Is there a theoretical limit to how hard it can rain? Yes.
Yes. Before it can rain, there must be water vapor in the air that can later condense and fall to the ground as precipitation.
Water is constantly evaporating and condensing, but at higher temperatures, the molecules are more energetic. When the water molecules don’t have a lot of energy, they start clumping into each other, which is how you get condensation. For a given temperature, at some point, there’s a maximum amount of water vapor that can be present before before condensation overtakes evaporation.
We call that point the saturation vapor pressure, or SVP. When you see descriptions of relative humidity, what they’re referring to is how large the current vapor pressure is to this maximum. For example, if you see that the relative humidity is 80%, that means that you’re at 80% of the pressure where condensation would begin to happen.
When the temperature drops, the rate of evaporation begins to slow. The excess condensation falls to the ground as rain, snow, or another form of water depending on temperature, pressure, and wind conditions. That is the underlying reason why cold air masses hitting warm air masses cause rain.
So the theoretical limit of how hard it could rain would be a hypothetical extremely hot air mass at equilibrium with water vapor meeting another hypothetical extremely cold air mass, also at equilibrium with water vapor. When the two collide, the water vapor at the boundary falls as rain.
The hottest temperature ever recorded on Earth in recent times is about 130 F / 55 C. Likewise, the coldest temperature is -115 F / -83 C. Imagine two air masses (H and C respectively) at these temperatures colliding. How much water will condense?
Well, the vapor density of water at low temperatures is effectively zero, so C doesn’t hold much water at all. H could hold about 120 g of water per cubic meter. If the two air masses were perfect 1 km wide, 1 km long, 4 km high columns that suddenly rammed into each other, then about 480,000 metric tons of water would suddenly rain down on you over a 1 km2 area in a few seconds.
This is roughly equivalent to about half a meter of water materializing in the atmosphere and blanketing a 1 km2area. It is likely that you would be knocked out (in which case you would die shortly thereafter from drowning). It would be like standing in a waterfall, except that the waterfall is everywhere.
Update: Just wanted to reiterate that this is purely a theoretical scenario. In reality, two air masses that had such radically different temperatures would not mix well, so they couldn’t discharge their water near-instantaneously, in the way that my deadly-waterfall does above. Also, the column of air is so high that all the water in it couldn’t leave at once.
As a weaker version of this, try opening your car door on a hot day after it’s been out in the sun for a while, and look at the ground near the edge of the shadow of your vehicle’s open door. You’ll see distortions in the air leaving your car — that’s the very hot air from your car trying to mix with the cooler ambient air outside. Here the air has a difference of perhaps 25 or 30 degrees F (maybe it’s 90 outside and 120 in your car), and it already has trouble mixing.
via jxf comments on Is there a theoretical limit to how hard it can rain?.
