Vallejo (CA) Temperature by Month

Vallejo, California, United States of America has an average annual maximum temperature of 21°C (70°F), ranging from 14°C (57°F) in December to 27°C (81°F) in September. Below you'll find a full monthly breakdown and a comparison with cities worldwide.

With significant temperature fluctuations, Vallejo enjoys distinct seasons year-round. Nighttime lows range from 12°C (54°F) in September to 4°C (39°F) in December.

The chart below illustrates the average maximum day and minimum night temperatures in Vallejo by month:

Temperatures tend to bottom out between 4 AM and 6 AM, then climb to their daily peak around 3 PM. September, the warmest month, sees 270 hours of sunshine.

The chart below shows the average temperature throughout the year:

Vallejo's average annual maximum temperature is 21°C (70°F). To put that in context, here's how it compares to a few well-known destinations:

Lisbon, Portugal averages 21°C (70°F) annually, warm summers, mild winters, and rain mainly in the cooler months.

Reykjavík, Iceland averages 9°C (48°F) a year, mild summers by Icelandic standards, but cold winters and frequent wind.

Osaka, Japan averages 22°C (72°F) annually, with hot humid summers, mild winters, and pleasant spring and autumn seasons.

Adelaide, Australia averages 21°C (70°F) a year, with warm summers, mild winters, and relatively low rainfall year-round.

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 Vallejo, 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 27°C (81°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.

For cities and regions with significant elevation, altitude is one of the biggest factors shaping local temperatures. As a rule of thumb, temperatures fall by around 6°C for every 1,000 metres gained, so a city at 2,000 metres will typically be around 12°C cooler than a city at sea level in the same region. Higher ground also tends to see more dramatic day-to-night temperature swings, since thinner air loses heat faster after sunset.

For more on Vallejo's weather, including monthly rainfall, sunshine hours, and humidity, visit our Vallejo climate page.