If you manage a company that has employees working outside, such as delivery drivers, you want them to look professional and have nice outfits. That usually means long trousers, even in a relatively casual environment. But if it tends to rain a lot at your worksite, you might want to encourage outdoor workers to wear shorts. This has both practical and health reasons behind it.
It's simply easier to dry off bare legs and change socks instead of struggling out of completely wet trousers or slacks. If someone is working outside in the rain, and they're wearing long work pants, those pants will be soaked. If the person doesn't have many chances to change or go inside, he or she would have to stay in the soaked pants for the entire time. With shorts, a quick wipe down once the person goes inside a structure is all he or she needs to dry off. And it's much easier to stash clean, dry socks at a work desk than it is to keep several pairs of spare trousers.
If the temperature outside is cool -- not necessarily cold, but just cool -- the person's legs can begin to feel cold when surrounded by wet cloth, especially cotton or its many variations like denim. Even at temperatures above freezing, that can put the person at risk of developing hypothermia.
The pants won't actually shrink on the person and trap them, despite what some urban legends might have you believe, at least about denim. But without the insulating factor of dry material, legs can really become too cold.
Shorts don't always seem like the most professional way to dress, but a lot of the success regarding what you wear is about attitude. All you and your employees have to say is that it's easier to dry off skin than it is to dry off trouser cloth when you've been working in rain, and they shouldn't get any more questions about it. If it will help, you might want to check about having your company's logo stitched into the shorts so they look more official.
If you'd like to see the shorts options available for workers, check out online clothing sites that have work pants for sale online. They should have a selection of short pants that will be fine for rainy days. Give your workers a few options that fit in with the rest of your company's dress code. And remind them to keep a stash of dry socks once the rainy season starts.