What causes wind

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Wind is the movement of air.
For us wind means:

  • Fresh breeze
  • my hat flew away
  • a door slam
  • the one who knocked my bike over
  • it blew my umbrella away
  • a tree falls over

Wind also does more than what we immediately notice and impress. Wind transports clouds from the place of their formation to where they deposit precipitation, transports pollen from plants over many kilometers, or dust from deserts to neighboring areas.
Wind also brings fresh air into polluted cities, carries dust and pollution thousands of kilometers, can damage homes and streets, and affects daily air traffic.
Wind is the basis for some human activities and sports: it drives sailing ships, balloons and gliders, is used by windsurfers, kite surfers, paragliders and model pilots. Surfers also depend on the wind: the waves they ride are caused by winds.

Without wind there would be no weather and our earth would look different.

Where does the wind come from?

Cold air flow in the valley under a bridge (France). In the valley basin (right), the sun's rays dissolve the fog and interrupt the flow of cold air.

What exactly is the wind now? To put it simply, wind is the movement of air in the approximately 8 or 20 kilometers high troposphere, the lowest layer of the earth's atmosphere. The wind in this "weather layer" arises due to the following properties of the air:

  • Air consists of a mixture of gases (nitrogen, oxygen and a few others).
  • Air is a poor conductor of heat and therefore only gives it off slowly.
  • Air is hardly heated by direct sunlight.
  • Air becomes thinner towards the top.
  • Air can absorb water vapor in large quantities (on average around 1%).

The creation of wind - i.e. air movement - works like this in simplified terms:

  • The earth's surface warms up significantly due to solar radiation - often by 10 ° C and more per day.
  • The lower layers of air heat up significantly when they come into contact with the ground and expand in the process.
  • The warmed, thinner air now rises, displacing cooler, heavier air.
  • Cooler, heavier air flows into the "bubble" created by the rising air (mostly from the side or from surrounding hills, which are cooler).

This leads to the movement of larger air masses, and thus - just - to wind.

Importance of the wind

We observe the same phenomenon when we light a candle. The air around the candle is (strongly) heated and rises to the top. If you hang a small wind turbine (or a few thin threads) over the candle, the movement of the wind shows the rising air.
In the atmosphere these processes are more extensive and much more powerful. The driving power plant, the sun, is much larger than our candle: more than five thousand times more solar energy than the energy needs of all of humanity always hits the earth.
Wind can now arise very small-scale or in large dimensions. Four types of wind are presented below as examples.

1. Local winds:

The rays of the sun warm the landscape on a very small scale. Let us observe a building: the wall of the house on the sunny side warms up more than that on the shadow side. If we now open the windows on both sides, we will soon have "wind". There is a draft because the cooler (heavier) air flows from the shady side to the warmer (lighter) air on the sunny side and displaces it. This process continues when the sun is shining, as the cooler air that flows in warms up relatively quickly and is in turn displaced by more cool air.
Hundreds of other examples of local wind systems can be found: any environment in which small-scale temperature differences of more than 2 ° C arise thereby triggers a measurable flow of air. A table surface or a street are sufficient if they are heated by the sun.
Stronger local air movements propagate as "climbing winds" into the higher air layers. If one observes birds, such as buzzards or hawks, on a sunny day, one sees these birds circling, whereby they usually gain height quickly: They circling around the center of a "updraft tube" that carries them upwards.
Local wind systems rarely get faster than 30-40 km / h. However, if local winds bundle, e.g. in the vicinity of thunderstorms, they form regional systems whose speeds increase significantly and, under certain conditions, can reach storm strength.

2. Regional winds

If temperature differences regularly occur over a large area, regional wind systems arise that extend over many kilometers and last for days. At times they reach the highest wind speeds that can be measured on the ground, but are not permanent and therefore often occur surprisingly. It is from these systems that the greatest dangers emanate.

Types of wind - examples

Foehn wind

Cloud and precipitation map of a foehn situation (Switzerland, April 27, 2010, 2:00 am): heavy precipitation pouring down on the southern Alps. The fall wind (foehn) "blows" the cloud cover aside from the Alpine ridge to the Black Forest and Lake Constance.

Schematic representation of a local downwind in the east from Lake Geneva (Yvorne): On Saturday 03:00 the wind blows from the south-east from the Valais towards Lake Geneva. At a height of 3 kilometers (from 750 hPa pressure) there is a counter-westerly wind. During the day (12:00) the wind in the valley turns to the west, as the cold air flow in Valais stops when the sun shines.

The foehn is a special form of downwind, which is created by the influx of very humid air on the flank of a mountain range. The air is lifted by the mountains, cools down in the process and loses a large proportion of its moisture. There are heavy, "monsoon" or "torrential" rains on the humid flanks of the mountains. As soon as the air has overcome the crest of the mountains, it flows down on the other side and warms up much faster than the cooling on the humid side, since it has lost the moisture it contains on the ascent. The resulting fall wind is drier, stronger and much warmer than would be expected for the surrounding area.

thunderstorm

Schematic representation of a thunderstorm formation
Source: Wikimedia Commons

Thunderclouds are caused by the lift of warm and humid air. This requires (1) a moist layer of air with a greater extent near the ground; (2) a clear vertical drop in temperature and (3) a trigger for the rise in humid air (elevation), so that, for example, a tube of updraft is formed. The warm, humid air rises quickly because of the greater temperature difference and cools down in the process. From a certain height (condensation level = cloud base), water droplets are created (see Figure 2). The lift increases so much with large vertical temperature and humidity differences that winds close to thunderstorms reach speeds of up to 100 km / h and the updrafts in the thundercloud reach speeds of well over 100 km / h. If the differences are particularly high, the rare, notorious tornadoes occur, whose small-scale wind eddies can reach speeds of over 500 km / h and destroy buildings.

Cyclones

Cyclones usually form in tropical areas when high solar radiation causes major temperature and pressure differences. Particularly above warm sea surfaces, when the sun is shining strongly, large amounts of humid air rise up, which can develop into large thunderstorms (see 2.1.) And combine to form larger cells. In middle latitudes (around the tropics) such cells start rotating around a center of lower pressure (the "eye" of the storm): Under certain conditions this rotation increases so much that wind speeds of over 150 km / h can occur at the edge of the cells . The center moves mostly to the west and away from the equator at the same time. Cyclones can stretch hundreds of kilometers in diameter, last for weeks and devastate areas of thousands of square kilometers. Overland, they usually lose their strength quickly, so that coastal areas are predominantly affected.

The jet stream

The large-scale temperature differences between the poles and the equator lead to the development of large-scale wind systems that occur over a long period of time and across entire continents and help determine the earth's climate.

The "jet stream" or "jet stream" is a hundreds of kilometers wide band of strong winds at the upper limit of the troposphere, which forms as a result of global compensatory movements between different temperature and pressure areas and, like the trade winds, is very reliable and stable in its occurrence. The strongest natural winds occur in the jet stream at speeds of over 600 km / h. The jet stream is very difficult to measure and can only be displayed or evaluated in altitude weather maps. For example, the bands of the jet stream can be seen on a map of the high-altitude winds at a height of 10 km (see figure below).

World map of the winds at a height of 10 kilometers. Jet stream ribbons over the Pacific and Indian Oceans

In the animation below, the world map is viewed from ever higher layers of air. How does the wind speed change in height?

The animation starts in the lower layers of air and then moves upwards. We can observe that the speed increases with altitude, especially in the area of ​​the jet stream.