Recent comments in /f/askscience

AerodynamicMarrow t1_j2oz73b wrote

Reply to comment by uber_snotling in Does cold temperature make vistas more 'crisp' looking? by colorado_hick

This is an important point, but as a commenter down below has noted, this explanation alone actually produces a result in the wrong direction - better visibility when it is warm!

The haze you see is scattering of light, primarily from atmospheric aerosols. Things like sea salt, desert dust, sulphate droplets from burning fossil fuels. These scatter light as haze and reduce the visibility (some examples here).

As the relative humidity (RH) increases, these aerosol particles take up water and swell, making them larger. These larger particles are even better at scattering light, making the atmosphere hazier as the relative humidity increases (even if you have the same number of aerosol particles).

You can actually see this effect near clouds (where the humidity is very high) - (Koren et al, 2007)

However, if you keep the absolute (or 'specific') humidity the same (i.e. keep the same number of water molecules), the relative humidity increases as it gets colder. This means that following the explanation above, the visibility actually gets worse in colder weather (all else being equal)!

The full reason for clearer skies when it is colder comes from a variety of effects.

  • Cold conditions often follows a cold front and the rain from this front 'cleans' the atmosphere of aerosols, increasing visibility.
  • Several processes in the Earth system, particularly trees and other vegetation, produce more aerosol as it gets warmer (reducing visibility on a warmer day).
  • Warm air promotes atmospheric mixing and convection, which reduces visibility a bit.
  • On cold days, the wind may be blowing in cleaner air from a different direction, avoiding sources of aerosol in nearby regions.

These factors may not be true in all places. Some of the biggests and most damaging smogs in London were in winter, as local sources of aerosol particles (from buring fuel for heat) increased significantly in cold conditions.

Obviously clouds have an effect too, but that would be a much longer post!

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BrobdingnagLilliput t1_j2ovrhh wrote

Reply to comment by Aseyhe in How do galaxies move? by modsarebrainstems

Well, now my curiosity is piqued! I think I understand how ordinary black holes form as a result of stellar evolution, but can you point me to some decent resources on supermassive black hole formation? I'd prefer something closer to a book than a web page.

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Aseyhe t1_j2ot4uq wrote

Reply to comment by BrobdingnagLilliput in How do galaxies move? by modsarebrainstems

Supermassive black holes form because of galaxies, not the reverse.

It has been suggested that supermassive black holes might form from "seed" primordial black holes, which would have existed before galaxies. But even then, it's the galaxy-scale initial density variations that allow galaxies to form around these seeds and grow them to supermassive scales.

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RVA_RVA t1_j2or55q wrote

Reply to comment by dcgrey in Does cold temperature make vistas more 'crisp' looking? by colorado_hick

East coaster here. I went out to the west coast to hike the PCT some years ago. I felt uneasy the entire time. Low humidity, strong wind, and completely exposed was a mixture we don't have over here. I'm used to the safety of the forest.

I loved it out there, I've been back a bunch of times to hike. I just have never been able to shake the uneasy feeling if being so exposed.

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DarkTheImmortal t1_j2oi5l9 wrote

Reply to How do galaxies move? by modsarebrainstems

Gravity.

For short distances, the pull of the expansion of the universe is weak. The closer the 2 objects are, the weaker the expansion. I like to use a rubberband as an example because even if you can't visualize it, you can easily do it to see. Take a rubber band and cut it so it's not a loop. Place 3 dots on it, one "main" dot, one that's close to the main dot, and one that's far away. Now stretch the rubber band. You'll notice that the near dot doesn't move away from the main dot nearly as much as the far dot. The expansion of the universe works in the exact same way.

Inversely, gravity gets stronger the closer 2 objects are. Like magnets.

Andromeda, for example, is close enough to where the gravitational pull of our 2 galaxies is significantly greater than the expansion of the universe so our 2 galaxies will eventually collide in the distant future. However, anything outside our local group of galaxies is far enough away where the expansion of the universe is significantly stronger so we will NEVER collide with anything out there.

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