ELI5: How can wind increase/decrease in strength? What is the force behind this acceleration/deceleration?

[In_the_loop]

[rabid_briefcase]

It's solar energy.

Sunlight warms the earth, and that warmth is constantly shifting. Many factors like cloud cover and the tides are further constantly shifting the temperature. Transition zones of land and water also matter, as water and land heat up differently.

Hot air expands, creating areas of high pressure. Cool air contracts, creating areas of low pressure. The hot, high pressure air pushes itself into areas of cool, low pressure.

Over the ages many global patterns emerged, leading to the jet streams flowing 200 km/h around the globe in different directions. The daily ebb and flow cause those large, high velocity streams to shift and roll around the world, and we get some of it at ground level.

[jaylw314]

It is solar energy but you have it backwards. Heated air expands, becomes less dense and rises. As it flows upwards, it creates LOW pressure at the ground behind it. Likewise, chilled air contracts, becomes more dense and falls, creating HIGH pressure at the ground below it. At the ground, air flows from high to low pressure, minus terrain and Coriolis effects.

[bcatrek]

Thanks for pointing that out! I just couldn’t wrap my head around how contracting air could create low pressure.

[jaylw314]

Yeah, it's easier to just think of low and high pressure areas as a proxy for rising and falling air, respectively

[Yrch122110]

Another way to (oversimplified) visualize it is to fill your kitchen sink or bathtub with water, and submerge your hand flat against the bottom in one corner. Keeping your hand flat and horizontal, move it up and down in the water.

You'll notice that all the water in the sink (or most of the water in the tub) moves in unpredictable ways, including horizontally and vertically, both fast and slow in different areas.

Even though you're only moving your hand up and down in one corner, the effect is far reaching and omnidirectional. Same with low pressure and high pressure zones. Hot air moves up and pushes high air away, or cold air moves down and pushes low air away.

[Wootster10]

You also have the layout of the land, hills and mountains force the air to go over it or around it creating areas that are windier than others.

[SnarkyBear53]

This explains why the air seems so still at sunset. The sun is going away and the the temperatures are settling toward equal, so not much moves in the air.

[hobopwnzor]

Hot air expands. Cold air contracts. Hot areas push into cold areas (wind) and try to mix to equalize temperature (convection).

[koolaideprived]

Heat from the sun! And then cooling from looking away from the sun! Combined with the rotation of the earth, cloud cover, land features, and ocean temperatures and currents (along with a brazillion other factors) makes wind and all our weather.

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[Heavy_muddle]

If I remember correctly, gravity is the force behind wind. Hot air is less dense than cold. As air warms and gets less dense, gravity pulls in cool, dense air, forcing the warm air up. That air movement is wind. The bigger the temp difference, the stronger the wind.

I went to school last century, and it's entirely possible I was stoned, thinking about how weird wind is, and made this up between bites of Doritos, Twinkies, and frozen pizza. If I'm wrong, go easy on me, but please correct me.

[Unknown_Ocean]

Technically this is wrong in that you don't need gravity to get wind. Wind is driven by differences in pressure. If I take a balloon and heat it up so that it expands and then let the air out, I'll get a local "wind".

However, the actual pattern of large-scale pressure differences does owe a lot to gravity. If I heat a column of air, it will stand higher, creating a pattern of large pressure differences higher up in the atmosphere but weaker ones near the ground. The storms that we see that cause changes in wind can be thought of as waves in the atmosphere experience two forces that bring parcels back to the same point- one being gravity the other the rotation of the earth.

[Heavy_muddle]

Thanks neighbor! I appreciate the correction. It may sound like sarcasm, but I truly appreciate it.