The references both to colder air holding less water vapor, and relative humidity, might be confusing. The water vapor, when it stays in the form of water vapor, has minimal impacts on visibility. A larger impact, captured in the equations above, occurs when the water vapor combines with pollution in the air, specifically ammonium nitrate and ammonium sulfate in those equations. Those are hygroscopic salts that will become hydrated if the relative humidity is high enough, forming larger particulates that will scatter light more, especially if they start to clump together as they can do. Whether they become hydrated is controlled by the relative humidity, not the absolute humidity.

For more on this, see the EPA's guidebook on air quality impacts on visibility, written with an emphasis on views in national parks. https://www.epa.gov/sites/default/files/2016-07/documents/introvis.pdf Section 4 is the most directly relevant, particularly starting on p. 22 (p. 31 of the pdf).

The relative humidity in Colorado tends to be low in both summer and winter, and also varies greatly with time of day, so I'm not sure to what extent humidity explains what OP is seeing.

Edit: the monthly average RH in Limon CO (on the plains, east of the mountains) is a little bit higher in the winter than in the summer, but the swing in relative humidity over the course of a single day is often much larger than that seasonal variation.

Tens of years shift in a process that took tens of thousands of years.

It's less the refractive index, and more variations in the refractive index, which makes the view less crisp.

Think of how air shimmers over a hot road. The heat rising off the road causes differences in the refractive index, which causes the shimmer.

Over long distances, with a lot of these together, the view would be fuzzier.

Generally no. They will usually go with whatever route is expected to be used for the final drug. That depends on things like oral bioavailability (if you swallow it, does it actually reach the bloodstream), and is it most likely to be used in a hospital setting or by patients themselves. Generally, oral is preferred where possible, because it has higher patient compliance levels, but if that’s not suitable (you need something shorter or longer lasting, say, or it breaks down in the gut to something useless), you’d probably do some in vitro and in vivo DMPK to figure out what route would be better. Early DMPK testing will give an idea on which route is to be used. Thereafter, the testing is done by that particular route, plus some IV injections here and there for Pharmacokinetic comparisons.

It sounds like you're talking about a parameter that would describe the visibility of stars, etc., which you have to look through the whole atmosphere to see, whereas OP is looking horizontally, just through the lower atmosphere. The humidity would matter for both, but specifically counting the total in a vertical column would be less relevant for the horizontal view.

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”)

Thanks for these useful links.

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No, the FDA does not run these tests. They'd have to pay for them then, rather than get the company to do them and pay for them.

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(Actually, come to think of it, shouldn't the formula be sqrt(4pi)/(lmax+1)? Alms from 0 to lmax have (lmax+1)² degrees of freedom (sum_0^lmax (2l+1)). This is enough information to split the full sky into (lmax+1)² pixels, which would then have a side length of sqrt(4pi)/(lmax+1). This works out to 1.13*pi/l, so very close to your formula and far from the 2pi/l I had been using.)

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I appreciate that story!! I’m sure the FDA must have received lots of copier drama without even knowing it.

Made me remember someone who accidentally sent their yearly development plan to the authorities in a handful of countries. Not her best working day.

I used to prep drug master files for the API. They’d be smaller of course, but still with three copies printing, etc was a chore. And yes, a lot of it was executed batch records and the environmental statement, which i guarantee no one ever read.

I once accidentally discovered a note in the middle of one I was compiling yelling at my group for monopolizing the printer. It could easily have gone to FDA. I would’ve loved to get that deficiency letter. “Please explain the copier/printer drama…”. Same guy who put in the note made a complaint about me to HR because I let the FDA ask him a question on a tour. (We were a small shop.). They had to explain the law to him. He was still pissed. “Hey, bud I answer their questions 8 hours a day, you can answer one about a lab balance.” Good times.

See this comment! About 56000 years.

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They used to be about a truckload of paper. That included a couple of copies for the reviewing agency’s convenience.

The ones I work with nowadays are around 100.000 to 1.000.000 pages as pdf.

To be honest, most of those pages are tables tabulating detailed test results (eg every single blood test taken in every study). Those are then summarized to more high level documents which are fortunately shorter.

As noted above, water content within the air is the dominant effect. Colder air holds less water, so in general, winter atmospheres have less water and are more transparent.

The variable usually used to capture this (i.e., modeling the effect of water on atmospheric clarity) is water column content, measured in grams per square centimeter. This is the mass of water in a column of atmosphere 1cm by 1cm, from the Earth surface up to the top of atmosphere. It typically ranges from 0 to 5 g/cm2, at least for non-cloudy atmospheres.

At 5 g/cm2, your naked eye will notice haze and muted colors. The colors are muted because some spectrally distinct light from distant objects is being intercepted by water and scattered as white light. E.g., a fraction of the greenish light reflected from a tree is absorbed by water and re-radiated as panchromatic light, before finally reaching your eye. So you see 90% photons from the tree, and 10% photons scattered off water between your eye and the tree. And of course this scattering causes blurred edges as well.

This variable (column water content) can be measured by scientific instruments in several ways, fairly directly, by its impact on the spectrum of sunlight. NASA has several satellites that make this measurement regularly, sometimes just to calibrate out the effect of atmospheric water on other measurements.

Here’s a nice map of water content, animated monthly: https://earthobservatory.nasa.gov/global-maps/MYDAL2_M_SKY_WV

You can see the decease in atmospheric water in Colorado in the winter months, when the atmosphere there is dominated by cool, arctic air; and then increasing in summer as you get air from the tropics. (As the narrative explains.)

You can also notice stronger seasonal effects over land than over oceans, partly due to thermal regulation by the ocean’s water mass. Again, this makes mid-Continent areas like Colorado have a stronger seasonal “swing” than, say, coastal California.

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People have done good explanations of why the colder air is "more clear," but you can also check with your friendly neighborhood amateur astronomer, who will tell you that cold, dry nights are the clearest for stargazing. And that, sadly for us stargazers, the best nights for stars are often the ones where your fingers get too cold to operate the scope. :)

Where I live, when it's around 20-35*, there's always a lot of moisture in the air, and it's usually cloudy. When it's cold, crisp, and clear as a bell, it's usually below 10 degrees; the conditions are AMAZING for stargazing, but my poor fingers freeze within half an hour.

Clear Dark Sky is a Canadian government forecast service that gives a stargazing forecast for North America .. they give good and clear explanations on atmospheric transparency and "seeing." I expect if you peek at the astronomy forecast,you will find your view of the mountains is the most crisp on days when transparency is high. https://www.cleardarksky.com/c/Ottawakey.html

> 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.

Up to a point. It's not that the air needs to be cold, it's that it needs to be cooler near the ground than above it. Sound travels faster in the hotter air, and sound waves curve and refract from the zone of higher speed into the zone of lower speed.

So in the night, when the air gets colder, the sound from up above tends to curve towards the ground and propagate farther. During the day is the other way round. More to read: https://blog.thermaxxjackets.com/refraction-of-sound-waves-acoustic-shadows-explained

I live in the north. Is this why the stars are amazing at -40?

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Does the refractive index of the air also play a role at all? I wouldn't imagine it would be too noticeable just looking at mountains, but if looking out over the horizon I would imagine it might.