In terms of speed:

* Calm: This refers to the absence of wind entirely.

* Gentle breeze: This is a light wind, much slower than fast wind.

* Light wind: Similar to a gentle breeze, this is a wind that's easily felt but not strong.

In terms of direction:

* No wind: This implies no movement of air at all, regardless of speed.

* Downwind: While not technically opposite, this refers to the direction a wind is blowing, as opposed to the direction from which it originates.

In terms of effect:

* Still air: This refers to the absence of any wind movement, creating a sense of stillness.

* Calm conditions: Similar to still air, this refers to the lack of any wind movement, creating a peaceful atmosphere.

Ultimately, the best opposite depends on the context and what you're trying to communicate.