San Ysidro (CA) Temperature by Month

The average annual maximum temperature in San Ysidro, California, United States of America is 23°C (73°F), with daytime highs ranging from 19°C (66°F) in December to 27°C (81°F) in September. This page covers monthly averages, day-night differences, and how San Ysidro compares to cities worldwide.

San Ysidro experiences balanced seasonal shifts, with noticeable but moderate temperature variations. At night, minimum temperatures range from 17°C (63°F) in September to 8°C (46°F) in December.

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

The minimum temperature is often recorded between 4 AM and 6 AM, while the highest temperature is usually reached at 3 PM, when the sun's heating effect is strongest. September, the warmest month, gets 272 hours of sunshine.

The chart below shows the average temperature throughout the year:

San Ysidro's average annual maximum temperature is 23°C (73°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.

Beijing, China averages 20°C (68°F) annually, but with big seasonal swings, very cold winters and hot summers.

Melbourne, Australia averages 20°C (68°F) annually, known for unpredictable weather, with four seasons sometimes happening in one day.

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 San Ysidro, December is the coolest month, with average highs of 19°C (66°F) and humidity around 69%, considered high. In September, the warmest month, temperatures average 27°C (81°F) with 79% 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.

Whether a city sits on the coast or deep inland makes a significant difference to its climate. Coastal areas tend to have more stable temperatures year-round, large bodies of water absorb heat slowly in summer and release it gradually in winter, keeping extremes in check. Cities far from the sea don't benefit from that buffer, which is why continental climates tend to have hotter summers and colder winters than their coastal counterparts at the same latitude.

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