I'll take the bate here and bight.

Air outside the house has a mostly slowly varying quantity of water vapour in it. It can take only so much water vapour before the vapour condenses into microscopic droplets of water - fog to you and me.

The amount of water vapour that air can hold as a vapour (that is to say as a gas) increases with increasing temperature of the air.

The air inside a house is very similar to the air outside a house because the ventilation deliberately built into the house lets it in, along with the gas-boiler pulling the air that it needs to operate. The amount of water vapour in the air inside the house is not far off the amount outside (unless you use a dehumidifier as I sometimes do).

In the UK it is usually warmer inside the house than outside. If you opened the doors and let fog in would it stay as fog? No, because as the air warms up it is able to cary more water as gas, and the droplets eVAPORate back into gas.

But there's still the same amount of water in the air in the house, if not more because of your breathing.

A digital humitity meter will read out in relative humidity, not the actual degree of water content in the air surrounding it. That's the difference between the amount of water vapour it could hold as against the amount it does hold.

So, for the same air, having come in through open doors and widows or just ordinary ventilation will read a lower relative humidity on the instrument in the house although it contains the same amount of water because .... the air has been warmed up by the time it arrives at the instrument.

With all the violence that the ususpecting air experiences as it is pumped into an airgun, all that ends up mattering is the amount of water in the air, not the 'relative humidity' of the air when it it was at peace and tranquility before it was abducted.

IMHO, best place to use a pump is a cold frosty high pressure day outdoors - or manage the vapour with dryers.

Here endeth this morning's lesson and I hope I haven't f....d it up.