Sometimes you’re out and it’ll start to rain. Doesn’t make sense to avoid it, because sometimes you just can’t. Better to be ready for it. It’s done all the time, and like other outdoor elements you can adjust for it (within reason).
Slow down. Drop your speed by a couple miles per hour, more as the rain gets harder.
Stretch the line. Add space between you and other riders, giving yourself more reaction time.
Stay slightly off-line. This helps to avoid wheel spray in your face, and gives you an extra margin of safety if the rider in front of you has to slow down. Since you’re going slower, the draft won’t be as effective anyway, so might as well choose safety over speed.
Clear the brakes. Feather the brakes once in a while (especially if you haven’t braked in a while, or before a large descent), not to slow down but to just clear debris that gathers on the pads. Don’t just squeeze the brakes, as you’ll only end up grinding road grit into the pads and your rims. Press/release lightly several times before you need your brakes.
Take puddles straight-on. Don’t try to turn through a puddle. Take the straightest line you can until you’ve cleared deep water. Try not to brake going through large puddles if possible.
Avoid leaning. Riders tend to lean more than necessary to take turns. Avoid leaning as much to maintain your weight over the bike, in turn maintaining more force on the tires against the road.
Don’t be fooled by light rain. If it’s been raining lightly for a bit, don’t be fooled into thinking “It’s not that bad, I can go faster.” A light rain will mix with oil/grime build-up on the road, giving you less traction than an ice sheet.
Zip-lock Bags. Unless your phone is water resistant (the newer ones tend to be, but I still wouldn’t risk it) place your phone and other non-water-friendly devices in a zip-lock bag before you ride, or at least keep one with you just in case you need it. This is also helpful in the heat, as pouring water over yourself or even lots of sweat can lead you to an early phone upgrade. [Thanks, Steve!]
Lower tire pressure. Drop your tire pressure by 5-10 PSI if it’s likely to rain. If you’re out and maxed-up on tire pressure, open the valve and tap it once or twice (very quickly) to drop the PSI a little bit (I’ve done this several times at home, and each quick tap drops the pressure by about 4-6 PSI typically, depending on your starting pressure; the higher the pressure, the more you’ll lose per tap). [Thanks, Ben!]
Lightening? Take cover. Go back to this mantra: “Don’t be stupid.” No ride is worth your life. On flat, open, rural roads (and we have many of those out here), you’ll be one of the tallest objects around, inviting a strike. Don’t believe the myth of carbon fiber not conducting electricity (it does), or that the rubber on the wheels keep you from getting hit (you can). And just because people survive lightening strikes doesn’t mean you will, or will enjoy it. You survive root canals… doesn’t mean you want one.