Weather and climate in Foster City, CA
What the weather is like in Foster City
The warmest month in Foster City is September, with average highs of 66°F. The coolest is January, when nights average 47°F. About 20.3 in of rain and snow falls in a typical year, most in December and least in July.
Skies are clearest in October (44% average cloud cover) and cloudiest in December (59%). Combining cloud with the length of truly dark nights, October, November, September offer the most clear, dark hours for stargazing.
Clear, dark hours per night
Hours of full astronomical darkness in mid-month, multiplied by the average share of clear sky. A rough guide to how much good observing time a typical night offers.
Monthly averages
| Month | High | Low | Precipitation | Cloud cover | Humidity | Wind | Dark hours |
|---|---|---|---|---|---|---|---|
| January | 55°F | 47°F | 3.5 in | 58% | 84% | 8 mph | 11.1 h |
| February | 56°F | 48°F | 3.4 in | 58% | 83% | 8 mph | 10.2 h |
| March | 57°F | 49°F | 2.9 in | 58% | 84% | 8 mph | 9.1 h |
| April | 58°F | 49°F | 1.5 in | 53% | 83% | 9 mph | 7.7 h |
| May | 59°F | 51°F | 0.7 in | 51% | 85% | 9 mph | 6.3 h |
| June | 61°F | 53°F | 0.2 in | 44% | 85% | 9 mph | 5.4 h |
| July | 63°F | 55°F | 0.1 in | 56% | 86% | 7 mph | 5.9 h |
| August | 64°F | 56°F | 0.1 in | 57% | 86% | 7 mph | 7.2 h |
| September | 66°F | 56°F | 0.2 in | 46% | 83% | 7 mph | 8.6 h |
| October | 65°F | 54°F | 1.0 in | 44% | 79% | 6 mph | 9.9 h |
| November | 60°F | 51°F | 2.2 in | 50% | 80% | 7 mph | 10.8 h |
| December | 56°F | 48°F | 4.5 in | 59% | 82% | 8 mph | 11.3 h |
Climate averages: NASA POWER, 2001–2025, for the grid area around 37.50, -122.50 (about 50 km across). High and low temperatures are estimated from the monthly mean and the average daily range. Hills, coasts and cities can differ from the area average. Sources and methods.