Actually humidity control feature does not exist in Evaporative Cooler. Think once how an Evaporative Cooler can control humidity where water continuously Evaporate. If Evaporation of water vapour takes place with the release of water vapours from the cooler in the form of air then we feel cool otherwise due to increasing in humidity water vapour loses its heat absorbing capacity from the surrounding air so we feel warmer than the actual temperature.
If Relative humidity is above 70% (practically when we feel that sweat is not evaporating). At that time we should stop Vaporisation process by switching off the Water Pump. But now the 'Evaporative Cooler' becomes just a name and it starts working as a SIMPLE MOBILE FAN.
Cooling Function of an Evaporative Cooler based on the heat loss property of water when it split in tiny water drops or water vapour. Water vapour is the gaseous state of water and is invisible to the human eye. The amount of water vapour present in Air is known as Humidity. Evaporative Cooler continuously Evaporate water and cooling take place. This is very effective in hot and dry air weather (humidity up to 50 %). Higher humidity causes more sweating due to reduced rate of evaporation of moisture from the skin.
Once relative humidity goes above 70%, the presence of water vapour in the air also increased and evaporation of moisture stopped. Then water vapours present in the air become warm and we feel like more warmer than the actual temperature.
Conclusion-Evaporative Cooler is more effective when the weather remains hot & dry. Between April to mid-June, it is more effective in India. This is the reason why an Evaporative Cooler is also known as Desert Cooler.