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What is Wind?

Wind is the movement of air caused by pressure differences and fluid dynamics in motion. While that may sound scary and technical, it’s not and is something you will absolutely be able to understand. We will also discuss how wind is measured and large-scale wind patterns around the globe.

What Causes Wind to Blow

The wind is air pressure—the weight of the air above us—balancing itself out, always flowing from high to low pressure. High and low pressure drive Earth’s weather, as our atmosphere is always , ebbing and flowing around our complex atmosphere. This movement of gas causes the wind to blow, especially in areas going from high to low pressure. 

It’s probably easiest to explain wind in a video, and also read more in the complete article below …

Air Pressure:

On average, our atmosphere pushes 14.7 lbs per square inch (psi) of your body. This air pressure varies and can be high or low, with the lowest atmospheric pressure on earth being under 13 psi and the highest approaching 16 psi (this is why your body may ache with low pressure; more to come on that in another post!).
 
The more air above us, or the deeper the atmosphere, the more it weighs. High pressure means the atmosphere is taller above you than average. The less air above us, or the more shallow the atmosphere, the less it weighs. Low pressure means the atmosphere is shorter above you than average.
 
High and low pressure drive our weather because they want to be the same height. The high pressure wants to come and the low pressure wants to rise up. Our atmosphere wants to be in equilibrium, which is not attainable. 

Why not? Because we are on a giant rock with molten lava in the middle, spinning through space at 1,000 miles per hour, covered by 70% water, half heated by the sun and the other half cooling toward open space, to name a few factors. But without these pressure differences, we wouldn’t have “weather.”&�Բ�����;

Fluid Dynamics of Wind

So, let’s simplify it a bit. Imagine a tall jar filled to the top with water. This is high pressure. Now, imagine a tall jar next to it half filled with water. This is low pressure. If you could join the bottoms of two jars together with a straw so the liquid could flow between them, what would happen?

The taller water in the jar would flow through the straw to the other jar, and the two would balance out at the same height. The fluid flowing through the straw is wind.

The wind is air pressure balancing itself out, always flowing from high to low pressure. This is why wind has no beginning or end, as our atmosphere is always full of high and low pressure, ebbing and flowing, around our complex atmosphere.

Pressure Gradient Force

If you have ever looked at a topographic map, you will be able to look at a weather pressure map and understand where the wind is blowing and how fast it’s going.

A topographic map shows you lines of constant elevation. Repeated lines eventually form unusual circles and shapes, showing everything from mountain tops to ravines and high ridges to deep valleys. Pressure maps are no different, showing where the atmosphere is high and low.

In topography, the closer the lines of constant elevation are, the steeper the slope.

To understand wind, imagine a ball rolling down the topography map of pressure in the atmosphere. The image below shows a pressure forecast, and the black lines are lines of constant pressure. High pressure is over the Great Lakes like a mountain of air, and low pressure is over the Rockies like a lake in a valley.  If we were to take a ball and set it on top of the high over the Great Lakes, it would gently roll downhill, maybe north, maybe south, but wouldn’t go too quickly.

Once the ball reached the tight pressure lines from North Dakota to Colorado, it would speed up because the slope of the hill (in the atmosphere) was incredibly steep. And the air does the same. The wind speed would increase in this area, going from high to low pressure. The image below shows the forecast wind speeds for the same time, with white showing low winds and pinks/purples showing high wind speeds.

How Wind is Measured

The wind has two components to measure: its speed and direction. The speed of the wind tells us how fast it’s going, and the direction tells us where the wind is coming from. Wind speed is measured by an anemometer, and wind direction by a wind vane.
 
Most anemometers take a number of rotations over time (like rotations per minute) from a propeller or cup and translate that into a wind speed. If the propeller spins slowly, say ten times in 15 seconds, that will be a low wind speed. In high wind speeds, the moving part of an anemometer may spin several revolutions each second. 
 
A wind vane is often in the shape of a flag, an aircraft fuselage with a large tail, or similar to catch the wind and always force the instrument to face the wind. This tells us the direction the wind is coming from and generally is how wind direction is communicated. A northerly wind is from the north heading south, not traveling to the north.
 
There is one type of anemometer with no moving parts, and that is a sonic anemometer. This weather instrument uses sound waves to measure the wind. By chirping a sound and listening for when it is expected to be heard, these incredibly sensitive and accurate anemometers can tell us speed and direction with no moving parts.

Global Wind Patterns:

Because we are on a giant rock with an atmosphere rotating at 1,000 mph, there are some large-scale wind patterns that are prevalent and semi-permanent. For example, wind and weather generally flow west to east across North America between 30-60°N. 

There are exceptions. Sometimes, the flow is more north-south oriented, and sometimes, it even goes “backward” (like our wind forecast example above!).
 
In the southern hemisphere, between 30 and 60°S, wind and weather also flow west to east. At the equator, 0°, weather flows east to west. The trade winds are a transition zone from the equator to 30° N and S. The polar easterlies flow from east to west above 60° N and S.
 
Bands where the winds come together have created areas of permanent low pressure, which causes clouds (another good future topic!) near the equator over what is called the Inter Tropical Convergence Zone (or ITCZ). More semi-permanent bands of low pressure are located near 60° N and S. Its counterpart, a subtropical high, is where winds flow away toward lower pressure, and these two bands are located at 30°N and S.
 
While we may have no control over where the wind blows, hopefully it is now much less of a mystery. Our constantly fluctuating atmosphere gives us pressure differences, and these force the air to try to reach equilibrium. The air always moves from high pressure to low, hoping to balance our atmosphere out in an atmospheric mission-impossible scenario, keeping the air on the go in the form of wind.

Wind may seem like a miracle … but there is a lot of science that goes into it! Do you love the wind?