Every since man moved inside from yurts, teepees, huts, etc we tend to look outside and if we see rain, assume it does so all day. I see this all the time being a cyclist. I commute to the office almost every day (except in temps below -10C) people often wonder how I put up with the weather. My resposne goes like this:
"Its pouring rain."
"No,... it rained,... a bit,... this morning but if you go outside now its dry'
"oh, gee,... will you look at that"
<< in fact,... it just happened as I was typing this>>
The fact is it rains in temperate climates every few days, even if just for a minutes. I can't tell you how many clouds I've passed under on 100km+ rides that dropp rain for ten minutes and then disperse. Dump a bunch of water on a field for 10 minutes and then march 10,000 guys across said field and you will have a morass in no time.