There's no way to answer "how much", but it does affect speed.

Joe

How much does humidity change greenspeed?

It changes it as the moisture from humidity gets into the surface but I'd say in any and all cases it would have to be less than a foot, maybe even half that at most.

About a year ago the USGA told me there can be things like a "wet 11" and a "dry 11".

My first reaction was for God's Sake an 11 is just an 11. But maybe they have a point.

All I know is whether it's wet or dry if the average stimp reading works out to an 11 then it's just a plain 11 wet or dry.  

On the other hand, if they were trying to tell me that a wet 11 and a dry 11 makes the golf ball roll differently or different distances for some reason then I'm going to have to get back on that claim.

It absolutely affects it.
by at least a foot.
throw in a little breeze and things get extra spicy.

Surely you would recognise that a super dry cut and rolled green will be faster than one prepped the same in high humidity (moisture)
The effect of cutting and rolling is cumulatve as well, so greens prepped the same on Fri-Sun will be faster on Sunday.
(assuming similar humidity)

very sad that golfers expect all this though-half the greens I've played in the UK weren't cut that day anyway.

Nothing beats a 19 handicap telling me the greens are slow though.

TEP,

That "wet" versus "dry" speed sounds to me like the trickle effect you speak of. "Wet" should have less trickle, but may be the same speed on the flat as "Dry".

Steven,
I'm not arguing with you, but here in NW Ct the day started out cool with clear skies and low humidity, this afternoon it's 'grey' from the increasing humidity. We get quite a few days like this.  

Jim,
If you had cool temperatures and clear skies, I would guess that you had dew on the grass in the morning?  If you had dew, then the humidity had to be high enough to condense onto the leaf blade.  Remember, the humidity always feels higher in warmer temperatures because your body is sweating to cool itself.

Jes,
It all depends on what is going on maintenance-wise.  A course can use walk mowers one day, use a triplex the next, then sharpen reels, and each one will have an effect on the speed of the green more that the weather conditions will.  Those are the maintenace practices that golfers will likely not know about.  Then there is verti-cutting, topdressing,  grooming, which may be more noticeable for the golfers, and also have a significant effect on green speed.

"TEPaul,
Perhaps it's like the the bow wave of a boat, it saps the energy from the ball. Two greens, both stimping at 11', one dry, one wet;  the longer the putt- the slower the wet 11 will be.
It's like the difference between two greens cut at the same height, but one is thatchy and one isn't. As the balls lose momentum the thatchy green grabs more and is, in effect, the slower of the two."

Jim Kennedy:

If a green is rolling at an 11 it doesn't matter if it's wet or dry----eg it's just rolling at an 11.

How can you say, then, that the wet one is rolling slower? In that case it either wouldn't be rolling at 11 or the drier one would be rolling at greater than 11.

Have you ever actually done a legitimate stimpmeter reading test?

I'll tell you something else too Sully about stimpmeter readings and that is I'm beginning to think a lot of people who do them don't do them the way you're supposed to.

Some probably don't even realize there's a limitation on the two way differential to be official. I see others stimping greens who pull the stimpmeter up way too fast basically bouncing the ball off the stimpmeter and firing it way farther than it would go if they did it right.

I even had one super tell me he was getting so pissed at his club asking for higher stimpmeter readings that he said he was going to take a hacksaw and take about six inches off his stimpmeter.  

What's the maximum differential for a reading to be official?

I know a super that does no stemp readings, but if a golfer should ask he says they are stemping at 9.5...or 10...or whatever speed suits him on that particular day...he knows that 99% of golfers don't know a 9.5 from a 10.5...or a 10 from an 11.

If I had a dollar for every time a golfer has approached me while I'm cutting pins and has said " wow, these greens are way faster than yesterday, what have you done?"  when we have done nothing different, I'd be able to quit my job and play more golf...speed is a perception...for example, a hole location can make a green seem fast or slow.

JimK:

I was sort of kidding when I said they told me that. Although what was said is 'there's a wet 11 and a dry 11.' Obviously, if one looks at that statement of course that can be true in either case. It just sort of sounded to me like they meant the ball rolled differently or perhaps different distances between a wet 11 and a dry 11 and of course that can't be true or one or the other wouldn't really be an 11 would it?  

I guess the difference is that the wet 11 has the potential to dry out and speed up...in this instance it makes sense for them to think along those lines as they are probably typically setting pins for their events in the early AM when it is usually wet from dew...