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What temperatures can one expect 50-150 meters below different types of deserts? The rocks in my campaign heavily feature iron sediments, like in Australia, but since this is a fantasy world I am open to changing types of rocks and environments.

This question is aimed at an underground cave system I am planning for a dungeon crawl.

Thank you!

EDIT: The climate zone is subtropical; as for the rock type, I have absolutely no idea yet. Its altitude ranges between 70m-150m over normal null, while the average temperature varies between 20°C-30°C in winter and 30°C-40°C in summer.

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  • $\begingroup$ what is the climate zone of the desert including an average surface temperature and what rock type? How high is the desert compared to normal nill? $\endgroup$ – Trish Nov 17 '20 at 17:25
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    $\begingroup$ You mention this is for a dungeon crawl, so I'll just note that cave temperatures are rarely so extreme that you'd see major effects on the PCs as dictated by the rules mechanics in most systems (it won't be blistering hot or freezing cold), although it could be useful for describing the setting better. A cave's climate is typically more mild than its surroundings, so if you can handle the trek to the cave, you can handle the climate inside the cave. $\endgroup$ – Nuclear Hoagie Nov 17 '20 at 17:59
  • $\begingroup$ @NuclearHoagie thanks! the question for the dungeon crawl was mainly bc of this other question I asked: worldbuilding.stackexchange.com/questions/190178/… and me wanting to know if the temperatures could drop that far. $\endgroup$ – ana_dm Nov 17 '20 at 18:03
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Deep ground temperature is reasonably easy:

At 15m down, all daily + seasonal fluctuations have nulled out, only longterm values matter. The temperature is the annual average of the AIR temperature of the surface above it. Surface type/color/texture/seasons do not matter.
For example, a black surface rock will bake warmer in the day, yes. But at night that same rock will radiate the heat faster.

Going any deeper, increase the ground temperature by 2.5C per 100m of added depth.
This is due to approaching the hotter interior of the planet, and adding a thicker layer of insulation above.

Exceptions: If the surface is under significant water(more than a shallow pond or marsh), skip the air and use annual average WATER temperature.

If in a region of geothermal activity, add X, where x varies wildly according to local conditions. Obviously, digging at Yellowstone will make for warmer tunnels.

For your query: the type of desert does not matter. Just take the average air temperature(!not! average of hottest per day, you need to average of the temp over the whole day. This will be surprisingly LOW for a desert!)
And add 1.25C - 3.75C

Example: near the middle of the Sahara desert, the average annual temperature is about 16.2C
(no, seriously! I said deserts are cold! Day peak air temperature could be 38C, but nighttime drops to -4C !)
So your tunnel at 100m under it would be at about 16.2+2.5 = 18.7C

Measured soil temperatures at depth. (this is in Indonesia, but same rules apply for deserts)
(image source: https://www.researchgate.net/figure/Temperature-variation-of-underground-soil-with-depth-for-typical-days-in-Malaysia-15_fig3_256838899) enter image description here

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    $\begingroup$ the ground type mattes for if there is to be expected water coming from above - which can alter the temperature in some cases. Type of desert matters because, you won't believe it, the Gobi has an annual rainfall that is pretty huge - just the whole water runs off! $\endgroup$ – Trish Nov 17 '20 at 18:10
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    $\begingroup$ remember to link your image source! it is a cool image and I want to know more. $\endgroup$ – Willk Nov 17 '20 at 18:21
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    $\begingroup$ @Trish The water ONLY matters if it carries temperature from a location with other weather. Example runoff from a glacier. And p.s., I do not consider the Gobi's 20cm(7.7inches) of rainfall at all relevant, much less "pretty huge"! $\endgroup$ – user79911 Nov 17 '20 at 18:29
  • $\begingroup$ For a desert, that is enormous. The egytian desert gest less than a millimeter - that's 200 times more! $\endgroup$ – Trish Nov 17 '20 at 18:31
  • $\begingroup$ why do I add the 1.25-3.75? I thought i added the 2.5 because of proximity to the earth's center $\endgroup$ – ana_dm Nov 17 '20 at 20:49
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The temperature of a cave is usually the same all year round and based on the average temperature of the year's day-night cycle - it is that perfect average of the whole year, if there are no other factors to heat a cave system, like being particularly deep (mines get hot).

Subtropical deserts have a rapid drop in night temperature, which has to be taken into account. So the daytime temperature is on average 30 °C, but if you take into account the night drop, which falls to freezing temperature, puts the expected underground cave to be a chill 15°C close to the surface.

Deep temperatures are getting warmer: 0.025°C per meter deeper. So our 15°C becomes about 17.5 to 18.75 °C.

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