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cryptotope t1_j2kzpez wrote

There are (at least) a couple of different factors at work that can affect how clearly you can see a distant object.

The first is 'stuff' in the air. Dust, pollen, water (or ice). These can all change with the weather (and the seasons). The behavior of humidity - the amount that air can carry, and whether it condenses out - depends strongly on temperature.

The second is the air itself. Non-uniform air temperatures between you and those distant mountains affect the density of the intervening air, and result in weak, transient lensing effects. As you noted, it's the same phenomenon that results in mirages or heat shimmer--or which causes stars to 'twinkle' at night. Greater variations (and sharper gradients) in air temperature, coupled with greater turbulence in the air, make for worse seeing.

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tylerchu t1_j2lpick wrote

I thought the twinkle was because stars have such a small solid angle, any fluctuation in their output is perceived as a huge difference visually. And since fluctuations always happen there’s always a change in what we see thus the twinkle.

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astro-sobrien t1_j2mb4t4 wrote

The twinkling of stars (technical term is scintillation) we perceive isn’t due to any intrinsic change in the star’s output*. It’s due to turbulent regions of our own atmosphere distorting the light from the star. This means that effectively light from the star is scattered into or out of our detector/eye and so we see the brightness of the star changing.

There’s some more complex stuff with the light wavefronts being distorted and interference patterns but I’m too rusty on it to remember and explain it, plus the simplified description above is enough (it got me through a Masters thesis on the topic!).

Edit: Also forgot to add, the reason we see stars twinkle while the planets in our solar system don’t. This is because stars are so far away that they’re point sources, whereas the planets are near enough that we can resolve them as disks on the sky with our eyes (it may not look that way but we do see them as more than single point sources with the naked eye). So while the light from the planets (well, reflected by them) is distorted in the same way as described above, it’s effectively being distorted across the extent of the disk from our point of view, so we don’t perceive any change in the brightness.

(*the intrinsic brightness of stars does vary but, on the timescale of twinkling, this variation is way too small to perceive. On longer timescales you can get variations that are large enough in magnitude that you could technically notice it, but again this isn’t what we’d call “twinkling”)

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Alis451 t1_j2m1v1a wrote

> any fluctuation in their output

the heat shimmer is what causes that fluctuation. there are other causes like gas pockets and dust, but the heat differences is the most common.

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Ebikeboi t1_j2oh0vv wrote

Sounds to me like you're seeing heat haze in the summer, very common down here In Australia.

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