Thanks to OLN I've been following the Tour this year and noticed that the riders have some type of cover over their shoes. What is the purpose of the thing? It looks kinda like a cover a scuba diver would wear. Is it for aerodynamics, weather protection, or something else?

BTW they zoomed in on one riders rear wheel today and good lord thats a small cassette. I'm pleased if I can maintain 20 MPH I can't imagine maintaining 30+, and they all look like they have such scrawny legs too.