Worms

I have a very serious question regarding worms. I have wondered this probably since I was in 2nd grade. I was in Mrs. Arnold's class and our classroom was on the far end of the school. To get to the bus we would walk around the outside of the school. There were wide sidewalks around the school. Whenever it would rain I had to dodge all of the worms on the sidewalk. I hated seeing those ugly gooey worms. I especially hated the idea of a worm being stuck to the bottom of my shoe.
The other day I was running from my car to the office because it was raining and I noticed that the parking lot had a ton of worms. As I dodged them- I asked myself again how do those worms get stuck on the cement? I could see if it was dirt they might come to the surface or something but how do they get all over the parking lot? Do they come from the sky? And why are they only there on rainy days?