1 The annual average temperature in Belize City, Belize is hot at 25.8 degrees Celsius (78.4 degrees Fahrenheit).2  Average monthly temperatures have a variation of 4.5 C (8.1 F) which is an extremely low range. 3  The range/ variation of daily mean temperatures is 7.3 C (13.2 F). The annual average temperature in Belize City, Belize is hot at 25.8 degrees Celsius (78.4 degrees Fahrenheit).