Air change during the night?

Something crossed my mind recently as I was monitoring the temperature & humidity profiles at various floors of a 4-storey bird house. During a coffee session with a few Malaysian bird farming friends a while ago, there was a mention of a blog suggesting it is better to effect air change during the night when the relative humidity is higher. Hmm.. sounds pretty logical to me then.

Well, I spared some thought on this subject again. How true is this proposition in general?

The concept of relative humidity can be misunderstood easily. It is technically more accurate to explain in terms of vapor pressure but I try to put in lay terms for the benefit of a wider audience.

Back to some basics to begin with... the graph below shows the amount of water (gm)/kg of air on the vertical axis, I simply referred as moisture content in the air. The red line shows the max. moisture content corresponding to a certain temperature. The green line shows the relationship between moisture in air and temperature for 50% relative humidity.


















Take a case on the graph above, consider the desired room temperature of 30 deg., air can hold about 28gm max. of water content per kg of air. This is the saturation point or commonly referred as the dew point temperature. If the air temperature is raised to about 42 deg. (assume constant atmospheric pressure), then the relative humidity drops to about 50% - N.B. without any increase of water content in air.

What's the significance of this? The converse also holds true. This means that as temperature drops during the night the relative humidity increases automatically even without any increase in the moisture content of the air.

So unless it is the winds that bring air with higher moisture content (e.g. from the sea or during rains), to think you get more moist air during the night is a misconception.

If you understand the point in discussion, it helps to make sense of the temperature and relative humidity cycles of the bird house during day and night.

Like wise there is no need to turn on the humidifier during the night. I don't like the motor noise and the air currents it generates. So my set up is allow the hygrostat to control the humidifier during the day and turn off during the night by a timer.