Given our relatively high humidity, is evaporative heat exchanging effective ?

The misconception originates from the false impression interpreted from the weather forecast which reports average daily value of relative humidity (RH).

The supply air temperature from the cooler depends on two factors, the outside air temperature and the outside air relative humidity (RH). At certain dry bulb temperature, if the Relative humidity is lower, more temperature reduction can be achieved. This is because more water can be evaporated into the air.

The fact is that RH fluctuates like temperature throughout the 24-hour day. Of interest to note is that when the temperature is relatively high during the day, the RH is relatively low.