Nighttime Cooling Pad Operation During Extremely Hot Weather

Tip Category: 
Evaporative cooling
Hot weather management

It is generally recommended that evaporative cooling pads should not be operated at night because the relative humidity of the air outside a poultry house tends to run 80% or higher at night.  Operating pads when the outside relative humidity is above 80% produces little cooling (less than 3oF) and tends to saturate the income air with moisture making it more difficult for the birds to cool themselves.

Outside air temperature and relative humidity are inversely related.  This means that as outside temperature rises in the morning, the relative humidity decreases.  Conversely, when air temperature in the evening falls, the relative humidity of the air increases.  If you graph outside temperature and humidity over a 24 hour period you will find that the two typically “cross-over” around 80oF and 80% Rh (Figure 1).  As a result, poultry producers will find that in the morning when temperatures climb above 80oF, the relative humidity will fall below 80 % and their evaporative cooling systems are able to produce a significant amount of cooling without saturating the incoming air with moisture.  In the evening when the outside temperature falls below 80oF, the humidity increases above 80%, and the cooling produced by an evaporative cooling system becomes very limited.

Under “normal” summer weather (daily high temperatures in the low to mid nineties) the crossover of temperature and relative humidity typically occurs midmorning (9 - 11 am) and late evening (9 - 11 pm).  As a result, the commonly used rule of thumb that evaporative cooling pads should not be used between 10 pm and 10 am is appropriate. But, it is a rule of thumb...not a law.  Though the crossover of temperature and relative humidity tends to consistently occur around 80oF and 80 % Rh, the time of day at which the crossover occurs is much more variable.  During very hot weather (daily high temperatures in the high nineties) the crossover tends to occur much later at night (12 am - 2 am) and earlier in the morning (7 am - 9 am) and therefore pads can be effectively used earlier in the morning and later at night.   Under very extreme circumstances (daily high temperatures above 100oF) the crossover may not occur at all because the outside temperature never falls below 80oF at night therefore it is possible to operate an evaporative cooling system nearly, if not all night long (Figure 2)....