engine is right. Your Pooleys will most likely be correct whereas the link you read is merely someone's badly written attempt at an explanation.
When I was at groundschool we were taught by bona fide meteorologists so I have no reason to doubt the explanation I was given. What follows is a summary of what I understood from those lessons and the notes provided:
Geostrophic wind at 2000' agl is parallel to isobars. Surface wind speed is less than geostrophic wind speed due surface friction. Less wind speed also means less geostrophic effect therefore the surface wind is approx 30º off the geostrophic. In the northern hemisphere the result is that the surface wind 'backs and slacks' from the geostrophic wind. Southern hemisphere it would veer and slack but that doesn't roll off the tongue so well.
So - in the northern hemisphere - the surface winds 'back and slack' whilst the winds aloft 'veer and increase'.
That is the basic theory. At night, due to the cool dense air, surface friction increases so the surface wind backs and slacks even more.
Last edited by oggers; 25th August 2016 at 11:10.