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Create or Destroy Water can create 10 gallons per upcast-level of rain in a 30 ft cube, up to a total of 90 gallons. It can also be cast so it falls as rain.

It looks like that would produce a rainfall of about 100 mm/hour or twice the violent shower listed here.

I've calculated that the water would reach terminal velocity almost instantly, and would fall at 0.01 m/s for the whole 30 ft height. That means the rain would last for ~15 minutes.

Would it count as difficult terrain, or some other effect?

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    \$\begingroup\$ I suspect you're using the terminal velocity of cloud droplets, not rain. Raindrops are much faster than this, and it certainly wouldn't take this much water 15 minutes to fall 30 ft. \$\endgroup\$ – 1600hp Mar 18 at 14:16
  • \$\begingroup\$ @1600hp that number did seem wrong \$\endgroup\$ – AncientSwordRage Mar 18 at 14:20
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    \$\begingroup\$ It looks like that is more like 9m/s and would be over in ~1 second. \$\endgroup\$ – AncientSwordRage Mar 18 at 14:28
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    \$\begingroup\$ @zeiss that equates to 475,000 mm/hour... \$\endgroup\$ – AncientSwordRage Mar 18 at 15:09
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    \$\begingroup\$ Or more like a bucket drop than actual rain. Call it rain if you like, for that much water to fall in a second, it has to arrive as a nearly "solid" volume, not as drops. \$\endgroup\$ – Zeiss Ikon Mar 18 at 15:51
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Not a lot

Assuming that the rain is "Instantanous[ly]" created uniformly throughout the "30-foot cube" where they represent about 1-9 in 400,000 of the volume (while not stated, this seems a reasonable ruling) and given that terminal velocity of raindrops is about 10m/s, they will hit this speed in about 1 second. However, a 3 m fall takes about 0.75s so all the rain is on the ground in less than a second.

When it hits the ground it is about 0.5-4.5mm of across the area. This is a lot of rain but it lasts for less than a second all up. For reference, the world record point rainfall event is 40mm over 1 minute - an average of 0.7mm/s - this is more than the 1st level spell but it is quite likely that there were seconds during that minute that approached the 9th level spell.

The impact would be less impactful than a bucket dump you get at a water park.

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One might rule it as heavy rainfall.

It would

extinguishing exposed flames in the area.

Additonally, it might create Heavy Precipitation based on the DM's rulings (DMG 110):

Everything within an area of heavy rain [...] is lightly obscured, and creatures in the area have disadvantage on Wisdom (Perception) checks that rely on sight [...] and imposes disadvantage on Wisdom (Perception) checks that rely on hearing.

Which further impacts Visibility (DMG 119, 243) of things in the distance as you try to view through it.

Note that the duration of Create or Destroy Water is instantaneous (PHB 203). So any effects caused by the spell, last for an instant. Ask your DM about how long it takes 10 gallons of clean water to fall in the form of rain in any given area (it only creates this amount of water once, not a steady stream of more than 10 gallons of rain).

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  • \$\begingroup\$ Following the comments on the question, it looks more like the rain will last for only 1 second. Does that seem like enough for heavy precipitation? \$\endgroup\$ – AncientSwordRage Mar 18 at 15:20
  • \$\begingroup\$ @Pureferret that entirely depends on the DM's ruling and the physics of the setting. I'd judge it on a case by case basis. When time (PHB 181) in fast paced situations is commonly measured in rounds which are 6 seconds, a turn is also 6 seconds it merely describes causal order (PHB 189). The spell ends as soon as it is cast (in an instant), it creates water that extinguishes exposed flames in that instant, but those 10 gallons of water still have to fall down and they linger (unlike so many other spell effects). \$\endgroup\$ – Akixkisu Mar 18 at 15:40
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The spell states:

Create Water. You create up to 10 gallons of clean water within range...Alternatively the water falls as rain in a 30-foot cube within range...

At Higher Levels. When you cast this spell using a spell slot of 2nd level or higher, you create or destroy 10 additional gallons of water, or the size of the cube increases by 5 feet, for each slot level above 1st.

This appears to be that if you choose the rain option and cast at higher levels the cube is increased instead of the number of gallons.

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    \$\begingroup\$ The "or" suggests you could pick to have more rain fall in the same area. \$\endgroup\$ – Erik Mar 18 at 20:35
  • \$\begingroup\$ Your conclusion is incorrect, and your answer merely cites the spell without stating any meaningful conclusion. \$\endgroup\$ – Akixkisu Mar 18 at 20:59

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