How does the wind come about?
You must find this question ridiculous – children know that air flows to form wind, and wind is formed because of air flow!
So, why does the air flow? Of course, I'm not referring to you turning on the electric fan, not that you puff up your cheeks and blow a breath, but the large-scale flow of air in nature, do you know the answer?
Today we will talk about the origin of wind, from a scientific point of view to explore the impact of atmospheric pressure, the rotation of the earth and the distribution of heat on the wind. Then you will know that the wind is not as simple as blowing.
Cherry trees swaying in the wind
<h1>atmospheric pressure</h1>
Our Earth's surface is covered with a thick atmosphere, which is mainly composed of nitrogen and oxygen, which account for more than 99% of the atmospheric volume. Gases are matter, they are also massive, and the total mass of the Earth's atmosphere reaches about 5.15×10¹⁸ kg. What we call air pressure is the atmospheric pressure per unit area, that is, the gravity of the air column above it.
You might say: It's simple! Everyone knows that a standard atmospheric pressure is 760 mm Hg high, which is 101325 Pascal, which is equivalent to 10.3 tons of pressure per square meter.
Measurement of atmospheric pressure
Very well, your answer is correct. But this is "a standard atmospheric pressure", which is both a measured value and a prescribed value. That is, when the temperature is 0 °C, at a latitude of 45 degrees sea level, the value on the mercury barometric gauge is 760 mm water mercury (equivalent to 1013.25 hPa), this is a standard atmospheric pressure.
In fact, at sea level at a latitude of 45 degrees, the temperature is not always maintained at 0 °C, and whenever the temperature changes, the air pressure also changes. This is because as the temperature increases, the movement speed of the air molecules will increase, and they will move away from each other, and the air density will decrease; conversely, the decrease in temperature will lead to an increase in the density of the air. While the thickness of the atmosphere in an area is basically unchanged, the increase in density means that there is more air over the area, and its atmospheric pressure naturally increases.
Atmospheric pressure changes with temperature
Due to a series of complex reasons on the earth's surface, such as differences in topography and landforms, changes in insolation intensity, and differences in the amount of water vapor evaporation, the surface air forms air masses of different temperatures. Where these air masses meet, due to different atmospheric pressures, air flows, and air pours from areas of high pressure to areas of low pressure, which is the wind.
The world's major air mass, the color indicates cold and warm
<h1>atmospheric circulation</h1>
Atmospheric circulation is the largest and most extensive wind that covers the Earth. Due to the large area of oceans covered near the Earth's equator, the sunlight here is also the strongest, and the sea water evaporates into the atmosphere in large quantities under the strong sunlight, while the high temperature of the sea surface also heats the air. The air mass above the sea surface is heated to form an updraft and the air pressure is reduced.
The gas rising from the equator does not accumulate, it cools at a high altitude on the one hand to form large-scale precipitation, and at the same time moves in the direction of the polar regions on both sides. By the time this air moves around 30° north and south latitudes, it has become dry and cold, and the dry and cold air sinks as the density increases, forming a subtropical high-pressure zone in this latitude zone. Reaching the ground and flowing again towards the equator, replenishing the rising air in the equatorial region, thus completing a cycle. We call this circulation the Hadley Circulation or Trade Winds Circulation Because it creates a large trade wind belt on both sides of the equator.
Causes of atmospheric circulation formation
As you may have noticed, on both sides of the subtropical high-pressure zone near 30° north-south latitude, the direction of atmospheric circulation is different: north-eastern trade winds blow south of the 30° north latitude line, while north winds blow mainly westerly. Why is this happening?
There are three forces that can control the direction of the wind: changes in the pressure gradient caused by atmospheric pressure differences in the high ground, rotation of the Earth, and friction.
From the analysis of the previous paragraphs, we know that air will flow from the high pressure zone to the low pressure zone, and it should be a straight line. At the same time, because the earth rotates non-stop, the rotation of the earth will exert a second force on the flowing air- the Coriolis force.
Simulation of the Coriolis force on a turntable, where the direction of pencil movement represents the direction of fluid motion
Due to the Coriolis effect, when the wind blows from the subtropical high pressure zone at 30° north latitude to both sides, it moves clockwise, and in the southern hemisphere it is reversed. Similarly, if a tropical cyclone or tornado occurs in the northern hemisphere ocean, it rotates counterclockwise.
Due to the Coriolis effect, the wind direction in the northern hemisphere high pressure zone is clockwise and the low pressure zone is counterclockwise
<h1>Atmospheric turbulence</h1>
Turbulence is an irregular atmospheric motion that often occurs within the boundary layer at the bottom of the atmosphere, inside the clouds of convective clouds, and within the westerly wind rapids in the upper part of the atmospheric troposphere. When there is obvious wind velocity shear in the air layer and the upper air temperature is lower than the convection conditions of the lower layer, random turbulence is prone to occur.
Because the sun's heating of various parts of the ground is uneven, the high and low pressure areas of the atmosphere are often randomly staggered, so the air mass will undergo random turbulence during the movement. Atmospheric turbulence, accompanied by the transfer and exchange of matter, energy, and momentum, moves much faster than laminar flow, so its diffusion and shear stresses are also very large. Coupled with the random occurrence of turbulence, the wind speed is fast and the wind direction is uncertain, and the sudden encounter of wind shear often poses a threat to the flight safety of the aircraft.
A vortex created by scientists to study the phenomenon of turbulence
<h1>Terrain wind</h1>
Normally, air flows from the high pressure zone to the low pressure zone, the wind speed is controlled by the pressure difference (pressure gradient), when the isobaric line distance is reduced, its speed is higher, and the wind's path is natural.
If there is an obstacle in front of it, the wind will not pass through, it will either detour in the shortest path, or forced to climb and oscillate, which is also one of the important causes of turbulence.
Mountains produce vertically oscillating winds
Sea and land winds can be important factors in the prevalence of winds in coastal areas. Since the specific heat of the water is greater than that of land, the sea is warmed more slowly by the sun. The surface temperature of the land rises relatively quickly, and the land heats the air above it by conduction. The rise of warm air causes a pressure gradient of about 2 millibars from the ocean to the land. High-pressure cold air flows inland to low-pressure areas, creating cool breezes near the coast.
At night, the land cools faster than the ocean due to differences in specific calorific values, and temperature changes cause the daytime sea breeze to dissipate. When the temperature on land is lower than the temperature at sea, the atmospheric pressure above the surface of the water will be lower than that of land, and land winds will be generated.
The generation of sea winds and land winds is caused by the difference in local air pressure along the coast, resulting in different wind directions between day and night
Cyclones usually occur in warm seas and occasionally cause tornadoes due to the heat of the land. When the sea surface temperature rises rapidly, the rapidly evaporating water vapor forms a rapidly rising low pressure zone, while the air has a non-zero absolute angular momentum due to the rotation of the Earth. As air flows radially inward, it begins to rotate cyclically (counterclockwise in the northern hemisphere and clockwise in the southern hemisphere) to maintain angular momentum. This results in a cyclone with a destructive force, which usually originates on the surface of the tropical sea, hence the name tropical cyclone. Tropical cyclones are called typhoons in Asia and hurricanes in the Americas.
Cross-sectional structure of a tropical cyclone
<h1>summary</h1>
Wind is generated by the flow of air, and the reason for the flow of air is that the uneven heating of the Earth's atmosphere leads to differences in air pressure at various latitudes and regions.
Atmospheric pressure is not constant, when the air is heated, it will expand and rise, the pressure of the area where the hot air is located decreases, and the adjacent cold air will make up for the space left by the hot air, thus forming a wind.
The largest range of winds on Earth is atmospheric circulation. Due to the Coriolis effect, wind bands in different directions have been created at different latitudes of the Earth.
Atmospheric circulation is not the only factor in the generation of wind on Earth, but is generated in various parts of the earth due to differences in topography and geomorphology, as well as uneven heating in various regions.
Wind is one of the most variable climatic factors on the Earth's surface and an excellent master of shaping topography, and we should study and understand it carefully.
Sculptural works of the wind