Cold air can hold much less water (hence why it's relative humidity). This is good if you take in cold air and heat it to 21°C or so because the air you end up

Cold air can hold much less water (hence why it's relative humidity). This is good if you take in cold air and heat it to 21°C or so because the air you end up with will have very low humidity, but if the outside is a drizzly 10°C and you are only heating to some 18°C, it can be a problem.