This is the Neopixel Giant Thermometer Scarf created by Instructables user caitlinsdad. It uses a digital thermometer, Arduino controller and strip of LEDs to display the current temperature. It also has a couple different cool light-up effects. Of course, I don't need a thermometer scarf to know what temperature it is. You know why? Because it's always 72°F. "Because you live in southern California?" What? No. Because that's what I have the thermostat set to and I never leave the house.
Keep going for a short video of the scarf doing its thing (its thing being telling the current temperature). One time i held a meat thermometer in my armpit.
Thanks again to Suede, who agrees wearing a scarf is temperature indicator enough: it's cold and you should bundle up.