Is WeatherKit wind direction backwards?

Yes - what's reported is a "west wind." But the wind bearing is east.

The wind direction is the compass heading from which the wind is blowing (so, 90.0 °, or West), but the wind bearing is the direction in which the wind is moving, so East. Put a weather vane in a west wind and it will point east.

In short, compassDirection = windBearing.

That's my understanding.

Actually according to the documentation both direction and compassDirection should refer to the direction the wind is coming from. Respectively,

Direction the wind is coming from in degrees, with true north at 0 and progressing clockwise from north.

...and:

General indicator of wind direction, often referred to as “due north”, “due south”, etc. Refers to the direction the wind is coming from, for instance, a north wind blows from north to south.

In the example Wind object:

WeatherKit.Wind(compassDirection: East, direction: 90.0 °, speed: 7.09 km/h, gust: Optional(18.18 km/h))

It’s indicating “wind out of the east” (as a simplified weather report may phrase it), specifically from compass direction 90º, which is due east.

A wind blowing from due west would be blowing from a direction of 270º.

I think the wind API should be interpreted as documented, and you’re just seeing bad data as mentioned in your other post.

In simple testing here, I get perfect agreement between the iOS 16 Weather app and WeatherKit for various test locations (including Penticton, BC), but disagreement between those and the iOS 15 Weather app. In particular the wind direction there is roughly backwards between the two.