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Highland or Alpine climates are largely defined by their low temperature, low humidity, and high elevation the prior two owing to the latter due to the lower pressure. As one goes up, the temperature, humidity, and pressure go down.

My planet, however, has a higher pressure than Earth's, at around 2.2atm, and its gravity is higher, at around 3.6Gs (not sure if that is even useful here but just in case/yes that is very high, but humans aren't going to live here so don't worry ;)). However, it is slightly colder than Earth, at around 8.9°C or 48°F.

How do I calculate the elevations my highlands will start at?

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    $\begingroup$ 3,6G seems a bit high, no? Jupiter has 2,64G $\endgroup$
    – Rodolfo Penteado
    Aug 6, 2020 at 1:52
  • $\begingroup$ @Rodolfo Penteado My world is extremely rich in heavy metals, ergo super dense. Yes, it is high, but I'm not looking to put earth-life on it. $\endgroup$
    – Aezyc
    Aug 6, 2020 at 4:49
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    $\begingroup$ @Aezyc Many astronauts have endured 3.8 g and even higher for a few minutes during launch. I doubt whether they would want to spend hours or days in such high gravity. On page 12 at rand.org/content/dam/rand/pubs/commercial_books/2007/… it is said that: "On the basis of the available data, one might conclude that few people would chose to live an a planet where the surface gravity was greater than 1.25 or 1.5 g." Does your story require humans to spend much time there? $\endgroup$
    – M. A. Golding
    Aug 6, 2020 at 17:43
  • $\begingroup$ @M. A. Golding No, humans are not going anywhere near this world. There is a native intelligent species there, but they are virtually nothing like us. $\endgroup$
    – Aezyc
    Aug 6, 2020 at 20:37
  • $\begingroup$ @Aezyc just to clarify, is this a hard-science (following Earth science, with need of citations) or a science-based (based on hard science, no need for citations) question? $\endgroup$
    – Enthu5ed
    Sep 15, 2020 at 20:00

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Short Mountains and stubby aliens

Let's first establish the general elevation of the mountains on your planet. Most things, due to the high gravity, will probably be shorter here, both the lifeforms and the mountains.

From the physics exchange on the effects of gravity:

In general, the bigger something is, the less it can deviate from having a smooth surface. The reason is tall mountains will be crushed under their own weight, and valleys will not be able to support their walls.

Also relevant is the angle of repose, where materials have a specific angle at which they can naturally rest. With higher gravity comes a smaller angle of repose, due to the materials forming the mountain naturally being 'pulled down'. This means that mountains will be naturally less sloped, and shorter.

Finally, the cold climate tends towards easier erosion, meaning any high mountains will be quickly eroded down by the colder than usual global temperature and weather.

Thick Atmosphere

Relation between gravity, temperature and atmosphere:

The thickness of a planet's atmosphere depends on the planet's gravity and the temperature of the atmosphere. A planet with weaker gravity does not have as strong a hold on the molecules that make up its atmosphere as a planet with stronger gravity. The gas molecules will be more likely to escape the planet's gravity. If the atmosphere is cool enough, then the gas molecules will not be moving fast enough to escape the planet's gravity.

Another excerpt about Mars' atmosphere height compared to Earth's:

The scale height of the atmosphere is about 10.8 km, which is higher than Earth’s (6 km) because the surface gravity of Mars is only about 38% of Earth’s, an effect offset by both the lower temperature and 50% higher average molecular weight of the atmosphere of Mars.

From the above articles we can garner that higher gravity and colder temperature means, in general, a slightly shorter atmosphere height (that is to say, the atmosphere here is shorter and more condensed compared to that of Earth's).

Molecular weight of the atmosphere here seems to be different than that of Earth's due to only 2.2atms of pressure despite 3.6g's of pressure, but despite this the high gravity and cold temperature will contribute to a much shorter atmosphere scale height. We'll ignore the effects of having/not having a magnetic field on the atmosphere loss to space as well.

Highland Height Estimate

Thus, using the conclusions from the two sections above, we know that (1) the mountains are shorter on your planet, and (2) so is the atmosphere.

This means that most likely, the highlands on your mountain will be significantly less tall than the ones on Earth.

In general, the highlands on Earth are roughly considered highlands at 500m and above, some reaching even 2500m.

Thus, considering both the lower mountains and shorter atmosphere height of your planet, the highlands will be shorter in height than that of Earth. A reasonable estimate would probably be anything 250m and above.

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  • $\begingroup$ Will update with some rough calculations based on the Ideal Gas Law later so we can make a more accurate second estimate. $\endgroup$
    – Enthu5ed
    Sep 15, 2020 at 20:15
  • $\begingroup$ Might not be able to come back to finish the calcs for a while, but comment if this interests you enough to warrant it. $\endgroup$
    – Enthu5ed
    Sep 21, 2020 at 13:48

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