Last updated: June 24, 2025
In Fullerton, United States, the average temperature in December hovers around -1°C (31°F). You might experience a chilly minimum of -22°C (-7°F) and a milder maximum of 20°C (68°F). Expect about 42 mm (1.6 in) of precipitation spread over roughly 4 days, with an average humidity level of 79%.
In December in Fullerton, the average temperature is -1°C (31°F), with maximums reaching 20°C (68°F) and minimums plummeting to -22°C (-7°F). This wide temperature range makes it essential to pack layers and be prepared for both chilly and warmer spells while traveling.
In Fullerton in December, the precipitation amount is 42 mm (1.6 in) spread over a period of 4 days.
In Fullerton in December humidity is 79%. With an average temperature of -1°C (31°F), this humidity feels like a heavy, damp blanket wrapping around you, making outdoor adventures challenging as the cold moisture seeps into your clothes, leaving you feeling chilled to the bone.
In Fullerton in December, the maximum UV index is 3, which corresponds to a moderate Exposure Category. The time to burn is approximately 30 minutes. For more details on the hourly UV index in Fullerton, you can check the information here.
UV Risk Categories
Weather in Fullerton varies notably across the year, with each month offering distinct climate conditions. This page compares December’s weather to other months in Fullerton, focusing on differences in temperature, rainfall, humidity, and UV levels.
| Temperature | Precipitation | Humidity | UV | |
|---|---|---|---|---|
| January Weather | -4°C (26°F) | 28 mm (1.1 inches) | 73% | moderate |
| February Weather | -4°C (25°F) | 20 mm (0.8 inches) | 81% | moderate |
| March Weather | 4°C (40°F) | 89 mm (3.5 inches) | 69% | high |
| April Weather | 9°C (49°F) | 57 mm (2.2 inches) | 71% | very high |
| May Weather | 16°C (61°F) | 142 mm (5.6 inches) | 80% | very high |
| June Weather | 24°C (75°F) | 97 mm (3.8 inches) | 83% | extreme |
| July Weather | 25°C (78°F) | 65 mm (2.5 inches) | 84% | very high |
| August Weather | 23°C (74°F) | 84 mm (3.3 inches) | 84% | very high |
| September Weather | 20°C (69°F) | 57 mm (2.2 inches) | 77% | very high |
| October Weather | 11°C (52°F) | 83 mm (3.3 inches) | 72% | high |
| November Weather | 4°C (40°F) | 22 mm (0.9 inches) | 80% | moderate |
| December Weather | -1°C (31°F) | 42 mm (1.6 inches) | 79% | moderate |