Dave Liquorice formulated on Monday :
A centrally heated house at this time of year will be around 30% to
40%. If it's very close to, or below, 0 C outside for a day or three,
the inside humidity may drop to 25 %.

If the humidity drops very low indoors in winter, I tend to suffer from
dry lips etc. - then I add some humidity by placing trays of water on
the radiators.