Not sure about your specific setup. I had experience with swamp coolers while living in Utah.
Personally, I found them really to be wet, especially when standing right under the main vent.
They work on the principle of turning over large volumes of air -- a whole house every hour or 2 and low humidity and high temperatures.
"To answer this, we always use the swamp cooler temperature humidity chart. It will essentially tell us at what temperature and humidity it’s too humid to effectively use an evaporative cooler.
Before we check out the evap cooler chart (and, even more importantly, explain how to use it), let’s have two things in mind:
Evaporative coolers are most effective in high temperature (85°F or more) and lower humidity (below 50% relative humidity) situations.
Evaporative coolers are least effective in lower temperature (below 75°F) and high humidity (above 60% relative humidity) situations.
The swamp cooler humidity chart spans from 75°F to 125°F temperature and from 2% to 80% relative humidity levels. It tells you at which temperatures and moisture levels using an evaporative cooler is effective. What is more, it tells you how many degrees does a swamp cooler lower. For HVAC professionals: This is based on the Mollier diagram and uses dry-bulb and wet-bulb temperatures for a direct evaporative cooler process." from the net.