Hello, Bob.

Seeing a woolly bear caterpillar on a sub-zero day is a wonder! You’re not the only person from who’ve observed them lately, especially near leaf litter and plant debris (their usual hibernation spots) on dark pavement in the strong sunshine. Is that where you observed it?

Woolly bear caterpillars (Pyrrharctia isabella) have an incredible ability to withstand sub-zero temperatures (some sources indicate -90 degrees F) because they contain glycerol, their own internal antifreeze. They typically hibernate in the winter, but I’m guessing the one you saw was residing near where the sunshine touched and warmed it, awakening it from slumber. They can freeze and thaw several times in a season until finally they pupate, forming a cocoon. 

Interestingly, the long-told myth about the width of their bands being a predictor of winter extremity has not been proven. But I think it would make for an engaging citizen science project or one to do with children.

I applaud you for observing and encourage you to keep your own phenological calendar to track what you notice in our world. 

Sincerely,

Julie

Sources:

https://www.weather.gov/arx/woollybear

https://neycenter.org/winters-fuzziest-survivors-woolly-bear-caterpillars/

https://www.threeriversparks.org/blog/phenology-natures-calendar