Evaporative Coolers in Florida: How They Work and Why Humidity Matters

How an evaporative cooler works

A direct evaporative cooler is a simple box. The Building America Solution Center is a Department of Energy funded guide hosted by Pacific Northwest National Laboratory. It describes a housing with a large blower fan and a thick, sponge-like filter pad that a pump keeps soaked with water. The fan pulls outside air through the wet pad and into the building. The guide says the flowing air evaporates some of the water, which can reduce the temperature of the air entering the home by 15°F to 40°F.

Two points set it apart from an air conditioner. First, our reading is that it has no refrigerant loop, only a fan, a wet pad and a pump. Refrigerant is the fluid an air conditioner uses to carry heat out of a house. The Department of Energy's Home Cooling 101 guide says an air conditioner transfers heat from a home's interior to the warm outside environment. An evaporative cooler instead uses water that turns to vapor. Second, the guide says direct evaporative coolers increase rather than decrease the humidity in the air. That is the opposite of what an air conditioner does at its indoor coil, covered in a house that feels humid.

The guide also describes indirect coolers, which use a heat exchanger so they do not add humidity to the cool air. Our reading: those are larger, built-in systems, and the small units sold as personal or room "air coolers" are the direct kind.

Why the cooling depends on dry air

The guide ties performance to two thermometer readings. The dry-bulb temperature is what an ordinary thermometer shows. The wet-bulb temperature comes from a thermometer wrapped in a wet cloth, and the guide calls it the lowest temperature that can be reached through evaporation. The gap between the two is called the wet-bulb depression. The guide says that the greater the wet-bulb depression, the better an evaporative cooler works.

Humid air shrinks that gap. The guide says that when the relative humidity is 100%, the wet-bulb temperature equals the dry-bulb temperature, so evaporative cooling cannot occur. Relative humidity is the share of moisture the air holds compared with the most it could hold at that temperature.

The guide gives two comparisons. For hot-day design conditions in Phoenix, 108°F dry-bulb with a 70°F wet-bulb is a 38°F depression, and the guide says evaporative coolers would work very well. For Sarasota, Florida, 92°F with a 79°F wet-bulb is only 13°F, and the guide says an evaporative cooler would not work very well there. It also gives a rule of thumb: areas with wet-bulb temperatures of 70°F or lower are appropriate for evaporative cooling.

A second example in the guide shows the same effect. At 100°F and 10% relative humidity, the air entering the home is 73°F, a 27°F drop. At 95°F and 50% relative humidity, it is 84°F, an 11°F drop. The United States Geological Survey's Water Science School shows this on a map. Evaporative coolers work best in the dry areas of the country, and in the humid east normal air conditioners must be used.

What that means in Tampa Bay

The National Weather Service Tampa Bay Area office says to judge how humid it feels by the dew point instead of relative humidity. The dew point is the temperature at which water vapor turns into liquid water. It says the higher the dew point, the muggier it feels, and that West Central and Southwest Florida see sweltering summer days when dew points are above 80°F. For Tampa, it records a highest dew point of 86°F on August 6, 2000. It also records seven straight days with dew points of 75°F or higher from August 5 to 11, 2024.

Our reading: the Sarasota comparison is the closest example in the guide, since Sarasota is on the same Gulf coast. It suggests a summer afternoon here sits nearer the 13°F end than the 38°F end. We did not measure any cooler in Florida. A product's advertised temperature drop is worth asking about only together with the outdoor temperature and humidity it was measured at.

Two other points from the guide matter here. Because an evaporative cooler brings in outside air and does not recirculate it, the guide says doors or windows must be open to give that air a path out. Our reading: in Florida that path lets humid outdoor air through the house. The guide also says standing water left in the pan can become a breeding ground for mosquitos and bacteria, including Legionella. It says to empty the pan whenever the cooler will be idle for more than a few days.

The range indoor humidity should stay in is covered in good humidity for a house. This page does not repeat it, and it makes no health claim about humidity.

Why a "portable air conditioner without a hose" is usually an evaporative cooler

A federal rule defines a portable air conditioner. Title 10 of the Code of Federal Regulations, section 430.2, calls it a portable encased assembly that delivers cooled, conditioned air to an enclosed space, and says it includes a source of refrigeration. Our reading: a source of refrigeration is what moves heat out of the room air. Home Cooling 101 says an air conditioner transfers that heat to the outside environment.

Our reading: a refrigeration-based unit needs somewhere outside to send the heat, which is the job of the exhaust hose in a window kit. Our reading: a unit sold with no hose and no outdoor connection has no place to send it. Our reading: in practice such a product is usually an evaporative cooler, or a fan with a water tank, and the humidity limits above apply to it. We have not tested any of these products, and some may be sold under either name.

The product page or the manual will say what kind of unit it is. Look for the words "evaporative," "water tank" or "cooling pad," and for any mention of a compressor or refrigerant. A cooler that adds water vapor will not remove humidity from the room, so it is the wrong tool for a room that already feels damp. The moisture sources and fixes for that are in our dehumidifier page.

What to ask before you rely on one

A short list for the product page, the manual or the seller:

• Is it an evaporative cooler, or does it contain a refrigerant system and a compressor?
• What outdoor temperature and humidity was the advertised temperature drop measured at?
• Where does the extra moisture go, and does the room have an open path for air to leave?
• What does the manual say about draining and cleaning the water tank?

The Department of Energy's Home Cooling 101 guide lists two drawbacks for an evaporative cooler: it needs more frequent maintenance, and it suits only low-humidity areas. Our reading: for a bedroom or a whole house in a Tampa Bay summer, the options that work with humid air remove heat with refrigerant, such as a ductless mini-split system. Sizing and installation of any refrigerant system is a job for a licensed contractor.