why does hair get frizzy in high humidity?

Hair absorbs moisture from the air and expands (and swells). The exact structure of your hair will determine what it actually does when it gets humid, but it's basically a buckling from absorbing the moisture.

So why doesn't that happen after a shower or swimming on a non-humid day? I rather like the look. Thanks, tho.

Oooh, good point! Your hair should be curlier while drying from swimming or showering, but it's straighter, generally. (Mine is anyway)

I retract my explaination =)

usually curly hair is really dry so when it needs moisture then it expands in order to get the moisture from the air. that’s probably why it isn’t frizzy when it’s completely drenched because it’s just been submersed in water.