Union, CA climate
Union has warm winters and pleasant summers, with very little rain. A typical July afternoon reaches 83°, a January night drops to 40°, and snow is almost unheard of.
NOAA 1991-2020 climate normals Updated once a decadeWhat a year in Union feels like
Winter
Spring
Summer
Fall
Union weather by month
Average highs, lows, rain and snow for every month, from NOAA’s 1991–2020 normals.
| Month | High | Low | Rain + melt | Snow | Wet days | Snow on ground | 30-yr range |
|---|---|---|---|---|---|---|---|
| January | 59° | 40° | 3.6" | 0 | 9 | 0 | 46° – 53° |
| February | 62° | 43° | 3.6" | 0 | 9 | 0 | 48° – 57° |
| March | 66° | 45° | 2.8" | 0 | 9 | 0 | 50° – 61° |
| April | 70° | 46° | 1.4" | 0 | 5 | 0 | 54° – 62° |
| May | 75° | 50° | 0.7" | 0 | 3 | 0 | 59° – 67° |
| June | 80° | 54° | 0.1" | 0 | 1 | 0 | 63° – 69° |
| July | 83° | 56° | 0.0" | 0 | 0 | 0 | 67° – 73° |
| August | 83° | 56° | 0.0" | 0 | 0 | 0 | 67° – 74° |
| September | 82° | 55° | 0.1" | 0 | 1 | 0 | 65° – 72° |
| October | 76° | 51° | 0.9" | 0 | 3 | 0 | 61° – 70° |
| November | 66° | 44° | 2.0" | 0 | 6 | 0 | 49° – 59° |
| December | 58° | 40° | 3.7" | 0 | 9 | 0 | 46° – 54° |
| Year | 72° | 48° | 18.9" | 0.0" | 55 | 0 |
“Wet days” have at least 0.01" of rain or melted snow. “Snow on ground” counts days with 1" or more. The 30-year range is the coldest and warmest that month’s average temperature got between 1991 and 2020.
Rain and snow in Union
Union gets 19 inches of rain and melted snow a year. December is the wettest month and July the driest. Rain or snow falls on about 55 days a year.
A year-round growing season
Heating and cooling a home in Union
Engineers use “degree days” to estimate how hard a furnace or air conditioner works. In Union, 84% of that work is heating and 16% is cooling, so the furnace matters far more than the air conditioner.
Degree days measure how far each day’s average temperature falls below or rises above 65°, added up over a year. Source: NOAA station normals.
Union and its neighbors
Union borders 2 towns with climate data. January nights across them vary by 1.6°. Fremont has both the coldest January nights (41°) and the hottest July afternoons (82°). Union’s January low is 40° and its July high 83°.
Temperatures and rain come from NOAA’s 5 km grid, so each town gets its own value.
How Union compares
Compared with every U.S. town and city on TownTapestry, each counted once.
Warmer than it used to be
The last 30 years in Union averaged 1.6° warmer than the 1900s. The biggest change is in January (+2.2°). The shift is larger than in 75% of U.S. towns.
Change by month1991–2020 average minus 1901–2000 average, at Union’s spot on NOAA’s grid
How much a month can swing
A normal is an average, and real years wander around it. March swings the most: between 1991 and 2020 its average temperature ranged from 50° to 61°. June is the most reliable, staying within 6°.
Common questions about Union’s climate
What is the coldest month in Union?
December, with a typical high of 58° and a low of 40°.
What is the hottest month in Union?
July, with a typical high of 83° and a low of 56°. About 18 days a year reach 90°.
How much snow does Union get?
Almost none. A normal year brings 0.0 inches.
When is the first frost in Union?
Frost is uncommon. Only 88% of years see a night at 32° or below.
What is the rainiest month in Union?
December, with 3.7 inches of rain and melted snow. The year totals 19 inches.
Temperatures and rainfall come from NOAA’s gridded climate normals for Union itself (about a 3-mile square). Snow, frost dates and day counts come from NOAA’s weather station at Redwood City, CA (15 mi) away. Normals are 30-year averages, not a forecast. Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000.