How Many 90°F Days San Jose Gets, 1999 to 2025

Home › Research and Data › San Jose hot days

By Cool Aid A/C & Refrigeration · Published November 12, 2026 · Data: NOAA National Centers for Environmental Information (GHCN-Daily), 1999 to 2025

San Jose’s airport weather station recorded an average of 20.4 days a year at 90°F or hotter from 2016 to 2025 (complete years; 2018 left out), up from 15.0 a year from 1999 to 2008. The rise is uneven: 2024 had 39 such days, the most of any year from 1999 to 2025, while 2005 had 4. Swings from one year to the next are larger than the rise, and over 1999 to 2025 the counts do not show a statistically clear upward trend.

90°F days at San Jose’s airport, year by year

NOAA keeps a daily record of the high temperature at San Jose International Airport. We counted the days each year when the high reached 90°F or more, and 100°F or more. Across the 25 complete years from 1999 to 2025, San Jose averaged 17.0 days a year at 90°F or hotter and 1.2 at 100°F or hotter.

A hotter decade, but not a steady climb

Comparing the two ends of the period, San Jose averaged 15.0 days at 90°F or hotter a year from 1999 to 2008 and 20.4 from 2016 to 2025. Days at 100°F or hotter went from 0.7 to 2.2 a year. The later decade mixes very hot years, such as 2024 (39 days at 90°F or hotter) and 2020 (32), with mild ones, such as 2021 (8) and 2025 (10).

A straight line through all 25 complete years rises about 2.6 days a decade, but neither statistical test we ran finds that rise clear (p = 0.20 for the line and 0.50 for a Mann-Kendall rank test; the usual bar is 0.05). Put simply, the counts are higher in the later decade, but they swing so much from one year to the next that a rise this size could come from chance alone. Livermore’s rise, about 5.5 days a decade, is clearer: it passes the least-squares test (p = 0.037) and just misses on the rank test (p = 0.055).

September brings the most 90°F days in San Jose

Of the 426 days at 90°F or hotter at San Jose’s airport in the complete years, 119 (27.9%) came in September, more than in August (86) or any other month, and 40 more came in October. Inland at Livermore the peak is earlier: July had the most (25.4% of its 90°F days).

Inland, far more hot days

Livermore’s airport, inland in Alameda County, averaged 59.1 days a year at 90°F or hotter from 2016 to 2025, about 2.9 times San Jose’s 20.4, and 13.2 days at 100°F or hotter against San Jose’s 2.2. Its most in one year was 85 days at 90°F or hotter, in 2020, and 2024 brought 28 days at 100°F or hotter, the most of any year from 1999 to 2025. Even its coolest year, 2010, had 37.

What this means for Bay Area homes and businesses

• Plan for September, not only July. At San Jose’s airport, September brought more 90°F days than any other month, and October still had 40 over the complete years. Have air conditioning and refrigeration serviced before the heat, in spring, and do not treat the end of August as the end of the cooling season. Our summer HVAC prep guide covers what to check.
• Hot days are when cooling equipment works hardest. An air conditioner or a refrigeration condensing unit sheds heat into the outdoor air, and the hotter that air, the longer and harder it has to run. A dirty condenser coil makes it worse. Santa Clara County food inspectors also cited food held at the wrong temperature more often in summer than in winter; see our cold-holding study.
• Inland sites get far more heat. Livermore’s airport logged about 2.9 times as many 90°F days as San Jose’s from 2016 to 2025. A system that copes in San Jose can be pushed much harder inland.
• One mild summer says little. San Jose went from 39 days at 90°F or hotter in 2024 to 10 in 2025. A cool year does not mean the next will be cool, so a repair put off because last summer was easy can still be needed this year. See our AC maintenance plans.

This study describes weather records. It does not forecast future summers.

Method

Data. Daily maximum temperatures from NOAA’s Global Historical Climatology Network daily (GHCN-Daily) for two stations: San Jose International Airport (USW00023293) and Livermore Municipal Airport (USW00023285). We downloaded them from NOAA’s National Centers for Environmental Information on November 12, 2026 and keep a dated copy. The San Jose airport record in GHCN-Daily starts on July 4, 1998, so the study starts with 1999, the first full year, and ends with 2025, the last full year.

What we counted. Days each calendar year with a daily high of 90°F or more, and of 100°F or more, as the temperatures are published (whole degrees Fahrenheit). We did not adjust any values.

Complete years. Every 90°F day at both stations in these years fell between April and October, so a year counts as complete when no more than 5 days of April to October readings are missing. At San Jose, 2012 and 2018 fall short. They are shown in the chart and table, marked with an asterisk, and left out of every average, the trend, the month totals and the most and fewest. Livermore has no incomplete years.

Averages and trend. We compare the averages for 1999 to 2008 and 2016 to 2025. For the trend we fitted a least-squares straight line of days per year against year and ran a Mann-Kendall rank test, using complete years only. A p-value says how often a trend at least this strong would turn up by chance if there were no real trend; under 0.05 is the usual bar.

Independent check. To check our counts, we asked a separate service, the Applied Climate Information System (ACIS) run by NOAA’s Regional Climate Centers, to count the same days for the same stations. Every yearly count at both stations, the set of complete years, every month total, and each average, most, fewest, trend figure and peak month on this page matched.

Sources

• Daily maximum temperature, San Jose International Airport (station USW00023293), NOAA National Centers for Environmental Information, GHCN-Daily, downloaded November 12, 2026: www.ncei.noaa.gov/cdo-web/datasets/GHCND/stations/GHCND:USW00023293/detail
• Daily maximum temperature, Livermore Municipal Airport (station USW00023285), NOAA National Centers for Environmental Information, GHCN-Daily, downloaded November 12, 2026: www.ncei.noaa.gov/cdo-web/datasets/GHCND/stations/GHCND:USW00023285/detail
• Global Historical Climatology Network daily (GHCN-Daily), NOAA National Centers for Environmental Information: www.ncei.noaa.gov/products/land-based-station/global-historical-climatology-network-daily
• Independent check: Applied Climate Information System (ACIS) web services, NOAA Regional Climate Centers: www.rcc-acis.org/docs_webservices.html

Limits

• Each place is one airport weather station. Temperatures vary across a city, so other neighborhoods can see more or fewer hot days than the airport.
• 27 years is a short window for climate. The averages and the trend describe these years; they do not forecast the next ten.
• A station’s instruments or exact site can change over the years, which can shift its readings. We used the values as published and did not adjust for that.
• NOAA runs quality checks and can revise past values. This page uses the data as downloaded on November 12, 2026.

More research from Cool Aid A/C & Refrigeration