You need a hygrometer to calculate your humidity. This comes close though
http://www.weather.com/outlook/homeandgarden/home/weather/tenday/10021?lswe=10021&lswa=HomePlannerForecast
I try to keep my house at above 30% humidity, using a humidifier. This seems to work fine for the solid body electrics. For my acoustics I put in the case, a humidifier, which is essentially a sponge, and it gets it above 40%.
Of course, once the humidity in the house is over 50%, I no longer use a humidifier.