for winter, with January and February being the coldest, and not December. This is called “seasonal lag” and occurs because it takes time for the earth and oceans particularly to warm up in that area.

This cycle repeats itself near-exactly, year in year out, because the Earth returns back to the same point exactly one year later, as regular as clockwork (if human-made clocks could ever be so precise). I say near exactly, because the Earth is in fact wobbling by a few degrees on its axis, in the same way a spinning top might wobble. But this wobble has a cycle of almost 26 thousand years, so… you’re not going to notice anything in your lifetime.

Atmosphere

Ok, so that explains the origin of the seasons, but the effects of this on daily weather depend upon whether you live inside or outside of the Tropics of Cancer and Capricorn. Within the Tropics, the changes in the angle of sunlight from one solstice to the other has little or no influence on surface heating and so the tropics see very little change in temperatures across the year.

Seasons in the tropics are in fact determined by the presence or absence of rain. When the sun is at right angles to the earth, heating is at its greatest, leading to thermals of hot moist air, which, when rising into the upper atmosphere, cool, losing their moisture, and depositing rain in the form of thunderstorms. As the sun tracks back and forth across the equator throughout the year, this band of rain follows it. This band is called the doldrums, or Inter-Tropical Convergence Zone (ITCZ), as it is the convergence of tropical winds, known as trade winds, being sucked into it from surrounding areas. When these winds carry moisture, they also bring rain, and so this complex pattern determines what areas will be wet or dry during what season. Only where the ITCZ is stable, or where trade winds carry moisture regardless of direction do we have the Tropical Rainforest, where rainfall and temperature are near constant – the only climate on earth without seasons.

Outside the tropics though, these sunlight angles become important. If you know anything about trigonometry, then you’ll get the idea that as the angle of sunlight to the surface becomes shallower, there is an acceleration of the effect of that heat spreading out across the surface and being diluted in power. So that by the time you reach the poles, the heating effect of the sunlight is only a tiny fraction of what it is at the equator.

In the mid latitudes, the change in seasonal angles is significant enough to produce large changes in temperature. In summer the sun is high above the horizon, concentrating heat and delivering warm or hot days. In winter, it’s much closer to the horizon and so surface heating is much less, with the result of significantly colder days.

This variation in temperature in the mid-latitudes is diluted, however, by the proximity of oceans. Being a large fluid mass, kilometres deep, the oceans do not heat as much in summer as the land, and do not cool as much in winter. So land areas near an ocean have much smaller winter/summer temperature ranges than those in a continental interior. This difference in coastal and interior continental temperatures is at its greatest in the mid-latitudes and is the essence of why we have more climate types in this region of earth than any other.

In the polar regions, the effect of axial tilt is at its extreme. Above the polar circle at 67°N, there is at least one day each winter when the sun does not even climb above the horizon, and at least one day each summer when the sun never sets. As one approaches the pole, these number of days increase until at the North Pole, the sun doesn’t rise for 6 months in winter, and stays above the horizon for 6 months in summer. The same occurs at the opposite time of year in Antarctica.

Global Seasons and Climate in One Chart

In the Koppen climate classification system, we have categorised patterns of temperature and rainfall determined by the seasons. You can learn more about this in my intro article or by watching my Secrets of World Climate series which covers each zone in detail. What you almost certainly haven’t seen before, is a representation of seasons across all Koppen zones in one image. And here it is.

For readability, I have grouped similar zones together or omitted rare or isolated ones. This list shows the most important and distinct zones, starting at the equator at the top and finishing at the pole at the bottom of the page. For each zone I have depicted the change in temperature across the year in a colour spectrum, from red as the hottest to purple as the coldest. Below each temperature progression is a progression of precipitation, with rain in blue, snow in white, and intensity of such precipitation as proportional shading. These patterns are for the northern hemisphere and will naturally be reversed for the southern.

Up top we see the unchanging single hot and wet season of the Tropical Rainforest. Next we see the change in rainfall across the year in the Tropical Monsoon and Savannah climates, and a slight bump in temperature just before the wet season.

After this we see the unique Subtropical Highland climate, where temperatures are reduced due to altitude. Note that there is a form of this climate where the rain is constant year round, but is not shown here.

Then we come to the Hot Desert, where there is little to no rain, but intensely hot conditions in summer.

We then have the unique three season climate of the Subtropical Monsoon – a mild dry winter, then a short and very hot dry summer, followed by hot wet season.

As we move into the mid latitudes, we get our first cool winter temperatures in the Humid Subtropical, followed by the unique dry summer /wet winter pattern of the Mediterranean climate zones. This latter, together with its westerly ocean facing sibling, the Oceanic, demonstrate mild temperature ranges from winter to summer.

By contrast the Continental climate zones that follow show much more extreme temperature ranges, with subdivisions of these being the cool deserts, with absence of rain, Humid Continental, where we see snow and rain defining seasonal precipitation and Continental Monsoon, where this pattern is varied by an absence of winter precipitation. The final continental climate of the Subarctic is then seen with extreme temperature ranges featuring very cold winters yet warm summers.

As we reach the poles, freezing temperatures dominate with only a brief cool summer in the tundra bringing any respite to these forbidding lands.