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(Edited for focus; see history for previous version)

Following up on this question, let's say I have a "coastal" mesa (relatively flat, high land not far from an ocean) with taller mountains behind it.

Here's a rough illustration:

elevation profile

Let's say that the climate near the bottom (elevation: 100m or less) is basically San Diego — 14°C - 22°C seasonal, ±5°C daily high/low — but perhaps wetter.

Let's say the mesa is about 1500m high (note: previously I was using 1000m) and about 50km inland. From Mark's answer (thanks!) we can estimate that the overall average temperature on the mesa will be 10°C lower.

...But that implies a summer high of a measly 15°C. Denver, which is at a similar elevation and much further north (though also much further inland) manages to get up to 30°C.

So... realistically, what sort of climate and plant life would I expect to find on this mesa? Could it sustain forests? Could deciduous trees grow here (transplanted by humans, if necessary), and if so, would they change colors in autumn? Would the mesa get snow, and if so, how much and how often?

Matthew
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    Have you thought about prevailing wind direction and precipitation? How much temperature difference between Summer and Winter? – Patricia Shanahan Feb 14 '20 at 22:38
  • Either tropical plants will evolve to colonize the mesa (and not be deciduous), or deciduous plants will evolve to colonize the lowlands (and not look tropical), or you will get a weird mix as both occur. Either that, or simply say that people changed the environment from whatever was native beyond recognition. – user535733 Feb 14 '20 at 23:01
  • Consider Cannes on the shore of the Mediterranean (where it rarely snows, almost never more than three days per year, and snow never endures) and Grasse, 15 km north of Cannes, average elevation 333 meters, where it snows every year (occasionally covering the whole town in snow); Grasse has mixed vegetation, combining evergreen and deciduous trees -- like this. – AlexP Feb 14 '20 at 23:06
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    @user535733 this area, as far as I understand, it a fraction of the world, not the whole world. So, the coastal area and mesa would be occupied by different plant species, common to other areas. OP's main problem would be to have deciduous trees (not conifers) in the mesa. – Alexander Feb 14 '20 at 23:06
  • @AlexP, I'm getting mixed opinions as to the elevation of Grasse, though I think I see what you're getting at. Care to maybe repost that as an answer? – Matthew Feb 15 '20 at 02:17
  • @AlexP, another useful data point: San Diego vs. Pine Valley. Similar latitude, 2000m elevation difference. Still looking for something near there closer to 1000m. – Matthew Feb 15 '20 at 02:23
  • Argh, didn't notice that was Pine Valley, Utah, not Pine Valley, California . Prescott is closer to the right latitude, but much, much further inland. – Matthew Feb 15 '20 at 02:32
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    Pennsylvania highlands top out at about the same height, elevation does not make that much difference as long as they are still on the coastal side of the range, and they absolutely support forests. the alpine tree line does not start until around 3500meters, unless you get close to the poles. – John Feb 15 '20 at 04:49
  • You're perhaps confusing average and high temperatures. Also, this is not really a mesa, it's more of a plateau: https://en.wikipedia.org/wiki/Mesa You might get an idea by looking at some of the flatter areas of northern California: the Sierra Valley or the area north of Mt. Shasta. – jamesqf Feb 15 '20 at 07:15
  • @Alexander, sorry, I finally figured out what you were saying... Yes, this is just a small section (maybe 1000km × 1000km, if that) of an otherwise Earth-like planet with a comparable variety of plants. Let's say this is an unusual spot where the biomes have a sharper-than-usual divide between them. – Matthew Feb 18 '20 at 18:13

1 Answers1

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Assuming there's nothing unusual about the area (no katabatic winds coming off the mountains, warm currents heating the shore, etc.), the key parameter you're looking at is the environmental lapse rate, the rate at which temperature drops with altitude. Here on Earth, it's about 6.5°C per kilometer.

Given that the coastline is tropical with a 10°C winter low, the mesa is going to have winter lows around 3.5°C, or possibly slightly lower because it's further from the moderating effects of the ocean. You might get the occasional overnight freeze, but snow will be rare (once or twice in a lifetime).

If you want regular snow, you need to make it higher. 1500 meters should get you snowfall most years, but it will tend to be a thin, short-lived layer. To get a continuous winter blanket of snow, you're probably looking at a minimum of 3000 meters elevation.

A good model to look at is the Coast Ranges of California. You can probably find a situation similar to what you want somewhere in there, though the high mountains of the Sierra Nevada are somewhat further from the coast than the backing mountains of your mesa. The key thing to note is that the presence of the ocean will tend to moderate the daily and seasonal temperature variation in the lowlands, but not up on the mesa.

Mark
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    Just for knowing the phrase "environmental lapse rate", you get a cookie. I doubt I would have found that on my own! If I go further north, how much somewhat-localized (falling off after say 20km or less) heating could I plausibly claim from some geothermal and/or oceanic source? – Matthew Feb 15 '20 at 02:59
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    Okay, I guess no one else is chiming in. I ended up declaring the plateau to be about 2000m; that should be low enough that the atmosphere isn't unreasonably thin, but high enough to have moderate snowfall. Worst case, summer should still hit almost a 15°C average, but the altitude also seems to bring more variation, so I think I'm still okay claiming forest up there? – Matthew Feb 18 '20 at 18:05
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    With the high mountains you've got backing the mesa, you should get plenty of rainfall to sustain a forest (assuming the prevailing winds blow the correct direction). And yes, altitude and distance from the ocean increase the temperature variation, both daily and seasonal. – Mark Feb 18 '20 at 21:27
  • Right; from my limited understanding of weather and comments on my previous question, my understanding is that the lowlands might by "dry-ish" ("Mediterranean"), but the elevation changes are going to cause all the moisture to fall out, if not on the plateau, then on the mountains themselves where it will get there as rivers if not direct rain. I'm going to call it "plausible" and avoid mentioning any specific altitude. Thanks again! – Matthew Feb 18 '20 at 21:37