Recent comments in /f/askscience

blind_ninja_guy t1_j675xlf wrote

Reply to comment by imbluedabedeedabedaa in If I had two cups of water, one normal size and one as big as a swimming pool and stirred them both with proportionally sized spoons, would the larger pool of water keep spiraling longer than the smaller? by r3volc

Good to know that the dog sized elephants that you could hold in whatever that moovie was where they go find atlantus before it again falls into the see is not realistic.

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_Greetings_Friends_ t1_j66t529 wrote

Reply to comment by Krispy_Kolonel in Why does road salt accelerate corrosion in a vehicle's underbody? by nebula828

When I was a kid there was a busted down VW bus that was just kind of left on the beach as debris but it was atleast fairly unworn, I went there a couple years back and the bus was still there but theres now a gaping hole in the top and every last inch of it is covered in rust or somekind of plantlife, the saying "It was a shell of its former self" is so real, it had only been about 10 years but if left in my yard 10 years it would be nearly identical

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agate_ t1_j66q62p wrote

Reply to Why is wind energy generation greater during the night? by ZeroTheHero524

This engineering question needs a meteorologist! The answer lies in the turbulent mixing of the lower atmosphere.

The planetary boundary layer is the turbulent layer of air near the ground. Turbulence in this layer mixes air near the ground with air higher up. This mixes all sorts of air properties from top to bottom of the layer: for example, humid air near the ground is mixed with drier air higher up, making the ground-level air less humid and the upper air more humid.

The layer also mixes momentum, or air speed. The air at ground level -- down among the grass blades -- isn't moving at all, but it's moving very fast several kilometers up. Turbulent mixing transfers momentum across the planetary boundary layer just like humidity, making the ground-level air go faster and slowing down the air higher up.

And now for the kicker: the amount of turbulence in the layer depends on solar heating. When the ground is heated by the sun, hot air rises. The rising plumes of air increase the turbulent mixing in the planetary boundary layer and cause it to extend higher up.

So, during the day, mixing in the boundary layer is more intense, so more slow-moving air at ground level is stirred up to the height of the wind turbine blades, so they experience slower wind speeds. At night, the PBL doesn't carry slow-moving air up to the turbines, so they get the full force of the upper-level winds.

You may have noticed that for you as a human, nights seem to be calmer, and it's windier during the day, which is the opposite of what wind turbines feel. This is the same effect in reverse! You're so close to the ground that you don't feel much wind unless turbulence in the planetary boundary layer brings it down to your height.

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Duros001 t1_j66m2rq wrote

Reply to Why is wind energy generation greater during the night? by ZeroTheHero524

I don’t know if there is a marked difference in density, but I would presume the air is colder (and therefor denser) at night, so a 15mph breeze of denser air has more mass than 15mph of warmer air. That coupled with the sun heating the ground and causing Eddys and other disruptions during the daytime could all be contributing factors

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Narrow_Competition41 t1_j66feiy wrote

Reply to Why is wind energy generation greater during the night? by ZeroTheHero524

It's greatest during the transition from night to day and day to night, notwithstanding any kind of atmospheric disturbance like a storm. Basically it has to do with the sun heating the surface during the morning hrs, and during the evening hrs the surface releasing that heat. Keep in mind air aloft, where the turbine blades are (~300ft) situated, is always more active than at the surface level.

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