Straightened Hair and Humidity
A: Humidity generally has the greatest effect on porous hair (hair that absorbs moisture most readily) because the cuticle layer of porous hair tends to be raised to allow moisture to pass both in and out of the hair shaft. What you need is to work to close the cuticle layer and seal the hair shaft to prevent the moisture from your humid environment from penetrating into the shaft.
After shampooing and conditioning in the shower, use a final cold water rinse on the hair to further contract the cuticle layer. This will also help to deter and delay sweat production by the glands in the scalp.
Towel-dry the hair and apply a smoothing serum or anti-frizz serum which are designed to further smooth the cuticle layer of the hair. Comb the serum through the hair using a wide-tooth comb to evenly distribute the product. You can either allow the hair to dry naturally or blow-dry the hair using a cool setting. The cool setting on your dryer will mean that your need more time to dry the hair, but the results will be much smoother.
Once the hair is completely dried, if you use a flat iron to smooth and flatten the hair, be sure to use a misting of hairspray on the hair before ironing, to help hold the straight style and to further seal the cuticle layer against moisture and humidity. You can also get extra hold and moisture sealing in the hair by applying a styling gel at the same time as the smoothing serum (after towel-drying and before the blow-drying steps).
Once finished with the styling steps, reapply hairspray and let it dry before brushing it through to loosen the hair strands and give your hair freedom of movement.
This should help you achieve a more humidity-resistant hairstyle.
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See also:
Hair straightening
How to keep a sweaty smell out of your hair
How to maintain your hairstyle in humid weather