Ladies (sorry, this really doesn't work for the men) - if you commute in office wear, when it is raining hard, wear a skirt. Or rather, take the skirt with you, but wear a cheap pair of waterproof trousers over your tights. This is comfortable enough for a short commute but not as hot and annoying as waterproofs over trousers. In the office just nip into the toilet to change. Do not forget your skirt...
(This may be counterintuitive, but if it is only raining a bit, a skirt is better too as tights dry out more quickly than trousers do.)