Humidity swings

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Clyde

Our summer humidity can very from 90+% for a day or more then drop to less
than 60% all in one week. This yoyo pattern depends on the speed and frequency
of weather fronts. We can have hight temps (85-95+) and high( 70+%) humidity
for a week or more at a time, only to be followed by a few days of 70's temp
and 60% humidity, with that pattern repeating itself several times.  The
winter temps typically drop below freezing for 2-3 months and below 0 for 1-2
months.  This can bring the humidity down to 15% or lower in homes without
humidification devices or means-schools are worse. These swings can push
pianos sharp by 30+ cents and do the opposite in the winter. 
As an example, I've tuned a dozen school pianos this week. All were somewhere
between 20 and 40 cents sharp.  This winter they will be at least that flat.
The typical school cannot afford to air condition the entire building(s) and
the typical piano is "stored" in non-air conditioned areas. Pianos in homes
aren't as bad, but suffer from the same effects of the seasons. BTW I live in
southeastern Minnesota, 25 mlies NE of the world famous Mayo Medical Center.
Where are you located?