Question: An odd discussion came up on a role-playing game forum recently. Some people were speculating about how a mediaeval society would be different if it had wizards, with the spells described in the game rules. One of the spells creates heavy rain - the rules specify 1 inch per hour, over a certain area. Somebody was commenting whether or not this spell would have a big effect on agriculture, and added "unless the crops can't take an inch of rain in an hour, in which case we're looking at this spell all wrong".

This turns out to be surprisingly difficult to look up. Maybe I'm looking in the wrong places, but I'm having difficulty finding out how commonplace one inch of rainfall in an hour is or what it actually looks like in practice. A lot of information about rainfall averaged over a month, and the Met Office classify rainfall greater than 4 mm per hour as "heavy", but don't say how common "heavy rain" according to this criterion actually is.

What does 1 inch of rain per hour mean in practice, how often does that much rain or more happen (say, in the UK), and what would it do to a field of crops?

Answer: 
Just one inch of rain in one hour, and that's it, or an inch of rain per hour that is sustained over multiple days? Keep in mind that rainfall rates of over an inch of rain *per minute* have been observed. On the other hand, one inch of rain per hour for two or three straight days, nonstop -- been there, done that, don't want to do it again.

Also, time of year is important. If that semi-heavy rainfall (I would not call a rainfall that drops one inch of rain over the course of an hour and then stops a heavy rainfall) falls right after farmers plant their fields or right before they were planning on harvesting, that might be damaging. Farmers expect their seeds to not wash away, and they depend on plants drying out prior to harvest.
There are crops that do require periods with little or no rain to be successful. A wheat crop for example can be ruined or degraded by untimely rainfall; dry conditions are needed for the wheat seeds to mature and the wheat heads need to be dry for harvesting.

Whilst not affecting plant growth, hay crops can be ruined by rainfall between the cutting and baling - it needs time without rain to dry out.