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Context / technology level:

  • I have a more-or-less Age of Sail airship. It is not intended to be entirely self-sufficient, but it should be able to make long trips with small stops to restock/repair; analogous to a naval sailship.
  • There is magic which allows it to overcome the problem of an airship being unable to move relative to the wind.
  • The same magic also enables the sailing airship to reasonably control its altitude without routinely jettisoning ballast or lifting gas.
  • There is a fantastical material available to use for the gasbags; greatly reducing passive leakage and allowing one fill of lifting gas to last a long time.
  • The magic and the fantastical material do not make the ship itself sturdier against weather or other causes of damage.

Motivation for the question:

Given the technological context, it seems to me that the limiting factor for such a vessel's range between stops would be drinking water. The range could therefore be (significantly?) extended if the ship were able to collect water from rain or clouds while in-flight. It is mentioned in passing on Wikipedia that collecting rainwater in an airship is/was done as a redundant buoyancy compensation system. While I haven't (yet) been able to find detailed information about the setup of the system, it's enough for now to take it for granted that it's possible.

However, it seems to be common knowledge that airships are highly vulnerable to bad weather (expressed clearly in this answer), and just steering for the nearest cloud to harvest rain from it might not always be a good idea.

Given that my Age-of-Sail airship will not have weather information updates by radio, and will be relying primarily on visual observation and instruments that give temperature, air pressure and altitude, how can it best determine which weather formations are safe for water collection?

In your answer, please specify, to the best of your ability:

  • Types of weather formations that ought to be avoided, and why they are a threat (what might go wrong if we entered them?).
  • Types of weather formations that ought to be useful for rain collection, and why you are convinced they are safe.
  • Enough context or scientific terminology that I will be able to research your indicated weather formations to identify where and when one can expect them to form.
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    $\begingroup$ I'm not enough of a meteorologist to give a good answer, but if you use a net to harvest condensation (à la "fogmobile": newscientist.com/letter/mg14018996-100-letters-fogmobile) you can use just about any cloud, whether it's raining or not. I'd be inclined to fly above the clouds most of the time to avoid rain/hail, then spread the nets and drop down into the cloud when I can see the weather is calm, keeping the speed up to maximise the volume of cloud harvested. $\endgroup$
    – Pastychomper thanks Monica
    Dec 16, 2021 at 13:51

3 Answers 3

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Avoid thunderstorms due to the strong vertical shear.

Avoid cold/warm frontal boundaries due to the strong vertical and horizontal shear.

Look for low (not too low), scattered cumulus clouds. Focus on darker clouds that already emit rain or virga (rain that doesn't reached the ground), generally in the afternoon. Those are your safest target. Avoid cumulus congestus clouds -- while they are more reliable rain producers, they are also more reliable wind shear producers.

The Graf Zeppelin harvested rainwater occasionally using a system of gutters. The water went to ballast, not drinking. Ballast was needed because daytime would heat the lifting gas, causing the ship to ascend (solution: Valve a bit of gas). But then at night the gas would cool, causing the ship to descend (solution: Drop some ballast). The ship did NOT depend upon this method for replenishing ballast regularly -- it was an occasional opportunity.

The specific method the Graf Zeppelin used was to brush one side of the ship against the rainy part of the cloud. This permitted them to control the amount of water collected, and prevented the ship from becoming too heavy (that would be bad). They did NOT dive through the cloud. That was considered much too risky by experienced airship officers. (Those guys weren't wimps -- many had previously demonstrated their courage night-fighting repeatedly over the skies of Britain during World War I, and many were fiery-crash survivors. To them, clouds weren't fluffy and pretty. Clouds meant the troposphere was trying to kill them.)

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  • $\begingroup$ the strong vertical sheer is the most dangerous. these winds might be able to push the vessel down fast enough that it is unable to get out. so it ends up hitting the ground. modern day aeroplanes have these same problems but they are less prone to accidents since they can reach the next column blowing upwards much faster. your 2nd best bet for safe clouds would be stratus clouds. the problem with these is that they can look like fog as seen from above. $\endgroup$
    – Postlim Fort
    Dec 17, 2021 at 9:01
  • $\begingroup$ @PostlimFort Is vertical shear dangerous only in the sense that it can change your altitude too quickly, so that you crash or burst your gasbags? Is it ever (realistically) strong enough to actually threaten the structural integrity of the vessel's frame? $\endgroup$
    – Qami
    Dec 19, 2021 at 19:11
  • $\begingroup$ @user535733 Thank you for your answer! $\endgroup$
    – Qami
    Dec 19, 2021 at 19:12
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    $\begingroup$ @Qami see the 1925 mid-air destruction of the USS Shenandoah by the wind forces of an otherwise unremarkable thunderstorm. $\endgroup$
    – user535733
    Dec 19, 2021 at 19:15
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    $\begingroup$ @Qami one excellent first-person account is in The Golden Age of the Great Passenger Airships, Graf Zeppelin and Hindenburg by Harold G, Dick, 1985. Horizontal wind is generally a nusiance, but shear in any direction is dangerous to a large, fragile airship. $\endgroup$
    – user535733
    Feb 22, 2022 at 12:35
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High above the raincloud. You need a wide funnel, a barrel, strong rope, and a sturdy winch.

A steel cable might seem better than rope, but there is going to be some possibility of drawing lightning using it. Silk rope is strong, so would probably be a good choice.

Best case: A barrel full of fresh clean rainwater.

Worst case: Something goes terribly wrong: Ship destroyed, crew dead, casualties on the ground, demands for reparations, diplomatic missteps, declarations of war, allies called in, a world war.

Let me know how it turns out. 😁

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Framing challenge. Your aim is to get drinking water, not to "collect it from clouds"

Solutions

  1. You're running it through your engines. Assuming your engines run on Hydrogen, burning your fuel produces pure water. If they run on Petroleum, it produces water and carbon dioxide. How much? This answer runs though some math and arrives at 1L of fuel combusts to produce approximately 1L of water. Sounds off, but most of the mass of H20 is in the oxygen atom and thus comes from the air. Thus if you stuck a reclamation system on your engine exhaust, you would continually be producing a small steady supply of it. Airships are not fuel efficient. This answer suggests a rating of 0.25 km/kg, or about 1L of water would be produced for every 5km traveled. The Graff Zepplin cruised at ~100km/hr, so you're producing 25L of water every hour, or 600L per day. As far as I can tell, the Graff Zepplin once carried a peak of 38 passengers, so they have about 15L each per day. This equates to a 1.5 minute shower (a shower uses about 10L/min), but if they shower every second day it's probably enough for drinking/cooking as well.

    Of course, if you are purely sailing, then this option doesn't work.

  2. Dehumidifiers. Rain falls when there is so much water in the air that it does not stay in gasseous state and instead condenses. There is a way to force this. Have you ever taken a can of coke out of the fridge and left it on the counter? The outside quickly becomes wet. This is the principle of a dehumidifier. There is plenty of water in the air - it's just a case of extracting it. Cobbling together a refrigeration system in the age of sail is no small feat, but if your magi-neers are successfully steampunking it, they can probably cobble something together out of brass tubing!

  3. Buckets and Lakes. See a lake below you? Lower a bucket. I seem to recall reading a story about a sailing ship doing this while at sea - they were by the outflow of some big river (Nile? Amazon?) so they were sailing in fresh water.

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