How it happens: Fine, straight hair can become limp when exposed to humidity. "Moisture from the air settles on the surface of this type of hair instead of penetrating it," says stylist Armando Corral. In other words, it weighs hair down.
To fix it now: Add volume at the roots with an antihumidity hair spray. (Try V05 Max! Hold Fast Drying Hair Spray, $3.50 at drugstores.) Divide the top layer of hair into four sections. Lift each section at the roots and spritz the underside. Give the spray a minute to dry, then gently back-comb each section.
To prevent it next time: Lay off the heavy styling products. It's tempting to douse fine hair with volumizing sprays to help combat limpness, but these products cause buildup at the roots and, in humid weather, flatten hair even more.
Bulk up with protein. "Fine, straight hair lacks protein, which gives hair body and helps it hold a style in humidity, says stylist Erica Colameta. To add it, shampoo and condition twice a week with protein-rich products, like Aveda Smooth Infusion shampoo and conditioner ($21 each, aveda.com).