I usually have straight hair, but on rainy days my hair gets curly, so it is like having a free perm hairstyle
I usually have straight hair=普段はストレートヘアーだけど
but on rainy days=雨の日は
my hair gets curly, ＝髪がクルクルになる
so it is like having a free perm hairstyle＝なので無料でパーマをしているようだ
Typically my hair is straight, but on rainy days it gets curly.
My hair is normally straight, but on rainy days it becomes curly, so it's like getting a free perm.
「普通」＝ typically, normally, generally
「ストレートヘアー」＝ straight hair
「私の髪はストレート」(my) hair is straight
「だけど」＝ but, however
「雨の日」＝ (on) rainy days
「パーマ」＝ perm -> curly (hair)
"Normally my hair is straight, but on rainy days it becomes curly, so I can enjoy getting a "free perm.""
これはもっと意訳です。MASARUさんの気持ちが入ってる。
日本 
アメリカ合衆国 
