How do I keep mascara from running in heat and humidity?

Have you tried using a lash primer before applying mascara? The real solution is in that, not just in the makeup product itself: a good primer (like Maybelline Lash Sensational or something similar) creates a base that holds up way better against humidity and heat, plus it makes it last twice as long. If it still runs on you, consider getting a truly waterproof mascara like the ones from Inglot or try lash tint instead of traditional mascara - those are a different thing entirely and they hold up much better at the beach.

Heat and humidity on the coast are brutal for any mascara, honestly. What actually works is combining techniques before you apply the product. First, make sure your lashes are completely dry and oil-free, because even though they might look clean, any leftover moisturizer or facial oil residue will sabotage them. Apply a very light translucent powder over your lashes with a thin brush before the mascara - that absorbs the moisture that builds up during the day.

Then comes the important part: use two very thin coats of mascara instead of one thick one. People always think applying more product will make it last longer, but it's the opposite. A thin coat dries fast and holds better, and the second coat on top seals everything without creating that weight that eventually runs with sweat and humidity. Let it dry well between coats, wait about 30 seconds without rushing.

And one last tip: seal with a transparent makeup setting spray when you're done with everything. It's not the same as a primer - it's literally a final seal that waterproofs. If the Maybelline one still doesn't work even with these steps, try mascara from another brand because sometimes certain products just don't work well with your skin type, even if they're "waterproof." Sometimes it's just body chemistry.