there are several threads relating to conversion factors, etc., currently floating around on the board.

many many topics/threads/messages _mention_ the effect of humidity on moisture content of flour(s) and how that may affect measuring and/or baking results.

the first thing that has to be recognized is depending on where (geographically / climate wise) you live - the pertinence of the question _will_ vary and "the correct answer" _will_ also vary.  seasonal variations in temperature and relative humidity - and how that will affect flour - are _not_ the same if one lives in Hawaii, Alaska, Georgia or the Great Plains states - along with everywhere else in the world . . . .

to complicate the matter further, regardless of geography/climate, "storage conditions" vary - even within a similar geography / climate.  and "storage conditions" is not absolutely limited to "what kind of container" and "do you put the lid back on"

for example, I live in central PA.  in the summer we use whole house air conditioning, as needed.  in the winter we use a whole house humidifier attached to the forced hot air heating system.  I bring flour home from the store, (generally) open it and dump into my flour storage - a 'sealed' Tupperware bucket.  got a couple - one for AP, one for bread flour, one for pastry.

here's a table I found at www.medlabs.com [1] - link is broken, but here's the 'then' data:
%RH   - - %H2O
00.0  - - 00.53
11.1  - - 05.90
22.9  - - 07.65
32.9  - - 07.65
32.9  - - 08.95
43.9  - - 10.11
53.5  - - 10.90
64.8  - - 12.21
75.5  - - 15.68
86.5  - - 18.80

if anyone has better data - by all means post - I've not had a lot of success finding quantitative info.

the table does not speak to what 'grade' of flour, what kind of wheat, what kind/fineness/etc. of milling,  it is what it are and I have not found other sources.  (King Arthur rep... can you chime in?  youse guys certainly have valid info, but I can't find it on the web site.)

so here's where it gets interesting:

(for example) in my location the outside ambient conditions vary in extreme from 0'F/10%RH to 100'F/100% RH.   that's not what happens inside the house.  in the winter the house heats to high 60's, the humidifier holds 35-40%RH.  in the summer we're in the low 80's and hope to keep the RH less than 70% - the RH is a 'byproduct' of how often/long the central ac runs.

so looking at the above data, the moisture content of the flour would vary roughly between 9% and 13/14%

but of course, the flour is not directly / continuously exposed to the (outside or) inside extremes because it's in a sealed plastic bucket......  but of course, it got there (more or less 'instantly') from the store, which has other exposure conditions....  this is of consideration because regardless of wider/narrow temp/RH condition "in the house" what the flour experiences "in the container" is different.

I use a scale, I've tweaked my recipes for my flour type/grind/brand.  tweaked numbers written on the card . . .

so (for example) a specific recipe I weigh out 400 grams of KA Bread Flour and 320 grams of water.  nominally 80% hydration.

using the above "data" that means the flour could vary from
91% flour = 364 grams plus 9% water = 36 grams  (winter)
to
86% flour = 344 grams plus 14% water = 56 grams (summer)

as I do most of my bread baking in the winter, my 'tweaks' undoubtedly run to the 'dry' side of flour.  in 'tweaking' I consider not only the feel of the dough but also the blinking final result and perceived by _eating_ the bread (g)

so, assuming I'm working at the low/dry end and assuming the tabular data is close to 'right',

the winter recipe is:
364 g bone dry flour + 36 g water in the flour + 320 g water, or 356 g water / 364 g flour = 98% hydration.
the summer recipe is:
344 g bone dry flour + 56 g water in the flour + 320 g water, or 376 g water / 344 g flour = 109% hydration.

glitch:  an 80% hydration level recipe was _not_ developed at 0% RH using bone dry flour - the recipe used flour that had some level of moisture in it.

my take is the "recipe development environment" did not use bone dry flour, so the variation in water content of the dough is not 98% vs 109% but rather some flavor of winter dry to summer wet flour moisture of 20 grams (56 - 36, as above)

so the 'comparitive end content' is:
400 g flour + 320 g water = 80% hydration
vs
380 g flour + 320 g water = 84% hydration

I've not seen too many quibbles over a difference of 1% hydration; 2% invites a discussion.  this math sdys 4% difference summer to winter.

all the above number are subject to error and correction - so what is the experience? 
can a 1, 2 3, 4% hydration level become obscured by "bench flour'?