Here are a pair of convertible mittens that I bought last year. They must be popular because they raised the price of them since I bought them. But they're still an excellent cold weather mitten for riding in frigid temps. At 20f, your hands will be toasty warm. At 5f, you'll be grinning to yourself at how warm your hands are. Great versatility, excellent venting and finger dexterity, the gauntlet is easy to use one-handed. And they even have a pocket for handwarmers. Good design and well made.
http://www.onlygloves.com/winter-glo...yaccessmit.cfm