Wind is the movement of air.
For us, wind could be:
- my cap's flown off;
- nice breeze;
- the door slams;
- the one which threw over my bicycle;
- blow my umbrella away;
- knocks over a tree.
Wind also does more than affecting us. Wind moves clouds
from the place where they originate to the place where they drop precipitation
, transports pollen from plants
over many kilometers or dust from deserts
into neighbouring regions.
Wind also brings fresh air into polluted cities, carries dust and pollution over thousands of kilometers, damages houses and roads sometimes and influences air traffic every day.
Wind is the basis for several human activities and sports: it moves sailing ships, balloons and gliders, is used by wind- and kitesurfers and paragliders, and model plane enthusiasts. Surfers also depend on the wind: the waves which they ride are caused by winds.
Without wind, no weather would exist and our earth would look different.
Where does the wind come from?
What now is the wind? Wind is - simply said - the movement of air in troposphere, the lowest layer of the terrestrial atmosphere
. The wind in this " Weather layer" develops due to the following characteristics of air:
- Air consists of a gas mixture (nitrogen, oxygen and others).
- Air transmits heat badly, and thus releases it only slowly.
- Air is hardly warmed up by the sun rays.
- Air becomes thinner with altitude.
- Air can take up water vapour in larger quantities (on the average about 1%).
The emergence of wind - i.e. air movement - occurs in the following (simplified) way:
- The earth's surface is warmed up by sun radiation - often by 10°C and more on a sunny day.
- The lower air layers are warmed up by the contact with the ground and thereby expand .
- Warmer, thinner air now rises upward, by displacing cooler, heavier air above it.
- The " bubble" left by ascending air is filled by cooler, heavier air (which flows usually from sideways or from surrounding hills).
This process leads to the movement of larger air masses - and to wind.
Abbildung 1. Kaltluftstrom im Tal unter einer Brücke (Frankreich). Im Talkessel (rechts) löst die Sonneneinstrahlung den Nebel auf und unterbricht den Kaltluftstrom.
Significance of wind
The same phenomenon you can see, when you light a candle. The air around the candle gets hotter (extremely) and flows up. If you put a little windmill over it (or some thin wires), their movement shows the upflowing air.
In the atmosphere these processes are spacious and much more gigantic. The moving power station, the sun, is larger then the candle: the solar energy from the sun, arriving on the earth, is five thousand times more, than the energy, which is used by all human being.
Wind arises in little scaled areas and also in greater dimensions. Four types of wind will be introduced.
1. Local Winds:
The radiation of the sun already warms landscape in little scales . Let’s study a building: The wall at sunny side gets wamer than the one of the shadow side. If we open the windows of both sides, we shortly get wind. Draft develops, because the colder (heavyer) air flows from the shadow wall to the warmer (lighter)
air of the sunny wall. When the sun keeps shining, this effect keeps running, because the streaming cold air gets warm really fast and so more cold air follows.
More examples for local wind systems can be found easily: In each area, where you can find difference of temperature of more than 2°C, a flow of air can be mesaured. For this the top of a table or street,is enaugh, if it’s warmed by the sun.
Stronger local air movement procreate them selfes to „updrafts“ in upper levels of air. When you watch birds on a sunny day, like buzzards or hawks, you see them circiut for getting more hight: They circuit around the
Local wind systems rarely get faster than 30-40km/h. But when some local winds bunch theirselves, like near thunderstorms, regional wind systems develop. Those are getting stronger and stringer sometimes and under spezial conditions they will become storms.
2. Regional Winds:
If differences of temperature occure permanently, regional wind systems emerge, which flow over serveral kilometers and continue over days. Temporarily they reach the highest wind speeds ever measured on the bottom Those aren’t durable and because of this they are also surprising sometimes. Thats why they are the most dangerous systems.
Clouds of thunderstroms develop because of the uplift of warm and wet air. Required are (1) a wet air layer with a great enlargement near the bottom; (2) a distinct vertical fall of temperature and (4) a trigger fort the upflow of wet air (lifting), so that a upflow tube develops.
Because of the great difference of temperature the wet and warm air flows up really fast and cools down. Till a certain hight (cloud ceiling) where little drops arise. The uplift ist getting stronger by great vertical differences of temperature and wetness. This winds get up to 100km/h fast, when they are near thunderstorms. In thunderstorms the winds reach speeds which are beyond 100km/h fast. When these differences are especially great tornados arise ,which wind whirls reach speeds of over 500km/h. These storms are able to destroy buildings.
Abbildung 2. Schematische Darstellung einer Gewitterentstehung.
Types of Wind
Through insolation the land heats up during the day stronger than the ocean so the air flows from ocean to land. (see fig. 3). At night this progress switches because the land cools down while the oceans surface keeps its temperature. These kind of winds are well-known at every coast.
Abbildung 4. Schematische Darstellung eines lokalen Fallwindes im Osten vom Genfer See (Yvorne): Am Samstag 03:00 bläst der Wind von Südost aus dem Wallis Richtung Genfer See. In 3 Kilometer Höhe (ab 750 hPa Druck) herrscht gegenläufiger Westwind. Tagsüber (12:00) dreht sich der Wind im Tal auf West, da der Kaltluftabfluß im Wallis bei Sonneneinstrahlung abreißt.
2.4 foehn wind
Foehn is a special form of downslope wind, which developes by the inflow of wet air on to the side of a mountain. The air is raised up by the mountain while cooling down and loosing a great part of its humidity. What results is a heavy, "monsoon-like" and torrential rain at the humid side of the mountain. As soon as the air has passed the ridge of the mountain it flows downwards on the other side and is heating faster than the cooling on the humid side, because the air has lost its humidity by raising up. The resulting downslope is more arid, stronger and warmer than its surroundings may believe.
Abbildung 5. Wolken- und Niederschlagskarte einer Föhnsituation (Schweiz, 27.04.2010, 02:00): heftige Niederschlägen ergießen sich an den Südalpen. Der Fallwind (Föhn) "bläst" vom Alpenkamm bis zum Schwarzwald und Bodensee die Wolkendecke beiseite.
Hurricanes usually develope in tropical areas when a high insolation creats a large difference between temperature and pressure. Espescially above warm ocean surfaces big masses of humid air raise by strong insolation, they can develope to great thunderstorms and consolidate to big cells. In the mid latitudes (arround the tropic) such cells get in rotation arround a centre of low pressure (the "eye" of the storm): Under certain conditions the twist gets so strong that there can be over 150 Km/h of wind velocity at the surrounding of the cells. The centre moves simultaneously and usually to the west and and offwards from the equator. In this way, the hurricanes can expand over one hundred kilometres, they can exist for weeks and destroy areas which are bigger than one thousand of square kilometres. Over the land they quickly lose their power so the destruction concerns almost the coasts.
Global wind systems
Because of the great temperature difference between the poles and the equator there is a developement of spacious wind systems which exist over longer times and above whole continents and which have a determining influences on the climate of the earth.
3.1. Trade winds
At the equator the insolationon the earth is much stronger and so the air above the ground is warming more than in the direction of the poles. In the area of the whole equator, excessive warm air raises up to over 10 kilometres, flows in higher regions to the Tropic of Capricorn or the Tropic of Cancer while dehydrating and cooling down. In the area of the tropic the air descends and flows as a near-ground trade wind again towards the equator (see fig. 6). Trade winds are arid, because the air loses its humidity while cooling down and descending and because in the areas of the tropic there isn`t much humidity. Because of this most of the great deserts are located in these climes.
Because of these temperature stratifications of the very warm, from the equator approaching air above the comparatively less warm air of the higher latitudes, the steady "trade inversion" is formed. This trade inversion prevents the vertical air change between the high streaming towards the polar and the near-ground streaming towards the equator. Therefore this circulation is very stable.
Above the continents (Africa, Southamerica) there is nearly no trade circulation because it is interrupted or redirected through the local convections ( the airdraft) over the landmasses.
Abbildung 6. Schema der Passatwinde auf einer Weltwindkarte.
3.2 The Jet Stream
The "Jet Stream" is an one hundred kilometre wide belt of strong wind at the upper boundary of the troposphere which is formed as a conseqence of a global compensation movement between different temperature- resp. pressure regions. This belt is, like the trade winds, in its appearance very solid and stable. Within the Jet Stream there are the strongest natural winds, with wind velocities over 600 km/h. The Jet Stream is hard to quantify and it can just be illustrated and analyzed by high weather cards. On cards with high winds of 10km there you can see the Jet Stream for example. (see fig. 7)
Abbildung 7. Weltkarte der Winde in 10 Kilometer Höhe. Jet Stream-Bänder über dem Pazifik und dem Indischen Ozean.
Wind originates from pressure
In the following animation the world map is regarded from rising air layers. How does the wind force in the height change?