Why does monsoon only occur in Asia

Monsoons and monsoon rains

Dark clouds pile up, rain pelts down from the sky. Throughout the summer, from June to September, there is heavy rainfall in India. This is important for agriculture: Without the rain, rice and other plants would wither and the harvest would be gone.

A wind that changes direction twice a year is responsible for this heavy rain. In the summer, the Indian mainland heats up very strongly. Air rises and sucks in the humid sea air of the Indian Ocean. The wind then blows from the sea to the land, bringing clouds and the rain the farmers long for. But sometimes the rainfall is so heavy that it floods large areas.

In winter the wind turns: because the Indian Ocean is now warmer than the mainland, it blows from land to sea. It is very dry in India at this time of year. This changing wind is so regular and reliable that it has been given a name of its own. It is called monsoon and the rainfall it brings with it is called monsoon rain.

The monsoons bring India the heaviest rainfall in the world. Monsoons and monsoon rains are not only found in India and Southeast Asia. It occurs in many tropical coastal countries, for example also in Northern Australia or in East Africa.

1.8.2008

It should be a perfect spectacle: The Summer Olympic Games in Beijing. The construction of the stadium alone, which is also known as the “bird's nest” because of its shape, took five years and cost the equivalent of 315 million euros. Of course, the perfect Olympiad also means that the weather plays along: no rain shower or downpour should screw up the major event.

The concern of the Chinese rulers is at least justified in terms of weather: According to weather experts, it rains on average every 3 days in Beijing in August. So that the Olympic Games shine in permanent sunshine, China is now even taking up arms: The country is shooting at its rain clouds. The ammunition: silver iodide. The clouds can be “inoculated” with this yellowish salt. It is sprayed into the clouds from airplanes or from the ground. There, silver iodide causes the water in the clouds to collect around the fine silver iodide droplets. This is how larger drops are created: The cloud rains down. In this way, the rain clouds are to be intercepted before Beijing and made "harmless". So that the games take place under blue skies, there are 26 stations around Beijing and an entire army of farmers who use silver iodide to fight against approaching rain clouds.

Everything could be much simpler: A simple sliding roof over the “bird's nest” was originally intended to keep the spectators in the Olympic Stadium dry. But this solution was supposedly too expensive for the Chinese. And probably not spectacular enough.

Showers record in India

Nowhere on earth is it more humid than in Mawsynram. The village in the Indian Khasi Mountains has the most rainfall in the world. An average of 11,872 liters of rain fall here every year. In Germany, on the other hand, it is a maximum of 1,500 liters a year, even in the mountains. The place Mawsynram achieved the rain record mainly because of its extremely humid monsoon winds in summer. These are responsible for these record-breaking rains. And it is not for nothing that the Indian state in which the village of Mawsynram is located is called “Meghalaya” - the residence of the clouds.

High and low - the air pressure

The earth has a thick packaging of air, the atmosphere. We only notice this atmosphere when it is moving. Then we feel a fine breeze or a strong wind. But although it seems weightless to us, this air has a lot of weight: a whole kilo of air presses on every single square centimeter of earth. If you calculate what this puts on our shoulders, the result is astonishing: It's several hundred kilograms! The fact that we are not compressed under this weight is due to the counter pressure that our body creates.

Due to its weight, the air exerts a pressure on the earth's surface: the air pressure. The further one moves away from the surface of the earth, the lower it becomes. This can be clearly felt in your ears when you are sitting in an airplane that is ascending or descending.

But not only the altitude, the temperature also has an effect on the air pressure. Because warm air expands, is lighter and rises: The air pressure on the ground drops. Cold air, on the other hand, is heavier and falls down: the air pressure near the ground rises. If the air masses are heated differently in different places on earth, areas with high and areas with low air pressure arise: the high and low pressure areas. In the high pressure areas, the air masses sink and warm up. Clouds dissolve, the sky is blue and the sun is shining. Low pressure areas, on the other hand, cause bad weather: When the warm, humid air rises, clouds form when it cools down and it can rain.

The high and low pressure areas are marked on weather maps with the letters H for high and T for low. Areas with the same air pressure are delimited on the maps by lines, the so-called isobars.

The wind compensates for the pressure differences between high and low: From the high pressure areas it always blows in the direction of the low. Because it is deflected by the Coriolis force, the air masses cannot flow directly from high to low. Instead of flowing straight as a bolt, they create a serpentine line. In the northern hemisphere they turn to the right and therefore circle the high in a clockwise direction and the low in an anti-clockwise direction. In the southern hemisphere it is exactly the opposite.

How is wind created?

A fresh wind often blows on the coast. If it blows particularly hard, there is also talk of a stiff breeze. But not only by the sea - air is in motion all over the world. Only in a few places on earth does not the slightest breeze blow, as in the Kalmenzone at the equator - named after the French word for calm: "calme". This windless area was previously feared by seafarers, because the sailing ships stayed there for weeks. But why is it that sometimes there is calm and sometimes a violent storm sweeps across the country?

Wind is mainly created by the power of the sun. When the sun's rays heat up the ground, the air also warms up. The warm air expands and thus becomes thinner and lighter: the air mass rises upwards. This creates low pressure near the ground. In contrast, where it is cold, the air sinks and high pressure builds up on the ground. In order to equalize the pressure difference between neighboring air masses, colder air flows where warm air rises. This happens all the faster, the greater the temperature difference between the air layers. This is how the air gets into action - a more or less strong wind is blowing.

The formation of wind at the sea can be observed particularly well. During the day, the air over the land warms up faster than over the water. The warm air masses rise and suck in the cool and heavy air over the sea: The wind blows from the sea to the land. At night the wind changes direction. Because the water stores the heat longer than the land, the air above it is even warmer and rises. Then the wind blows from the land to the sea.

Where the wind blows from is always indicated with the direction of the compass. In our latitudes this is often from the west, we live in the so-called west wind zone. The hot trade winds, on the other hand, reliably blow from the east towards the equator. And the polar easterly winds transport icy air masses from the pole to the arctic circle.

Precipitation

It doesn't matter whether it's raining, hailing or snowing - it's “to blame” for the clouds. Because without clouds there would be no precipitation. However, it depends above all on the temperature, whether there is a downpour or a wild snowstorm.

Most of the precipitation on earth falls as rain. When small water droplets collide in a cloud, they combine to form larger and heavier droplets. Are they too heavy to float; if the temperature is above 0 ° Celsius, they fall on the earth as rain.

When the air temperature is very low, precipitation no longer falls as rain, but as snow. The snowflakes grow from hexagonal ice crystals that stick together in very cold clouds with water droplets. If the ice formations are big and heavy enough, they dance down from the sky like snowflakes.

If, on the other hand, strong updrafts pull through a towering cloud, there can be hail. Small drops from the lower part of the cloud are swirled upwards, where it is colder than below. There they freeze to form small ice balls, about the size of the heads of a pin. These ice balls are called sleet. If in a very high thundercloud with a strong wind the globules in the cloud are flung up and down several times, more and more raindrops freeze onto the globules. The more the ice balls are driven around in the cloud, the bigger and harder they become. From half a centimeter in diameter, these ice balls are called hail. Hailstones can grow larger than tennis balls and have often already done a lot of damage.

In contrast to precipitation that falls from clouds, there is also precipitation that occurs close to the surface of the earth. If the temperature on the ground drops overnight, the air can absorb less moisture. The excess water then settles on the ground, on plants or on objects: the moisture is clearly visible as dew. If the temperature falls below 0 ° Celsius at night, the water freezes on the objects and forms a whitish layer. Then one no longer speaks of dew, but of frost.

Tropics

It's hot and humid like a greenhouse. Around noon there are heavy downpours, the heat lets the moisture evaporate and hangs heavily in the air. No wonder that the humidity is almost always over 70 percent. In this climate, trees and plants grow up into the sky over several floors. Such tropical rainforests still exist in the Amazon, in the Congo or in Southeast Asia. But the rainforests only make up part of the tropics. There is much more to this climate zone!

The tropics form a wide belt on both sides of the equator and are limited by the tropics to about 23 degrees north and south latitudes. It is typical of the tropics that temperatures fluctuate more in the course of a day than in the course of a year. Because the position of the sun is almost uniformly high all year round, there are no seasons like ours. The length of the days also varies little over the year, at 10 to 13 hours.

There are different vegetation zones in the tropics. The always humid tropics with their daily downpours and a lush tropical rainforest are close to the equator. In the direction of the tropics it is getting drier. In this region between the rainforest and the desert, there are vast grasslands: the savannas. Depending on the amount of rain, trees still thrive or, with increasing drought, only isolated shrubs and thorn bushes. These savannas are particularly widespread in Africa.

A particularly well-known savannah is the Serengeti in Tanzania and Kenya. In addition to the typical savannah plants, impressive animal species live here. Herbivores such as the giraffe, zebra and elephant or the big cats cheetah and lion are at home in the Serengeti.