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After you sure that isn't because your skin is moist where sunblock was recently applied, and therefore cooler?

Sunblock is supposed to reflect UVA/UVB, but doesn't do much for infrared or visible light, which is why your skin gets warmer.

(Edited, fixed)

Sunblock is not supposed to reflect heat (infrared). It's supposed to reflect UV. Different spectrum. Kind of the whole thing the OP was about. I have no idea if sunblock also reflects infrared though, incidentally.

https://care360.basf.com/emea/en/industries/personal-care/co...

Sunblock mostly absorbs UV (and turns it into heat). Some reflection occurs but it's mostly absorption.

After looking into this even more, there are two types:

Mineral - Reflects

Chemical - Absorbs

I thought so too but it's not the case. Even the mineral sunscreens mostly absorb.

https://cen.acs.org/business/consumer-products/What-in-sunsc...

Video with a UV camera is interesting: https://www.youtube.com/watch?v=E9tEEDpeJoo

Black means absorbed, of course.

And, omg, you can buy an active version on amazon, for this exact purpose: sunblock coverage check!

https://www.amazon.com/s?k=uv+mirror+for+sunscreen+test

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