Although swamp coolers and air conditioners were invented to turn hot air into cool, the science behind each method is quite different.

What is a swamp cooler?
Its just another name for an evaporative cooler. Water is used to wet absorptive pads around the sides of the cooler. A fan or "squirrel cage" draws outside air through the pads and the air is cooled as the water in the pads evaporates. The major benefit of this type of cooler is that you only need to power a water pump and a fan to draw the air through the pads.

They typically use between three and 10 gallons of water per day. That's equivalent to a few toilet flushes or, on the high end, a short shower.

Unless the relative humidity is lower than 30% or so, swamp coolers are not very effective. They work well in the southwest, west Texas, and parts of Idaho, Washington, Colorado, Wyoming, and Montana. In those areas you should be able to achieve a 20 degree differential between outside and inside.

Also, for a swamp cooler to remain effective it must exhaust as much air as it takes in. This means keeping a window open. If you dont, the humidity will rise and so will the temperature of the air blown from the cooler.

Air conditioning, on the other hand, is popular because it will reliably dehumidify and cool the air, no matter where you live. This how an air conditioner works:

The compressor compresses cool Freon gas, causing it to become hot, high-pressure Freon gas. This hot gas runs through a set of coils so it can dissipate its heat, and it condenses into a liquid. The Freon liquid runs through an expansion valve, and in the process it evaporates to become cold, low-pressure Freon gas. This cold gas runs through a set of coils that allow the gas to absorb heat and cool down the air inside.

Mixed in with the Freon is a small amount of lightweight oil that lubricates the compressor.

Swamp cooler vs air conditioner: which is cheaper to operate?
In our friendly contest the swamp cooler wins handily. For similar sized units, even an air conditioner with and Energy Star rating uses 4-5 times the power needed to run a swamp cooler.

The difference is the amount of energy required to power an air conditioner compressor versus the electric motors used in a swamp cooler.

Which is better for the outdoor environment?
Unless you believe that soon to be outlawed R-22 refrigerant leaking into the atmosphere from your air conditioner is a good thing, the swamp cooler wins again. The swamp cooler maintains a Green profile by mimicking natures way of cooling.

The way in which an air conditioner or swamp cooler affects the indoor environment of your home is determined by the way it circulates the air. Although an air conditioner re-circulates the same air, this is helpful in controlling allergens such as dust and pollen.

Since an open window is necessary when running a swamp cooler, youll be letting in what the air conditioner keeps out.

Swamp cooler vs air conditioner: Which one wins?
I think swamp coolers are a good option since theyre relatively less expensive and much cheaper to operate than air conditioning as long as you live in Phoenix, Arizona. But if you call Little Rock, Arkansas your home, nobodys going to talk you out of your air conditioner.