The summer season in Fullerton, California lasts for almost three months, from July 3 to September 27. During this time, the average daily high temperature is above 82° F, with August being the hottest month, with an average high of 85° F and a minimum of 66° F. For outdoor activities, the best time to visit Fullerton is from late May to early October, with a peak score in the last week of June. The humidity level in Fullerton remains relatively constant throughout the year, with a 4% muggy, oppressive or miserable comfort level. For activities in warm climates, the best time to visit Fullerton is from mid-July to mid-September, with a maximum score in the third week of August.
The clearest month of the year is September, when the sky is clear, mostly clear or partly cloudy 90% of the time. The cloudiest month is February, when the sky is overcast or cloudy 41% of the time.