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Deserts are known to be hot and dry places. In my desert, I need a high humidity in the air. It doesn't have to be near the ground, but at least the atmosphere below 1 km should have a layer of high humidity. This should be a consistent condition throughout a season if throughout it is not feasible to maintain this condition throughout the whole year.

It is not necessary for my desert to be hot. In fact, having a high humidity and moderate temperature is preferable.

What could cause a desert to have high humidity?

A big oasis? Being near an ocean? Wind towards mountains? Seasonal wind?


This question graduated from the Sandbox.

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    $\begingroup$ What is your definition of a desert? Scientifically, Antarctica is a desert with high humidity. $\endgroup$ – Alexander Oct 13 '17 at 19:13
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    $\begingroup$ If all you want is the sand dunes and winds, would you consider using a very wide beach instead? $\endgroup$ – Henry Taylor Oct 13 '17 at 19:17
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    $\begingroup$ So you are wanting high humidity but no precipitation. Right? $\endgroup$ – Henry Taylor Oct 13 '17 at 19:18
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    $\begingroup$ Antarctica is a desert with low humidity. The reason there's snow and ice is that it doesn't warm up enough for what precipitation it does get to go away, therefore it builds up. Vostok Station averages 22 mm of water-equivalent precipitation per year. Generally speaking, if an area receives less than 250 mm of precipitation per year it's considered a desert. $\endgroup$ – Keith Morrison Oct 13 '17 at 20:33
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    $\begingroup$ Antarctica has fairly high relative humidity Seasonal mean AIRS RH over Antarctica $\endgroup$ – Alexander Oct 13 '17 at 20:57
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You're in luck! We already have this action on Earth, in the Atacama Desert. Fog rolls in from the ocean, but the mist is so fine it doesn't fall as rain, hence preserving its desert-hood. Only a few plants are able to take advantage of the mist, so it still comes across as "deserty". There is a little lizard which lives there which has weird horns around its head which allow water to condense and flow down to its mouth. Smart little devil.

Quick note on coastal fog... From the internet: "Coastal fog is usually a result of advection fog which forms when relatively warm, moist air passes over a cool surface." Like a warm wind going over a cool or cold ocean, which just happens to be near your desert!

Check out this article ... they're using condensing vanes to snag the water for use. http://www.bbc.com/news/world-latin-america-32515558

Stole a neat pic from the article:

Fog over desert

Here's the condensation rig:

enter image description here

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    $\begingroup$ Wow, seems like earth already has so many fantasy features available for us as inspiration! If you could add how the Atacama fog formed and one or two photos, this answer will be perfect! $\endgroup$ – Vylix Oct 13 '17 at 19:27
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    $\begingroup$ The Earth is by far the most awesome planet I've lived on. $\endgroup$ – Ghotir Oct 13 '17 at 20:07
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    $\begingroup$ @Vylix The saying is, "Fact is stranger than fiction." $\endgroup$ – jpmc26 Oct 13 '17 at 22:39
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At low temperatures “high humidity” does not produce the effects that you might expect.

Under low temperature conditions it is possible to have 100% saturation humidity, but because the air temperature is so low it is almost meaningless. At high temperature the difference between high humidity and low humidity is very significant, but at low temperatures there is very little difference between high humidity and low humidity because air’s capacity for holding moisture at low temperatures is very low.

humidity v temperature

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In addition to having this condition in a cool desert, you can have high humidity in a regular ol' desert too, as long as you have an ocean.

enter image description here

The Red Sea is literally the worst place on Earth, as any sailor who has gone through it can tell you. Not only is it brutally hot all summer, it is amazingly humid. If you are looking for humid and not too hot, allow me to throw Arabian Nights at you.

Right now, as I am typing, Jeddah, Saudi Arabia (which is just off the bottom of the attached picture) is 80 F (27 C) with 86% humidity at 5 am local time. Of course, it is October, which is basically the middle of summer (yesterday's high was 94 F (34 C), for example). It gets cooler towards the winter time.

A quick look at Djibouti, on the opposite side of the Red Sea from Jeddah shows typical conditions. Average humidity is in the 70s for most of the year, dropping in the summer months under the blazing sun. By comparison, a noted hot and humid summer locale like Jacksonville, Florida sees the same humidity for most of the year as Djibouti.

Any desert can be a humid desert, if you put enough ocean into it.

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  • $\begingroup$ That is my nightmare. I hate humidity (ofc i live in a humid environment :P) it is annoying $\endgroup$ – Christopher Oct 14 '17 at 14:24
  • $\begingroup$ October is the middle of summer? It is the Northern hemisphere... $\endgroup$ – Miguel Oct 15 '17 at 5:01
  • $\begingroup$ @Miguel Something of a joke. 94F / 34 C and humid is pretty mid-summerish in most places in the world. $\endgroup$ – kingledion Oct 15 '17 at 21:54
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Most people know that deserts are classified for lack of watery precipitation because water fluctuations replenish groundwater, and that feeds vegetation, springs, streams, and so on. Stagnant water becomes crudded; dry earth becomes parched and loses the multitude of pores and other soil structures which convey water through the topsoil layers.

I've always preferred to be more ontologically concise and simple: an ‘aridland’ is one which has less water in some form or another than certain standard definitions; a ‘desert’ is one which is limited in habitability to below certain standard definitions. I.e.

This permits you to classify the following regions as deserts:

  • smog–covered wastelands, where only intrepid mutants roam in few and small groups.
  • seasonal inland salt lake which receives loads of rainwater but supports nothing except nanoscopic halophiles.
  • an alien planet which is pristine and quite viable, but somehow devoid of life.
  • similarly, there could be much and many fertile nutrients, but radiation results in sparsity of life and lifespans.

Similarly, these would probably not be considered deserts:

  • smog–covered wastelands which are, never–the–less, densely populated.
  • somewhere artificially irrigated in sufficient quantities and with competent ecological management that enables it to support a jungle, or hydroponics and vertical farming, or whatever.
  • the aquatic frogfolk cities under the Europan ice ceiling.

Measuring the hospitality of a zone — the ease with which a certain group or class of lifeforms are able to thrive — should be an entirely different metric.


Ah, yes: you wanted a desert with “high humidity” and “moderate temperature”.
So, Arrakis but with much dissolved water in the air?

My suggestion relies on the composition of the terrain and earth.

So long as there is some water, there are species of vegetation which will take root with even very small amounts of vital minerals. They are variously called ‘pioneer plants’ or ‘weeds’. Eventually, you will get a parade — figure of speech — of animal critters and less–hardy vegetation which build upon the gradually increasing concentration of organic detritus and minerals brought in by the decomposition of the aforementioned parade.
Very simplified, but you get the idea.

Even very coarse earth suspended on a mesh grate over a vast subterranean river will eventually accumulate compacted crud on which mosses will be able to survive — and, so on. If, however, the terrain is of some chemical composition which is either poisonous or simply incompatible with simple things such as lichens, then you'll never get anything to take root. Such a chemical would need to contaminate whatever soil is deposited — ergo, simple copper–plated pebbles probably wouldn't work for very long.
Of course, if you don't mind having a desert which becomes less of a desert due to recurrent traffic, then the copper pebbles could be adequate. Why copper? Most microbes, lichens, and mosses can't grow on cuprous surfaces.
Indeed, any unlimited accretion of foreign material which supports life would eventually dilute whatever chemical compels the desert to be so deserted.

Probably the best way to go, if a worldbuilder chose to use my suggestion, would be to have coppery pebbles and thorough drainage in a region. Use the quantities of water to flood and wash the area clean.

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As @akaioi said.

Also, high air pressure would allow more moisture to be held by the air without precipitating.

So if the world had higher air pressure the humidity in the desert (while still being low compared to other places on that world) would still be high from an Earth point of view.

On world with Earth like air pressure, maybe the terrain is arranged in a V to concentrate the air pressure in an area. You might get a high humidity desert in the tip of the V but the water would then condense out as the air moved up the mountains which would cause rivers to head down to your desert. That might ruin the whole "dry" thing you are going for unless terrain also routed those rivers away from the desert area.

EDIT:

Vylix pointed out that I was unclear so:

By "V" shaped, I mean two mountain ranges coming to a point with prevailing winds running between them to the point. This would create a localized high pressure area. The mountains have to be low enough that the air can spill over them but high enough that it causes enough air to "bunch up" in the V.

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Coasts of Persian (Arabian) gulf have the highest dew points in the world. Now it is October and still dew point did not go below 27°C (80.4°F) and the end of August and first half of September it was above 30°C (86°F for continuous 25 days).

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    $\begingroup$ While this is interesting, perhaps you could expand on exactly what those dew points mean. As it stands, this does not actually answer the question one way or another. $\endgroup$ – Gryphon - Reinstate Monica Oct 4 at 5:46
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    $\begingroup$ How is it not clear? The Persian Gulf coasts have hot desert climate but have the highest humidities in the world, higher than even tropical rainforests. Relative humidity is not a very good measure of how much water vapour is in the air, dewpoint is. $\endgroup$ – شاهين حسن Oct 4 at 8:37
  • $\begingroup$ I think is a good comment but you should rephrase it as answer: i.e. "Deserts near the coast have a very high dew point (so the air is easily saturated with humidity). For example..." $\endgroup$ – Stormbolter Oct 4 at 8:48

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