Thank you, National Weather Service

Their forecast was correct: a cold front has pushed into the Chicago area, dropping temperatures rapidly and turning our sunny morning into a cold and grey last day of autumn. At 11:30 it was 11°C outside my door; now, just over 2 hours later, it's 4°C, and getting cooler yet.

This will not stop me from walking another few kilometers, though. It's still above freezing, after all.