How does this product hold up against sweat in a warm climate?

It’s not going to hold up very well for very long on skin. I used it for Shrek the Musical. The actor who played baby Shrek had this sprayed directly to his face while the actor who played adult Shrek and ogre Fiona had it painted on prosthetics over their face... baby Shrek is only in the opening number and it would be starting to come off from sweat/heat. On the last day of the production, the kid cried because he was ad the production was ending... he had streaks on his cheeks.

On the back it says it can be used on hair, does anyone know how to do this?

You can use a comb or paint brush to apply makeup to hair. Thank you for your inquiry!

Can it really glow very well in the dark without any blacklights?

It maybe had a slight glow simply in the dark - but keep in mind that there's virtually no cosmetic product that is going to safely brightly glow on your skin without a blacklight/blue light/UV light. It's not like you can crack open a glowstick and paint yourself with it, there's just no safe way for that to be done. Under blacklight, however, this product glows phenomenally well. Much much better than any cheap crayon or item you might get in a Halloween store.