http://toddborg.blogspot.com/2013/01/why-are-mountains-colder-than-valleys.html
My focus, as I recall, was on the rate at which air cools as you gain elevation (roughly 4 degrees for every 1000 feet of altitude gain.)
I got some things right, but I got some major points wrong. Six years later, a reader I haven't met, Neal Mielke, wrote a response to that blog. He was gracious in his corrections, and I appreciated his input very much.
Neal's smarts are self-evident. After I wrote him back, I found out what I suspected - that he was a Physics major back in the day. (It must grate on fellows like Neal when a physics dilettante like me rambles on without expertise). So I print his response here.
Thanks again, Neal.
Here is his letter:
Hi Todd,
I just discovered your mystery novels, and your blogs, and I am enjoying both. I’d like to comment on your 2013 blog about “why mountains are colder than valleys.” OK, it’s an old blog, and I’m years late in making a comment. But I just bought your 5th novel, and that’s even older, so hopefully you’ll forgive me.
You wrote: “The simple only-kind-of-techy answer is that the lower you are the more atmosphere is above you, and the more the air gets squeezed by all the air above it. The molecules in compressed air have more energy and they bounce around faster than they do in air that isn't so compressed.”
Pressure being the root cause just isn’t right, and thinking that it is will lead a person to be confused about a lot of situations where higher pressure doesn’t correlate at all with higher temperature. The air in a scuba tank isn’t hot, even though its pressure is more than 100 atmospheres. There are also a bunch of real-world meteorological effects that would make no sense if higher pressure led to higher temperature:
1) Christmas Valley was colder than Echo Summit last night, even though Christmas Valley is lower
2) Frost often forms on valley bottoms when the nearby hilltops are frost-free
3) Meteorologists often refer to inversion layers, when warm air is above cold air
4) An upstairs loft in a townhouse or condo can be baking when the bottom level is cold
5) Once one hits the stratosphere, air temperature starts to rise with increasing altitude
6) Water temperature drops as one dives further below the surface of water, while pressure dramatically increases.
Thinking in terms of pressure also misses out on explaining why thunderheads form over mountains rather than valleys. And why hawks (or a hanglider) circling overhead is a visual manifestation of the lapse-rate effect. Understanding the reality behind the lapse rate is pretty cool, I think, and it’s worth really understanding it.
Localized heating combined with convective heat flow (warm air rising and cooling as it expands) is the real reason for all of this: the lapse rate effect that you blogged about, the seemingly contrary examples that I listed above, plus thunderheads and circling hawks. If you want a simple non-techy answer to give people, it would be better to say “it gets colder as you go higher because you’re getting farther from the source of heat, which is the sun hitting and warming the Earth’s surface.” The atmosphere is nearly transparent to the sun, which means that the sun warms the earth rather than the atmosphere directly. The atmosphere gets warmed only indirectly, from contact with the surface. When the surface air gets warmed it rises and carries heat higher into the atmosphere, so the whole atmosphere gets warmed. You “see” that effect when you see hawks circling in a thermal – they’re riding the rising warm air. The lapse rate of (about) 4 degrees per thousand feet exists because that’s the natural rate at which warm air rising cools as it expands. That lapse rate can actually be calculated on a single sheet of paper, from a handful of equations including the Ideal Gas Law.
Think of the sun-baked earth (or surface of Tahoe) as a hot plate, and a lot of things will make sense.
Christmas Valley was colder than Echo Summit last night because the surface heating went away when the sun set – the hot plate got turned off. Even more than that, the Earth’s surface radiates more infrared energy than the air does (otherwise night-vision goggles would see nothing but murky air). So the surface cools faster than the air – the hot plate turns into a cold plate. This cools the air in contact with the ground, and the cold air is stuck there because cold air wants to sink rather than rise. Because of that there are no “reverse thermals” of warm air above flowing down to the ground. The cold air clings to a narrow boundary layer close to the ground, trying to sink. The cold air in Christmas Valley stayed there, and the cold air up on Echo Summit flowed down to join it. That cold-air-flowing-downhill effect is the same reason why frost often forms on valley bottoms but not on hilltops. And inversion layers happen because warm air rising is a one-way street. If anything causes warm air to be above cold air, it’ll tend to stay that way. Inversion layers are common in the morning, because of the cold-plate effect, and they can happen when a warm air mass moves sideways over a colder one.
Convection is also why upstairs lofts are hotter than rooms downstairs. The warm air from the heat registers rises. It wants to cool as it rises, but merely at 4 degrees per thousand feet, to that’s a negligible effect indoors.
Temperature starts rising with altitude once one reaches the stratosphere, because there’s actually a second hot plate up even higher. That’s the ozone layer, which absorbs most of the sun’s UV energy. Up there, the temperature rises with altitude because you’re getting closer to that source of heat. It’s an inversion layer (higher temperatures at higher altitudes), so this is stable – the warmest air is generated way up high and it doesn’t want to sink. Similarly, if you dive below the surface of the ocean (or Tahoe), it gets colder as you go deeper. That’s because the heat source is above you where the sun hits the surface, and you’re getting farther away from it as you go deeper. This too is an inversion layer. Warmer water is less dense than colder water, so warmer water stays near the surface. (This effect, famously, reverses near the freezing point, at which point colder water is actually less dense than slightly warmer water. Everyone knows that ice floats, but water that’s very near freezing also floats to the top.)
What about those thunderheads? Well, if you think about it, why should Tahoe be cooler than Sacramento, if sunlight is all that matters and pressure is irrelevant? The sunlight at Tahoe is just as strong as in Sacramento. Why doesn’t the surface of the Tahoe basin heat up to 100 degrees, the same as Sacramento? Well, it would ... if you surrounded the Tahoe basin with a wall that extended up into the stratosphere (and made it transparent so that it didn’t block the sun). But at 10,000 feet the temperature would be about 85 above Tahoe and 60 above Sacramento. If the wall went poof, the warm Tahoe air would rise and the cold Sacramento air would rush in, because cold air is heavier than warm air. This air movement would stop only when the temperature at 10,000 feet was the same in both places, which (because of the lapse rate) would mean that the temperature at Tahoe’s surface would be the same as above Sacramento at 6200 feet. So, horizontal airflow guarantees that Tahoe is cooler than Sacramento. There’s no wall going poof, so the effect is a gradual one, but it’s there, and it’s responsible for afternoon thunderheads. The sun “tries” to heat the air at the summit of Mt. Tallac just as much as it’s heating the surface air in Sacramento. But hot air at Tallac’s altitude doesn’t belong there – on all sides it’s surrounded by colder, denser air. So when the sun is out there will always be a thermal rising from the summit of Mt. Tallac. And if the humidity is right you’ll get a cloud forming above the peak. Thunderheads form in the afternoons because the effect builds and builds as the day progresses, until the sun starts to set.
I hope that this is interesting to you, and not an annoyance. Again, I’m enjoying your novels and your blogs.
Regards
Neal Mielke
(from the Bay Area ... love the Sierras though!)
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