Everyone loves a sunny day. But for taxi drivers, there’s now a reason to enjoy them even more: Sunshine drives up cab tips, according to research published this month in PLOS ONE. The study tracked weather and nearly 14 million taxi rides in New York City over a 10-month period and found that a shift from a dark sky to full sunshine was linked to up to a 0.7% increase in tipping. That’s no big deal for one ride, but over dozens of trips a day, it adds up—accounting for an estimated daily increase of $17,466 in cab tips citywide. With taxi trips being a fairly standardized experience (unlike restaurants, which range dramatically in food quality and atmosphere), the researchers argue that taxicabs are a great model for understanding general tipping behavior, a practice that makes up billions of dollars of the U.S. economy. Researchers couldn’t definitively say why sunshine drove up tips, but suggested it was linked to better moods, which have previously been shown to improve with sunlight. The take-home message for cab drivers? Combat gray skies with your own sunny disposition, the researchers say, to help beat that dip in tip.