Heat and UV are correlated but weakly. It’s entirely possible that an early spring day with cool temperature and light breeze has a higher UV levels than a cloudy day with 35°.

You don’t need specific SPF clothing to get protection.

This has a good discussion about spf clothing. www.nytimes.com/…/best-sun-protection-clothing/

My personal preference is to seek shade and wear sunscreen when it’s intolerably warm, rather than to add tight weave clothing, but to each their own.