July in Cayo Guillermo is the cloudiest month of the year. In July, the weather in Cayo Guillermo is typically hot. Average daily high temperature in July is about 28 degrees above zero. On average, Cayo Guillermo has 16 rainy, 8 partly cloudy, 5 sunny, and 2 cloudy days in July. The rains are usually moderate. July as well as June, August, September, and October is considered the most uncomfortable time to visit Cayo Guillermo.