"Is the wind cool?"

Translation: 風は涼しいですか？

[Jersebas]

風は涼しいですか？

[TyrantRC]

adding hiragana now:

かぜは　すずしいですか？

[MaxCrowe2]

Wa vs Ga here?

[TyrantRC]

You use は because you wanna ask something about it, the topic is "the wind" (the other person already knows the concept of wind) and you want to know if is cool.

If you were to use が it would be an strong emphasis on this particular wind, no idea what context you could use it like that though, maybe as you were guessing the answer to the question "take a guess: what is refreshing right now?"