When evaluating the need for humidification, it is important to know what happens to the air around us. As reference material we can use European Normal Year, which describes the climate in our part of Europe. The diagram covers the humidity need over a period of one year.

This actually shows that there is a need for humidifying on almost all days in the year; but of course the need varies from one day to the other.

In most of North America, dry air problems are seasonal in nature, typically encountered during the winter heating season.  Dry air is less of a problem in warmer months when indoor air is less frequently heated, although air conditioning can have a major affect on controlled RH levels. 

Dry, desert regions can experience adverse low humidity conditions year round.

In winter heating season, cold supply air is heated to normal indoor temperatures.  This process can reduce indoor RH levels to extremely low levels (in some cases less than 10% RH).  Warming supply air just a few degrees requires adding moisture in a controlled manner to maintain optimum RH levels.