At lvl 9 the Monk class gains this ability
[...you gain the ability to move along vertical surfaces and across liquids on Your Turn without Falling during the move]
Now my question is, if it can move acros liquids, why can't it run upward through a heavy rain?
I keep having this discussion with a friend and would really apreaciete some help
A definition for the English word "across" is:
From one side to the other of (a place, area, etc.)
To move across a liquid would thus require you to go from one side of the liquid to the other. This can easily be applied to concepts like walking over a lake, or a pond, or an actual body of water but if you were to ascend into the sky by walking on raindrops, I could not say you have walked across anything.
This really just comes down to how we read the standard English of the rules, and for me, if the ability were intended to allow you to ascend in rainfall, it would not have been worded the way it is. I simply cannot read that "the ability to walk across liquids" would include "the ability to ascend using raindrops".
Even if it did allow you to step on raindrops, you have to remember that in D&D falling happens instantly OR 500'/round depending on how your DM handles falling. Assuming your DM rules that the rain is falling at 500'/round to ascend you'd need a faster movement speed see Fastest a character can move in a turn.
So, with a permissive DM and a fairly specialized build, yes you could "climb the rain" however it'd probably be easier to just get a Fly speed.
Wuxia is "a genre of Chinese fiction concerning the adventures of martial artists in ancient China [and] is traditionally a form of fantasy literature". The abilities of monks in D&D have drawn on this literary and cinematic tradition since the earliest editions of the game.
One particular source of wuxia abilities is the practice of qinggong, which when "exaggerated in wuxia fiction [gives] martial artists...the ability to move swiftly and lightly at superhuman speed, and perform gravity-defying moves such as gliding on water surfaces, scaling high walls and mounting trees".
While in my limited experience of Wuxia I don't recall anyone running up the rain, there was a memorable scene in Tai Chi Master in which the antagonist is hurling javelins at superhuman speed and the protagonist is running on top of each javelin in flight in order to close the distance over uncrossable terrain to the antagonist.
So I certainly think that 'running up the rain' is within the spirit of the kind of abilities that a 5e monk gets at ninth level; I am comfortable with it being "rules as intended" (RAI). That being said, even for the explicit ability of running up vertical surfaces, there has to be a liquid there to run on - air doesn't count, and the ability does not give one the ability to fly, levitate, hover, or move vertically without a supporting surface.
In order to satisfy rules as written (RAW), you need to make an argument about running "across liquids". Rain itself is not a liquid, but it is composed of falling liquid drops surrounded by air. If the ability allows you to run across the surface of a still pond, a generous DM interpretation would allow you to run across* the individual upper surface of each liquid raindrop. However, you would still need to account for the fact that these individual raindrops, unlike a solid wall, are falling even as you are pushing off of them, in the same way that it might cost you more movement to run upcurrent on the surface of a swift river. Were I to allow such movement as a DM, I would say that running up rain counts as climbing where "Each foot of movement costs 1 extra foot (2 extra feet in difficult terrain) when you’re climbing, swimming, or crawling." A DM who wanted the allow this use of the ability but more strictly limit would be within RAW to allow you to ascend the rain, but as "climbing difficult terrain", such that if you had 45 feet of movement you would be permitted to ascend 15 feet on your turn.
They could also require you to make a Strength check to push hard enough or fast enough off the falling rain to reach the next drop or to not slip: "At the DM’s option, climbing a slippery vertical surface or one with few handholds requires a successful Strength (Athletics) check. Similarly, gaining any distance in rough water might require a successful Strength (Athletics) check."
Also to note is that this ability works only on your turn, such that as soon as you complete your movement, you begin to fall if you have run up off of the ground.
*@Medix2 "to move across a liquid" may invoke the meaning of across as from one side to another. But across can also mean 'from one thing to another in succession in order to cross something else.' Pitfall Harry crosses waterholes by moving across the heads of the crocodiles, even if he puts only one foot on each head. This use of 'across' could work with raindrops.
