Evaporative coolers cool air through evaporation, as the name of course implies. In drier air, an evaporative cooler is able to lower the air temperature more effectively than when the air is humid.
This is why evaporative coolers are best used in parts of the country with arid climates and are less effective in warm, muggy regions.
In fact, if you live in one of the many humid regions of North America, evaporative coolers, aka swamp coolers, might not be available from your local appliance or home improvement store because they simply aren’t effective enough to be recommended for use where you live.
Wet Bulb Temperature
This is a technical phrase you will come across as you research evaporative coolers and their effectiveness.
The definition of wet bulb temperature is the lowest temperature air can become through evaporating moisture out of it.
Wet bulb temperature is the temperature measured by a technical instrument called a psychrometer. The device includes a wet bulb thermometer, dry bulb thermometer and a psychrometric chart showing the relationship between the two.
The wet bulb portion of the instrument is wrapped in fabric and soaked in water. The bulb is then vented to create evaporation, which of course, removes heat from the surface and cools the thermometer. The dry bulb thermometer continues to show the temperature of the air around the device – it stays constant because moisture is not being evaporated off of it.
The psychometric graph demonstrates how the difference in wet bulb and dry bulb temperature changes as evaporation occurs at various humidity levels. As humidity rises, the evaporative cooling becomes less. At 100% humidity the wet and dry bulb temperatures will be the same.
Fun Fact! Ben Franklin famously did experiments with evaporative cooling in the 1750s, proving the effectiveness of this technology.
Note: Wet bulb temperature is considered the ideal temperature of air cooled through evaporation. In our research, we have found that an evaporative cooler has the ability to cool air to between 3 and 7 degrees above the ideal ambient wet bulb temperature.
Evaporative Coolers
This chart shows that as indoor air temperature rises, an evaporative cooler will become more effective at cooling air as the relative humidity drops.
In other words, an evaporative cooler makes the most sense, as we’ve noted, in dry air conditions usually found in desert regions.
Evaporative Cooler Optimum Humidity Level Chart
| Indoor Air Temperature | Optimum Humidity Level |
|---|---|
| 80 F | 40% – 60% |
| 85 F | 40% – 55% |
| 90 F | 25% – 45% |
| 95 F | 20% – 35% |
| 100 F | 15% – 25% |
| 105 F | 10% – 20% |
The next chart shows what temperature differences can be achieved with an evaporative cooler based on indoor air temperature and relative humidity.
For example, if it is 75 degrees in your home and the RH is 20%, air from an evaporative cooler can be 59 degrees. The air coming from the unit would be 26 degrees cooler than the indoor air.
But at a relative humidity of 70%, the cooling power would be just 5 degrees.
Evaporative Cooler Temperature Chart (Humidity to Efficiency)
| Air Temp | 20% RH | 30% RH | 40% RH | 50% RH | 60% RH | 70% RH |
|---|---|---|---|---|---|---|
| 75 F | 59 F | 62 F | 64 F | 66 F | 68 F | 70 F |
| 80 F | 63 F | 66 F | 68 F | 71 F | 73 F | 76 F |
| 85 F | 67 F | 70 F | 72 F | 74 F | 76 F | 79 F |
| 90 F | 70 F | 74 F | 77 F | 79 F | 82 F | 84 F |
| 95 F | 74 F | 78 F | 81 F | 84 F | 87 F | N/A |
| 100 F | 78 F | 82 F | 85 F | 88 F | N/A | N/A |
| 105 F | 81 F | 86 F | 89 F | N/A | N/A | N/A |
| 110 F | 85 F | 90 F | 86 F | N/A | N/A | N/A |
The above chart will assist you in determining if an evaporative cooler is a good air cooling option for your home.
Should I add ice to my evaporative cooler? A few manufacturers claim that adding ice to the water in an evaporative cooler can increase its ability to cool air.
Some homeowners swear by it too, but there is mixed evidence that it works. In fact, some HVAC professionals warn that adding too much ice to the water will prevent evaporation and will reduce the effectiveness of the cooler. It is also possible that ice could damage the pump of an evaporative cooler.
The “perfect” water temperature for use in evaporative or swamp cooler is 50F. So adding some ice can increase its effectiveness, just don’t over-do it or you might damage your cooler and you won’t get much benefit from it.
Evaporative Cooler Temperature Chart
How cool does the air feel coming from an evaporative cooler that is right in front of you and you’re in its airstream?
The answer is that it will feel cooler than it actually is. This is true for two reasons.
1). Evaporation, of course, is the first. If the air is moist, the moisture will hit your skin and evaporate, taking heat with it. Think of it like standing outside in a 75F day with a 10mph breeze. You’ll feel cooler if you just got out of water from swimming or are sweating than if you are dry.
2). Windchill – There is a little bit of a windchill effect. Moving air feels cooler. If you’re familiar with AccuWeather, the service uses something called RealFeel ™ to “advise people on how weather conditions really feel.”
Evaporative Cooler Feeling Temperature Chart
| AmbientTemperature | 20% RH | 30% RH | 40% RH | 50% RH | 60% RH | 70% RH |
|---|---|---|---|---|---|---|
| 75 F | 51 F | 54 F | 57 F | 60 F | 61 F | 63 F |
| 80 F | 56 F | 58 F | 61 F | 63 F | 65 F | 67 F |
| 85 F | 59 F | 62 F | 65 F | 67 F | 69 F | 72 F |
| 90 F | 63 F | 66 F | 69 F | 72 F | 74 F | 77 F |
| 95 F | 67 F | 70 F | 73 F | 76 F | 78 F | N/A |
| 100 F | 70 F | 74 F | 77 F | 81 F | 83 F | N/A |
| 105 F | 74 F | 78 F | 81 F | 85 F | N/A | N/A |
| 110 F | 77 F | 82 F | 86 F | 89 F | N/A | N/A |
As you browse the chart, it is easy to see what we have been discussing on this page – air feels cooler when the relative humidity is lower. As the air gets more “muggy,” has more moisture in it, it feels warmer.
Our chart is quite accurate, though you will see other charts with slight variations when researching this subject from scholarly sources.
If you confirm evaporative cooler is suitable for your climate, our best evaporative coolers on the market post can help you find a best unit for your home.
