Temperatures in Belize during December generally vary between an average high during the day of 80.0 degrees and an average low at night of 71.0 degrees.On any given day during December in Belize, you can expect early morning temperatures to be around 70.3 degrees.Temperatures in Belize typically rise during the day, to an average of 80.4 degrees in the early afternoon.By the early evening hours in December the temperature will generally fall to somewhere in the vicinity of 75.9 degrees.Temperatures in Belize average around 75.3 degrees during December.y the early evening hours in December the temperature will generally fall to somewhere in the vicinity of 75.9 degrees. Temperatures in Belize average around 75.3 degrees during December.