Does Humidity affect evaporation rates?

If so, does it do it noticeably? 

Yes.  So does wind.  Neither have a substantial affect on our scale as our surface area is very small.  Several factors affect every batch.  The factor you can control and will have a measurable affect is how vigorous the boil is. 

Yes, I agree, it does have an effect!

About all the environmental factors you can notice have an effect.
I need to start a very accurate log to figure the exact rates better but I know one thing that I'm sure can help to minimize the environmental effects is having a large enough boil pot.
If you are boiling seven gallons in a keggle for example, you have a nice stainless column coming up from the top of the wort.  When you have a full boil going, you have essentially created your own weather environment from the top of the wort to the top of the keggle. You have heat rising and that creates what amounts to a wind coming up off of 212° liquid.  This helps keep it consistent because their is a large section above the wort that doesn't change from one day of brewing to the next.

If you are near the top of the pot, you have the outside air effecting the top of the wort.
I'm going to buy a 26 gallon kettle this weekend.  The keggle is small for a 10 gallon batch by the time you account for boil off, hop absorption, and wanting 11 gallons for the fermentation. 
With the new pot I'll be able to brew 15 gallons and still have a rolling boil and plenty of free space above the pot making a consistent steam cannon shooting up from the pot.

This is an interesting conversation.  The reason I asked is because I've found recently, in the warmer more humid months that I'm not evaporating as much as I do when its less humid.  It might just be the flame isn't turned up as much as usual and I'm attributing the lack of evaporation to the wrong thing but it is interesting.  In North Dakota its cold much of the year, I wonder if outside air temp influences evaporation?  Not sure why it would since the wort is at 212 regardless of outside air temp but maybe it does have an influence?

I find that I boil off less when the humidity is high (as it is often where I live).  I might be exaggerating it in my mind, but it seems to be a pretty significant difference to me.  I would think it has something to do with moisture saturation in the atmosphere, much like humidity rises in hot climates after it rains.

The humidity created by the boiling wort just above the kettle will be much greater than the ambient humidity.  This layer just above the kettle reduces the evaporation rate effectively creating a buffer between the cooler air and the hot wort.  If your humid summer days also have a slight breeze, this diffuses the layer and increases the rate the wort evaporates.  If your cold winter brew days have little breeze, the layer stays close to the wort slowing the evaporation rate.  As heat also affects evaporation rate, hot summer days have a higher evaporation rate than cold winter days due to greater excitement of molecules as the heat rises. 

jomebrew said:

The humidity created by the boiling wort just above the kettle will be much greater than the ambient humidity.  This layer just above the kettle reduces the evaporation rate effectively creating a buffer between the cooler air and the hot wort.  If your humid summer days also have a slight breeze, this diffuses the layer and increases the rate the wort evaporates.  If your cold winter brew days have little breeze, the layer stays close to the wort slowing the evaporation rate.  As heat also affects evaporation rate, hot summer days have a higher evaporation rate than cold winter days due to greater excitement of molecules as the heat rises.

Click to expand...

This goes right along with my comments about a larger pot.
If you have a foot of pot above the wort, or if you have 6" of pot above the wort, there is a huge difference in how much of the atmosphere created by the boiling wort is protected from the outside weather.