January Weather in Fullerton, United States

Last updated: June 24, 2025

In January, the average temperature in Fullerton, United States hovers around -4°C (26°F). Expect minimums plunging to -25°C (-13°F) and maximums reaching up to 15°C (59°F). With an average precipitation amount of 28 mm (1.1 in) spread over 4 days, the weather remains quite humid, with an average humidity level of 73%.

January Temperature in Fullerton

In January in Fullerton, the average temperature is -4°C (26°F), with maximums reaching 15°C (59°F) and minimums dropping as low as -25°C (-13°F). Be sure to pack warm clothing and prepare for a range of temperatures if you're planning to travel during this month.

January Weather in Fullerton: Daily Temperature Highs, Lows, and Averages

The chart illustrates January temperature patterns in Fullerton, showcasing daily minimum, maximum, and average temperatures. It highlights the typical range of temperatures throughout the month, providing insights into weather trends. In Fullerton, January typically has an average temperature of -4°C (26°F), with highs reaching up to 15°C (59°F) and lows dropping to around -25°C (-13°F).

Data source: Temperature data for Fullerton generated using information from the Copernicus Climate Change Service.

January Precipitation in Fullerton

In Fullerton in January, the precipitation amount is 28 mm (1.1 in) over the course of 4 days.

January Precipitation Trends in Fullerton

The chart displays monthly precipitation patterns in Fullerton, with values for each month alongside January’s data, which is highlighted for easy comparison. It shows the amount of precipitation, providing insights into how January’s conditions compare to those of other months. In Fullerton, January typically receives around 28 mm (1.1 inches) of precipitation spread over approximately 4 days.

Data source: Precipitation data for January generated using information from the Copernicus Climate Change Service.

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Millimeters (mm) Inches (in)

January Humidity in Fullerton

In Fullerton in January humidity is 73%. With an average temperature of -4°C (26°F), this humidity feels like a biting chill that cuts through layers of clothing, making outdoor activities less enjoyable and travel quite uncomfortable.

January Average Humidity in Fullerton

This chart illustrates the average monthly humidity levels in Fullerton, with January’s data highlighted for easy comparison. It shows the fluctuation in humidity throughout the year, helping you understand how conditions vary by season. In January, average humidity in Fullerton is approximately 73%, offering insight into how humid the weather typically is during this month.

Data source: Humidity data for January generated using information from the Copernicus Climate Change Service.

January UV Index in Fullerton

In Fullerton in January, 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.

Maximum UV Index Levels in Fullerton for January

The chart shows the monthly maximum UV Index (UVI) for Fullerton in January, representing the highest UV radiation level. The UVI measures the intensity of ultraviolet radiation from the sun, indicating the potential risk of sunburn and UV-related health issues. In January, the maximum UV index in Fullerton typically reaches 3, which falls under the moderate exposure category.

Data source: Fullerton January UVI data generated using information from the Copernicus Climate Change Service.

UV Risk Categories

Extreme (11+): Avoid the sun, stay in shade.
Very High (8-10): Limit sun exposure.
High (6-7): Use SPF 30+ and protective clothing.
Moderate (3-5): Midday shade recommended.
Low (0-2): No protection needed.

How January Weather in Fullerton Compares to Other Months

Weather in Fullerton varies notably across the year, with each month offering distinct climate conditions. This page compares January’s weather to other months in Fullerton, focusing on differences in temperature, rainfall, humidity, and UV levels.

This table compares monthly weather conditions in Fullerton, showing how January’s temperature, precipitation, humidity, and UV index levels differ from those of other months.
	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