God indeed has a sense of humor. From Weather Underground:
Here’s the actual METAR data, showing 10°C (50.0°F):
According to the Mexican National Meteorological Service, the record low temp for December is 12°C (53.6F) see table here: Cancun_climate (PDF)
According to this page lowest for December is 52°F (11°C)
Either way, 50°F is a new record for December low temperature, and it couldn’t happen at a more inconvenient time for the Cancun COP16 climate conference.
h/t to Ross Hays