Landscapes are shaped by the elements and processes of our earth. What is the role of the wind in all this, and how does wind cause weathering?
Wind weathering is much more than just some dunes in the desert. It can take place far outside deserts, it can cause harm and damage and can even be caused by human activities.
The tiny particles that get picked up by the wind, following the saltation method, affect the natural landscape. Causing interesting rock structures, but also affecting cities.
To understand wind weathering, it’s important to know what wind is and how it can be caused. In essence, the wind is the movement of air in the earth’s atmosphere. Wind originates from differences in pressure around the globe. Temperature differences across the earth cause differences in pressure.
Typically, bands of high-pressure areas are formed near 30°N/S and at the poles, belts of low-pressure form at the equator and around 60°N/S.
These configurations change with the seasons and are influenced by other factors. Air moves indirectly from areas with high pressure to areas with lower atmospheric pressure.
The ideal model of the earth’s pressure belts is just a broad representation. In reality, these belts of low-pressure are built up of smaller pressure cores. These cores, also called low-pressure zones or areas, can also divert from their normal location.
Especially around the mid-latitudes, there are big differences between the pressure in winter and summer. The zones move mostly from west to east, and the global situation is evolving every day. That’s why wind speeds and directions can change quickly.
Note: Individual pressure cores decide the wind strength and direction.
How Does The Wind Move?
As said before, wind moves from areas with high pressure to areas with lower pressure. But this isn’t the whole picture. The earth is turning around her own axis, 360° every day. This causes the Coriolis effect, which states that winds bow when moving from one to another place.
The basic rule to keep in mind is that the wind moves clockwise around high-pressure zones in the northern hemisphere. For low-pressure zones it’s the other way round: anti-clockwise. For the southern hemisphere, it’s the opposite.
Fact: Wind around high-pressure zones in the northern hemisphere goes clockwise.
Wind can also be formed on a more local scale, without the influence of major pressure zones.
In deserts, the temperature differences between night and day cause a wind increase during the day. This is because the differences in temperature cause small and local pressure differences.
In mountainous areas, the wind slides down the mountains into the valleys at night and rises by the mountainsides during the day, because the mountain sides facing south warm faster than the valleys.
On warm summer days at the coast, there is a temperature difference between the relatively cold sea waters and the warmer land. The resulting pressure differences cause a colder breeze from over the sea in the afternoon.
Now that we understand where and how wind can be formed, we can take a look at the influence of wind on erosion. Wind weathering or wind erosion is a natural process that moves soil substances from one location to another.
When wind moves over loose soil, individual particles can be carried with the wind over a certain distance, before settling to the ground again.
This distance can differ a lot, some particles are moved for only a couple of feet, while others can travel hundreds of miles and get into the broader weather systems. There are several factors at play, like particle weight, wind speeds and soil moisture.
There are three main movement types of soil particles based on the weight of the particle:
The heavier particles will move based on the creep mechanism. When a particle creeps, it rolls as if it were over the surface in the direction of the wind. It doesn’t take flight, but it is getting moved.
The second form of movement is saltation. This is mostly the case with medium-weight particles. The particle briefly takes flight, before landing on the ground again. The distance that the particle travels depends on the wind speed and particle weight.
The difference between this and the last form: suspension, is that with suspension the particle gets suspended into the atmosphere and gets carried by the upper wind and weather systems. This typically takes place for only the lightest dust particles.
When a particle that experienced saltation lands again, it can initiate other soil and dust particles. It’s like a domino effect. Thousand if not millions of particles can be in one of these states at the same time in a small area.
Fact: Wind erosion is the combination of saltation, suspension and creep.
Wet soil won’t take flight that easily. Heavier particles will travel less far and need stronger winds to get picked up. The lightest dust and soil particles can suspend into the atmosphere and get moved by the global weather systems.
Sand from the Sahara desert in Africa can often (if the conditions are correct) make it all the way up to Europe. The skies in Europe can get hazy, and when it rains, the sand precipitates out. It’s not abnormal that Europeans find a thin layer of sand on their cars after one of these events.
When the wind blows through a landscape, the landscape gets affected. This is most notable in barren landscapes with no vegetation like deserts, mesas and coasts. But you might ask yourself, how does wind cause weathering and erosion? Sand or other tiny dust particles get picked up by the wind.
When these saltation particles hit soft rocks, like sandstone with relatively high velocities, tiny pieces of the rock break off. This can create beautiful structures in the sandstone like Tassili n’ Ajjer in Algeria or Arches National Park in the USA.
Sometimes a harder type of rock, that is less susceptible to erosion lays on top of softer rock. In this case, the softer rock gets weathered resulting in rocks balancing on a small sandstone pillar.
These processes can take up to hundreds or thousands of years to form. And eventually, these magnificent natural structures will collapse, because the weathering became too much for the structure to hold itself.
Note: Wind weathering is the cause for interesting rock formations that take thousands of years to form.
In deserts and other sandy areas, sand particles are picked up by the wind and transported. At the bottom of the dune or even in front of the dune, sand particles get picked up, what we call saltation.
They fly through the air, some particles are deposited on the windward side of the dune. Whilst other particles from the windward side are picked up by the wind because other saltation particles hit the ground like illustrated in a previous section. Behind the dune, the wind slows down, so the heavier particles fall back to the ground. The particles in saltation get deposited here. Dunes thus grow on the back (or leeward) side. Over the years, the dune will move slowly inland, because sand particles get transported from the windward to the leeward side. This slow process is called dune regression.
Dust storms are most commonly occurring in deserts. They can range from pretty harmless to storms resulting in damage and death, based on the location and intensity.
When the wind over a big arid area starts to pick up, the saltation process sends smaller particles in suspension. These particles will fly through the air in masses, creating typical dust clouds. When cities get covered by these storms, visibility and air quality are abruptly reduced.
These storms originate in the desert but can move over great distances. It can result in major economic impacts and health problems if the dust is inhaled. For example, in the 2009 East-Australian dust storm, due to the low visibility, flights were canceled in the region.
Note: Dust storms originate in deserts and can travel long distances.
Although wind weathering is a totally natural phenomenon, human influence can worsen the phenomenon. When humans clear forests and other vegetation, the earth is more susceptible to erosion and weathering from the wind.
So even far outside deserts in drier periods, the wind can pick up large amounts of dust from agricultural fields. This is a direct result of human activities, but climate change will also contribute to more wind erosion.
Important: Destroying forests does not only destroy habitats but also increases wind erosion.
With the warming climate, precipitation patterns change all over the world. Some parts get dryer, other parts get wetter. But the chance of prolonged dry periods is growing all over the world.
Mainly this will contribute to an increase in wind weathering because dryer soil gets picked up easier by the wind. Higher temperatures will also let the soil dry faster.
The saltation and resulting suspension processes cause wind weathering. Affecting not only landscapes but also human settlements. Humanity is also playing a part in making the problem worse.
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How does wind cause weathering? The combination of wind that can originate from weather systems or local differences in pressure cause particles to be picked up by saltation or suspension.
These particles can affect natural landscapes, resulting in majestic structures and dunes. Dust storms affect human establishments, and human influence is only making wind erosion and weathering more common.