China shoots off rain clouds

It should be a perfect spectacle: The Summer Olympics in Beijing. The construction of the stadium alone, which is also called the “Bird’s Nest” because of its shape, took five years and cost the equivalent of 315 million euros. Of course, the perfect Olympics also require the weather to play along: No rain shower or even downpour must be allowed to spoil the big event.

The concerns of the Chinese rulers are justified, at least in terms of the weather: According to weather experts, it rains on average every three days in Beijing in August. In order to ensure that the Olympic Games shine in permanent sunshine, China is now even resorting to weaponry: 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 aircraft or from the ground. There, silver iodide causes the water in the clouds to gather around the fine silver iodide droplets. This creates larger droplets: the cloud rains down. The rain clouds are to be intercepted and “rendered harmless” in this way even before Beijing. To ensure that the Games take place under blue skies, there are 26 stations around Beijing and a whole army of farmers who fight against approaching rain clouds with silver iodide.

Yet everything could be much simpler: a simple sliding roof over the “bird’s nest” was originally supposed to keep the spectators in the Olympic stadium dry. But this solution was allegedly too expensive for the Chinese. And probably not spectacular enough.

Record showers in India

Nowhere on earth is it wetter than in Mawsynram. The village in India’s Khasi Mountains has the most rainfall in the world. 11,872 litres of rain fall here on average per year. In Germany, by contrast, even in the mountains, the annual average is no more than 1,500 litres. Mawsynram achieves the rainfall record mainly because of its extremely humid monsoon winds in summer. These are responsible for these record-breaking rainfalls. And it is not for nothing that the Indian state in which the village of Mawsynram is located is called “Meghalaya” – the abode of the clouds.

How do clouds form?

How clouds are formed can be observed particularly well on cold winter days: When you exhale, steam comes out of your mouth – a whitish haze hangs in the air. It forms when the moist, warm air we breathe meets colder air. This is because warm air can store a lot of moisture – significantly more than cold air. When the warm air cools down, it can no longer absorb as much water. Then the excess water collects to form small water droplets that float in the air and become visible as a white veil. It is quite similar with the “real” clouds.

The power of the sun heats the land and the water on the surface. The heat turns some of the liquid water into gaseous water: it evaporates. Because warm air is lighter than cold air, it rises. If the moist, warm air cools down more and more as it rises, the excess water collects as droplets around tiny particles of dust or soot. This is also called water condensation. The droplets are still so small and light that they float in the air. A cloud has formed.

Clouds therefore always form when warm air cools down. This can happen when the ground and the air above it warms up and rises. Also, when the wind drives the air up a mountain range, warmer air is forced upwards. At altitude, it cools down and clouds form. The same happens when a zone of warm air meets a zone of cold air. The cold air causes the lighter warm air to rise and clouds form again!

But it does not rain immediately from every cloud. Only when the water droplets combine into larger drops due to air movement and are heavy enough, do they fall back to earth as rain. If the temperature is below 0° Celsius, the drops freeze into ice crystals. The precipitation then falls as snow, or in the case of thunderclouds as small graupel or large hailstones.

There are also clouds that form directly above the earth’s surface. This often happens in autumn when the air cools down more and more. The whole landscape then appears blurred white. If you can see less than a kilometre through this white haze, it is called fog.

What clouds reveal about the weather

White clouds float in the blue sky like thick cotton wool. Others, on the other hand, tower darkly and fearsomely into the air. Clouds can look completely different and change constantly. Depending on how and where they appear, they herald different weather. If you know a bit about clouds, you can tell from their shape whether it will soon rain or snow. The height of the clouds also tells you something about the upcoming weather.

Way up high, more than six kilometres above the earth’s surface, are the high clouds. They include the delicate feather clouds, which contain many ice crystals. If many of them are visible, they herald bad weather. Small fleecy clouds and veil clouds, which also consist of ice crystals, float just as high in the sky.

The medium-high clouds, for example the coarse fleecy clouds and stratus clouds, are found between two and six kilometres above sea level. If coarse fleecy clouds extend over wide areas, the weather will be bad. Grey stratus clouds also indicate that it will soon rain or snow.

In the lowest “cloud storey”, below two kilometres altitude, the low clouds are moving. These include the bright cumulus clouds, which are formed by water droplets. This type of cloud is the most common in the world. Because they bring good weather, especially in summer, they are often called “fair-weather clouds”. Grey, low stratus clouds, on the other hand, can also bring rain or snow. And the darker the cloud looks, the more rain or snow it carries.

Clouds that swell several kilometres high over all three “storeys” can carry all kinds of precipitation with them: Far below, the water is not yet frozen, and rain is formed there. But if the drops are whirled up into high and colder cloud layers, ice crystals form. Rain, snow or even hail therefore fall from the towering thunderclouds.

The cycle of water

The water on earth is always on the move. Huge amounts of it are constantly moving – between sea, air and land – in an eternal cycle in which not a drop is lost.

The motor of the water cycle is the sun: it heats the water of the oceans, lakes and rivers so much that it evaporates. Plants also release water vapour into the atmosphere through tiny openings. The moist air rises, tiny water droplets gather at high altitudes and form clouds. The water falls back into the sea or onto the earth as rain, hail or snow. If it falls to earth, it seeps into the ground, feeds plants or flows through the ground, via streams and rivers back into the sea. The eternal cycle of evaporation, precipitation and runoff starts all over again.

The water cycle has existed almost as long as the earth has. It ensures that living beings on our planet are supplied with fresh water. And not only that: without the water cycle, the weather as we know it would not even exist.

Precipitation

No matter whether it rains, hails or snows – clouds are “to blame”. Because without clouds there would be no precipitation. However, it depends mainly on the temperature whether there is a downpour or wild snow.

Most precipitation on earth falls as rain. When small water droplets collide in a cloud, they join together to form larger and heavier drops. If they are too heavy to continue floating; if the temperature is above 0° Celsius, they fall to earth as rain.

At very low air temperatures, precipitation no longer falls as rain but as snow. The snowflakes grow from hexagonal ice crystals that stick together in very cold clouds due to water droplets. If the ice formations are large and heavy enough, they dance down from the sky as snowflakes.

If, on the other hand, strong updrafts move through a cloud that is piled high, hail can occur. Small drops from the lower part of the cloud are whirled upwards, where it is colder than below. There they freeze into small ice balls, about the size of pinheads. These ice pellets are called sleet. In a very high thundercloud, when the wind is strong and the pellets are tossed up and down in the cloud several times, more and more raindrops freeze to the pellets. The more the ice beads float around in the cloud, the larger and harder they become. From half a centimetre in diameter, these ice balls are called hail. Hailstones can become larger than tennis balls and have often caused a lot of damage.

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

Monsoon and monsoon rains

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

Responsible for these heavy rains is a wind that changes direction twice a year. In summer, the Indian mainland heats up very strongly. Air rises and sucks in the moist sea air of the Indian Ocean. The wind then blows from the sea to the land, bringing clouds and the rains that farmers long for. Sometimes, however, the rains are so heavy that they flood large areas.

In winter, the wind shifts: because the Indian Ocean is now warmer than the mainland, it blows from the land to the 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 its own name. It is called the monsoon, and the precipitation it brings is called monsoon rain.

The monsoon brings India the heaviest rainfall in the world. However, monsoon and monsoon rains do not only occur in India and Southeast Asia. It occurs in many tropical coastal countries, for example also in northern Australia or in East Africa.