UKC

89% sounds a lot but is it too much on summers day?

I guess is depends, are you trying to steam a sponge pudding or cook some mangetout?

Depends how windy it is.

It depends on the temperature. If the temperature is, say 10 deg C then it's OK if it's 34 deg C like Singapore, you'll be sweating like a Scouser on rent day after 10 minutes of walking down the road.

Look up relative humidity for a more detailed explanation.

It really depends on the individual and their acclimatisation.  

Drop me in it now and I’d sweat like a pig.  But after spending a week in that sort of humidity and high temperatures in Alexandria (VA) refusing to use air conditioning, I enjoyed a gentle jog into D.C. to go museum spotting without looking like I’d been for a swim.

I take 80% as a threshold above which I don't expect to be on top form on impermeable granite or DWS where condensation is likely. Routes at Bosigran can feel a grade harder when it's humid. My ascent of Grendel in such conditions still causes laughter.

Look for the Heat Index on that particular day. For example, 30 Celsius at 75% humidity might have a heat index of 40 Celsius. As humidity rises, the skins ability to perspire goes down, and you experience much greater heat. If you have travelled in Asia during the summer you might have noticed people using cloths to mop their skin - this is due to the humidity stopping the sweat from evaporating. If you can't evaporate sweat, you overheat.

If you mop the liquid off surely you still won't get the cooling effect of it evaporating?

Yes. Sweat evaporation is constant though, so you'd pretty much be mopping and wringing constantly to achieve anything close to perspiration. And I assume OP is talking about humidity in regard to climbing - not exactly conducive with climbing, all that mopping...

Isn’t evaporation is dependent on the surrounding humidity?

Sweat production is pretty much constant just on a dry day it evaporates quickly on a humid day the air is already saturated so the air will not absorb the sweat. 

Humidity is relative. The air is like a sponge that is squeezed by cold and expands in the heat. 

If the sponge can be 100% saturated by say 100ml of water at 10’c, when you raise that temperature the sponge expands and is able to absorb more water. If it could absorb  400ml at the new, higher temperature, then that 100ml would make its humidity 25%. 

If you then made up the level of water to 400ml it would be at 100% humity. If you then cooled it back down to 10’c, the additional 300ml of water would be ‘squeezed out’ and condense on surrounding surfaces. It would still be at 100% humidity but hold less water for the same volume of air.