Seasonal Extremes Matter

Seasonal Extremes Matter: given enough time, any object will equilibrate to any environmental condition. When changes in humidity persist for extended periods of time, beyond temporary spikes or short fluctuations, then the materials may have time to fully adjust to the new environmental conditions. If inappropriate moisture levels persist, collection damage could occur. Consider, for example, the general pattern of relative humidity over the span of one calendar year.

Figure 6

Figure 6: Graph of the outdoor dew point for Rochester, NY during the year 2010.

Looking at the trends of the outdoor dew point (which indicates the absolute amount of moisture in the outdoor air), we will not be surprised to see that the moisture levels are highest in the summer and lowest in the winter, with spring and fall being transition seasons between the two extremes. Were the relative humidity inside a collection space to follow these outdoor trends (which is, in fact, what tends to happen) many collection objects would fully adjust to the seasonal humidity extremes, higher moisture content in the summer and lower moisture content in the winter.

Figure 7

Figure 7: RH inside and outside a book during a three-month period at room conditions (IPI Library).

Comparing the relative humidity levels, we see that the %RH inside the book does not repeat the short-term fluctuations of the macroclimate but does follow the larger, seasonal trends. When the lower-humidity of the winter persists between months 1 and 2 (December and February),  the %RH inside the book continues to decrease until the macroclimate drastically increases at the start of month 2 (February).

Our Conclusion

Periods of sustained high humidity in the summer and sustained low humidity in the winter are much more significant in terms of preservation than sudden or short term fluctuations in RH.