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Both water activity (materials) and relative humidity (gases) are referred to the saturation pressure (ps) or partial pressure of water vapor above pure water:

 

Aw = p / ps

 

%RH = 100 x p/ps

 

The saturation pressure (ps) is strongly dependent on temperature. At normal room temperature, (ps) increases by about 6.2% for a 1°C increase in temperature. In an open environment that is not saturated with water vapor, the partial pressure of water vapor (p) does not change with temperature. In a closed environment, (p) changes proportionally to the °K temperature (°K temperature = °C temperature + 273.16). At normal room temperature, the change in (p) caused by a small change in the °C temperature is practically negligible. Because (p) does not change with temperature while (ps) does, the relative humidity of a gas (%RH = 100 x p/ps) is strongly temperature dependent. At 95 %RH and room temperature, an increase of 1°C results in a relative humidity decrease of about 6 %RH. At 50%RH, the same temperature increase causes relative humidity to decrease by about 3 %RH.

 

The water activity of most hygroscopic products is not as strongly dependent on temperature. At room conditions, research data typically shows that water activity varies only by roughly 0.0005 to 0.005 Aw (0.05 to 0.5 %RH) when temperature changes by 1°C.

This is explained by the fact that the partial pressure (p) at the surface of a hygroscopic product does vary with temperature. Above most hygroscopic products, the magnitude of the change in the partial pressure of water vapor (p) with temperature is similar (but not exactly equal) to the magnitude of the change of the saturation pressure (ps) above pure water.

 

In summary, a change in temperature causes the partial pressure of water vapor above a hygroscopic product to change. At the same time, the partial pressure in the air above the product is practically unchanged. It follows that any change in the temperature of a hygroscopic product automatically causes the product to exchange moisture with the air (or gas) that surrounds it. Moisture is exchanged until the partial water vapor pressure at the surface of the product and in the air is equal. When measuring water activity, it is essential to keep temperature as constant as possible.