Winds of Venus

The Daily Mail reports, in a remarkably confused article (I'll spare them the embarrassment of a link), that the Venus Express orbiter finds that Venus' rotation period has slowed by 6.5 minutes since the last time it was measured 20 years ago.

That certainly sounds like a lot, until you realize that Venus' rotation period is 243 days. That's still a big change, but not as proportionately huge as you might at first think. And Venus' atmosphere is fiendishly thick and heavy and moving very fast--at speeds up to 100m/sec. If Wikipedia can be believed, the mass of Venus' atmosphere amounts to 1/10,000 of the total mass of the planet, and is moving far faster than the surface; up to 60x faster.

A back of the envelope calculation says that about a 10% increase in wind speed would decrease the planet's contribution to the total angular momentum enough to account for the slowdown.

Why would winds speed up? Maybe they didn't speed up, they just changed direction so that they were less cyclonic and more circum-planetary. If so, you'd expect them to change again down the line, and let poor Venus speed up a smidgeon again.