The wind carries the husk particles away. The separated grains accumulate in a mound near the platform for winnowing. Sieving permits tiny wheat particles to flow through the sieve's perforations while large contaminants stay on the sieve. These include rocks, weeds, and other grain species.
Husk particles are the covering of the seed (caryopsis). It protects the embryo within while it is growing in the soil until it reaches maturity. The hull or coat must be removed to reveal the white interior of the kernel (endosperm). Some types of cereal grains have two outer layers called aleurone cells that provide nutrients to the developing embryo when they rupture under pressure from the weight of the grain. Other types of cereal have one layer of skin cells called phytic acid which prevents metals such as zinc and iron from being absorbed by the plant.
When plants grow in fertile soil with plenty of water, the seeds develop into mature plants with all of their parts. However, most plants do not live in this way but instead die after producing more offspring like themselves. This is called reproduction. Different methods are used by different organisms including flowering plants (angiosperms) and ferns among others. While some organisms produce eggs and sperm that combine to form new individuals like bees and crocodiles, others use spores or pollen that contain the full genetic information for making babies.
Abrasion is done by wind-moved particles. When one grain collides with another grain or surface, it erodes that surface. Very small particles are carried great distances by air currents created by winds. As they travel through the atmosphere, these particles interact with light waves that reflect back and forth between the ground and clouds, causing clouds to form or dissipate as needed. These cloud-forming particles then become part of the water vapor in the air when they fall back to Earth. The particles are small enough to penetrate deep into porous rock surfaces, such as those found in mountains, where they are eventually removed by other processes. For example, rain washes sand from the mountain sides down to lower levels, where it is drained by streams.
Wind can also move soil. In areas where there is shallow soil, wind can blow it away completely, leaving nothing but rock exposed. This happens when wind moves large quantities of dry soil. The wind blows the soil away from its parent slope and into the valley. Over time, all that remains of the parent slope is a low cliff in the middle of the valley. Where there is deep soil, wind can also cut channels into the earth. These wind-eroded features can be very deep, with walls over 100 feet high. However, most wind erosion occurs on soils that are relatively thin.
Loess deposits arise when tiny particles of silt and clay are blown by the wind (Figure below). Vertical cliffs are formed by loess deposits. Loess has the potential to create a thick, rich soil. As a result, loess deposits are employed for agriculture in many places of the world.
Wind Transport of Sediment Clay and silt, for example, move through suspension. They can linger in the air for days. They can travel long distances and ascend far above the earth. Larger particles, such as sand, migrate as a result of saltation. This means they jump from foot to foot while traveling downwind.
Saltation is the vertical jumping or skipping of small particles, such as sand, across smooth surfaces, such as roads or beaches. The force of landing propels them again in the direction they were moving before. Sand, for example, may be transported by wind over large distances until it comes into contact with something solid, such as a cliff or ocean shoreline. At this point, the energy of the wind is converted into kinetic energy which causes the particle to jump up and away from the surface it was moving across.
Kinetic energy is the energy of motion. It can be thought of as the mass times velocity squared. For example, if you throw a ball hard enough, it will keep on going even after it hits the ground. This is because the mass of the ball is small compared to that of a vehicle and the speed it reaches when it hits the ground is very high. The same thing happens to particles transported by wind. Once they leave the surface they are being carried across, they become airborne again and continue along their path.
Wind transports particles of various sizes in the same manner as water does (Figure below). Clay and silt, for example, move through suspension. Creep is the movement of particles bigger than sand. It depends on the rate at which a particle is moving when it hits something solid (such as a tree trunk or another rock). The faster the speed, the more damage will be done to the object. Creepers are less visible because they don't fly far nor do they leave a track behind them.
The type of soil affects how much damage wind can do. Loose soils such as sand accumulate more rapidly than clay soils. This is because the wind blows right through them. Clays and other loose materials tend to stick together after they have been stirred up by traffic or water flow. That's why areas near roads or rivers usually have more severe erosion than others nearby.
Larger rocks can also protect smaller ones from being blown away. This is because rocks break up gusts of wind into small fragments that cannot reach far. Trees also play an important role in wind protection. Their wide branches create a roof that prevents wind-borne debris from reaching the ground. Also, their thick trunks act as natural bumpers that prevent heavy objects from being carried great distances.
Finally, wind can be used to destroy things too.
Finer particles can travel hundreds of miles before settling during a pause in the wind, and tinier specks can remain suspended in the air permanently. The finest particles are jostled this way and that by moving air molecules, and they float with the smallest currents. They can only be washed away from the atmosphere by rain and snow. Larger particles reflect light and heat from the sun back into space, keeping our planet cool. Smaller particles are more reflective than their size would suggest and they have an impact on how much sunlight reaches the earth's surface.
Particles from natural sources such as volcanoes, wildfires, and even oceans waves wash into the atmosphere but most come from man-made sources such as vehicles, factories, and fires. In fact, about 90% of the world's sulfur dioxide, 75% of its nitrogen oxides, and 50% of its dust comes from human activities. Climate change is expected to increase these emissions further. Particles from fossil fuel combustion reach the atmosphere through both direct emission and indirect processes such as ozone formation from chlorinated hydrocarbons emitted when fuels are burned.
Airports are one of the largest sources of atmospheric particulates. The engines of airplanes emit particles when they run on jet fuel or kerosene. The smaller the particle, the faster it will travel through the air. Some of these particles may be large enough to stay in the atmosphere for several days or weeks.