This is also the reason why the selling of Air coolers is not a wise business in areas with high humidity like Mumbai and the Assam-North east region.
The cooling effect of air coolers primarily depends on the difference between the Ambient temperature (Dry bulb temperature) and Wet bulb temperature. To get a feel of wet bulb temperature, you can put it like this- “The degree of coolness of a recently moped floor after it dries up completely”.
This difference between dry bulb temperature and wet bulb temperature is small on “Humid days” or the days in which we sweat a lot if the temperature is high.
Now you know why air coolers are not as effective on some days.