UV and heat in San Juan
In San Juan, the average daily peak UV index stays at 3 or above in every month of the year, from 6.6 in December to 10.6 in August.
The warmest month is September, with average highs of 32°C, and the air is most humid in November, at around 81% relative humidity. UV figures come from five years of NASA satellite data, 2020 to 2024, and temperatures from the nearest long-running weather station.
- 10.6Average daily peak UV in August, very high
- 12Months with an average peak of UV 3 or more
- 32°CAverage high in September
- 6.9 hA day at UV 3 or more in August, on average
When is the UV index highest in San Juan?
The sun is strongest all year round, with the highest average peak in August. In August, about 96% of days reach a UV index of 8 or more, which the World Health Organization calls very high.
How hot and humid does San Juan get?
Average highs range from 29°C to 32°C through the year. Relative humidity averages 73% to 81%. Day length ranges from 11.0 to 13.2 hours.
Month by month
| Month | Peak UV | Days UV 8+ | Hours UV 3+ | High | Low | Humidity | Daylight |
|---|---|---|---|---|---|---|---|
| January | 7.1 | 30% | 5.5 | 29°C | 23°C | 76% | 11.1 h |
| February | 8.0 | 76% | 6.5 | 29°C | 23°C | 75% | 11.6 h |
| March | 9.7 | 96% | 7.0 | 29°C | 23°C | 73% | 12.0 h |
| April | 10.4 | 97% | 7.0 | 30°C | 24°C | 74% | 12.6 h |
| May | 10.3 | 96% | 7.4 | 31°C | 25°C | 76% | 13.0 h |
| June | 10.3 | 97% | 7.3 | 32°C | 26°C | 77% | 13.2 h |
| July | 10.1 | 95% | 7.0 | 32°C | 26°C | 79% | 13.1 h |
| August | 10.6 | 96% | 6.9 | 32°C | 26°C | 79% | 12.7 h |
| September | 10.0 | 98% | 6.9 | 32°C | 26°C | 79% | 12.2 h |
| October | 8.3 | 80% | 6.4 | 32°C | 26°C | 79% | 11.7 h |
| November | 7.0 | 25% | 5.7 | 31°C | 24°C | 81% | 11.3 h |
| December | 6.6 | 1% | 5.3 | 29°C | 24°C | 78% | 11.0 h |
When should you protect your skin in San Juan?
The World Health Organization advises sun protection whenever the UV index is 3 or above, and the advice gets stronger from 8. In San Juan, that means every month of the year, for about 6.9 hours around midday in August. The forecast on any day can be higher or lower than these averages, so check it each morning.
How do you protect yourself?
Cancer Council Australia recommends five steps together when the UV index is 3 or higher: slip on covering clothing, slop on SPF 50 or SPF50+ broad-spectrum, water-resistant sunscreen, slap on a hat, seek shade and slide on some sunglasses.
Clothing covers skin all day without reapplying. Sleeves usually stop at the wrist and the collar, so the backs of your hands and your neck are easy to miss; the Meriggio · Italian · say it “meh-REED-joh”The shady spot where you rest through the midday heat. has hand covers and a neck shade for those two places. Women’s fit, men’s fit.
Sources and data
- UV index: NASA POWER Hourly API, NASA Langley Research Center, CERES SYN1deg (UV index), MERRA-2 (temperature, humidity). Hourly values for 2020 to 2024, around 18.47, -66.11. Satellite and reanalysis grid values for the area around the city, not a single weather station. Peak UV is the mean of each day’s highest hourly UV index. Fetched 18 October 2026.
- Temperature: NOAA NCEI Global Summary of the Month, weather station SAN JUAN L M MARIN INTL AP (RQW00011641), 11.3 km from the city centre, 2010 to 2024. Highs and lows are the average of each month’s mean daily maximum and minimum.
- Relative humidity is an area average from NASA POWER for a grid square about 55 km wide.
- Day length calculated from latitude for the 15th of each month.
- World Health Organization: Radiation, the ultraviolet (UV) index, 20 June 2022
- Cancer Council Australia: Be SunSmart