Foster City (CA) Temperature by Month
Foster City in California, United States of America sees moderate seasonal temperature shifts, with daytime highs between 14°C (57°F) in December and 24°C (75°F) in September, averaging 20°C (68°F) annually. Explore the full monthly breakdown below.
Foster City Monthly Temperatures
In Foster City, seasonal changes bring about a moderate variation in temperatures. Nighttime lows range from 13°C (55°F) in September to 6°C (43°F) in December.
The chart below illustrates the average maximum day and minimum night temperatures in Foster City by month:
The coolest part of the day is typically between 4 AM and 6 AM, while 3 PM is usually the warmest, when solar heating is at its peak. September, the city's warmest month, averages 270 hours of sunshine.
The chart below shows the average temperature throughout the year:
Temperature: Foster City vs the United States of America
The map below shows the annual temperature across the United States of America. You can also select individual months if you want to compare a specific time of year.
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Foster City vs World: Temperature Compared
Foster City's average annual maximum temperature is 20°C (68°F). To put that in context, here's how it compares to a few well-known destinations:
Rome, Italy averages 20°C (68°F) annually, with reliably warm summers and comfortable winters.
Queenstown, New Zealand averages 10°C (50°F) annually — remember seasons are flipped, so its coldest months fall in June and July.
Shanghai, China averages 21°C (70°F) a year, with warm summers, mild winters, and a noticeable spring and autumn.
Adelaide, Australia averages 21°C (70°F) a year, with warm summers, mild winters, and relatively low rainfall year-round.
What Does the Temperature Feel Like in Foster City?
Temperature alone doesn't tell the whole story — humidity plays a big role in how warm or cold it actually feels. High humidity in summer makes the heat feel more intense, particularly once temperatures climb above 25°C. In winter, the same humidity can make cold air feel sharper than the thermometer suggests.
In Foster City, December is the coolest month, with average highs of 14°C (57°F) and humidity around 78% — considered high. In September, the warmest month, temperatures average 24°C (75°F) with 73% humidity — conditions that feel high. For a full picture, see our humidity page.
Climate temperature data is typically calculated as a 30-year average. This smooths out year-to-year variability and gives a more reliable picture of what a place is actually like, rather than what happened in any single unusual year.
The readings come from a range of sources — land-based weather stations, ocean buoys, ships, and satellites. That data is collected by weather services around the world, then pooled, quality-checked, and averaged to produce the climate records you see here.
Global average temperatures have risen by around 1.2°C since the pre-industrial era, and the effects are visible across many regions. Winters are milder on average, with fewer frost days and less snow in many parts of the world. Heatwaves are more frequent and more intense, and Europe's summers of 2018, 2019, and 2020 all set records.
Summers are also getting drier in some areas, while winter rainfall has increased in others. This contributies to higher river levels and more flooding. In many countries, spring arrives earlier and autumn lasts longer. It has knock-on effects for wildlife, agriculture, and local ecosystems.
For more on Foster City's weather — including monthly rainfall, sunshine hours, and humidity — visit our Foster City climate page.
