Do Swamp Coolers Work in Humidity? Where the Cutoff Really Is

Swamp coolers do work in some humidity, just not much of it. Cooling comes from evaporating water into the air, and the drier the air, the more water it can take and the more heat it gives up. As outdoor moisture rises, the cooler's temperature drop shrinks and the air it delivers gets damper. There is no single humidity percentage where a cooler "stops working," but there are clear thresholds where it goes from excellent to marginal to not worth running. This page gives you those thresholds and the math to check your own weather.

Why humidity limits evaporative cooling

A direct cooler can, at best, bring air down to the outdoor wet bulb temperature. In practice a pad captures a fraction of the gap between dry bulb and wet bulb (its saturation efficiency, about 60 to 80% for aspen and 85 to 95% for rigid media):

Humid air has a wet bulb close to its dry bulb, so the gap is small. Doubling the relative humidity at the same temperature can cut that gap roughly in half.

Performance at different humidity levels

Two things stand out. First, rigid media buys you about 10 percentage points of humidity headroom over aspen. Second, the supply temperature climbs steeply between 20% and 40% RH at this temperature. That band is where most real-world complaints come from. Run your own numbers in the cooling temperature calculator.

The cutoffs that actually matter

Because RH changes through the day, it is easier to use wet bulb or dew point to decide whether a cooler will perform.

Dew point is handy because forecasts list it and it changes slowly through the day. In Arizona and New Mexico, when forecasters mention dew points in the upper 50s or 60s F, expect cooler performance to drop. See swamp coolers during monsoon season.

What happens to indoor humidity

Temperature is only half the comfort story. Here is the same cooler on a dry day and a humid day, tracking moisture all the way into the room.

Note that the temperature rise across the house (8 F here) is not fixed. More airflow and more relief air reduce it. People, cooking and showers add moisture on top of what the cooler supplies. For indoor humidity targets and health implications, read swamp coolers and indoor humidity.

Why RH alone is misleading

A "40% humidity" forecast in the morning can become 15% by afternoon with no change in actual moisture. The dew point stays put while the temperature climbs, and RH drops. That is why a cooler often performs better at 4 pm than at 10 am in the desert, even though it is hotter. Conversely, a 105 F day at 20% RH (wet bulb about 74 F) is harder on a cooler than an 85 F day at 30% RH (wet bulb about 64 F). Always translate to wet bulb before you judge.

Getting the most from a cooler on humid days

1. Increase relief air. Open windows wider, especially on the side of the house opposite the supply. More exhaust lowers indoor humidity and raises airflow. Aim for at least 2 sq ft of opening per 1,000 CFM on humid days. See venting and relief air.
2. Run high speed. More air changes mean less time for heat and moisture to build up indoors.
3. Consider fan-only mode. When the wet bulb is above about 75 F and the outdoor air is not much warmer than indoors, turning the pump off and running ventilation alone can feel better than adding moisture. If the outdoor air is hotter than indoors, close up instead.
4. Keep pads in top shape. Every point of saturation efficiency counts when the gap is small. Clear distribution holes and replace scaled pads; see aspen vs rigid media.
5. Cool at night and morning. Run hard overnight and early morning to pre-cool the house mass, then reduce relief and ride out the humid afternoon.
6. Use spot cooling. A room air conditioner in the bedroom covers the worst nights without giving up the cooler for the rest of the summer.

Where humidity rules out a swamp cooler

Most of the US east of the Rockies has summer design wet bulbs of 73 to 80 F. In those climates a direct cooler is not a practical whole-house solution, although it can still provide useful spot or outdoor cooling in shops, barns and patios where airflow alone helps and moisture does not matter. The Gulf Coast, Florida, the Southeast, the Midwest and the Mid-Atlantic all fall in this group. The Pacific Northwest is mild enough that cooling needs are small, and coastal California is often both mild and humid near the water.

Where coolers excel: the Great Basin, Colorado Plateau, Front Range, inland Southern California, the Central Valley, southern Idaho and the high desert of New Mexico and Arizona. The low deserts (Phoenix, Tucson, Las Vegas, El Paso) are excellent in late spring and early summer and marginal during monsoon surges. See best climates for swamp coolers and our climate index for city-level data.

Portable coolers in humid places: what to expect

Portable evaporative coolers are sold nationwide, including in humid regions, and often marketed as "air coolers." In a humid climate, expect them to behave mainly as fans. On an 88 F day at 60% RH, the wet bulb is about 77 F; a small portable with a thin pad at roughly 50 to 60% efficiency drops the air only about 6 F, and that air is now near 85% RH. In a closed room, humidity climbs within an hour and the cooling effect fades.

Where they can still help in humid regions:

• Open, well-ventilated spaces such as garages, workshops, barns and covered patios, where the moisture blows away and the airflow on your skin is the main benefit. See outdoor patio evaporative coolers.
• Dry spells and afternoons with lower dew points, which occur even in humid climates.
• Livestock and poultry houses using properly designed fan-and-pad systems, where design rules account for humidity; see evaporative cooling for livestock and poultry.

For bedrooms and living spaces in humid regions, a window AC or ductless heat pump is the right tool. Our sizing guide for portable and outdoor coolers covers airflow targets where a portable cooler does make sense.

Indirect and two-stage options for humid edges

If your climate is mostly dry with humid periods, an indirect or two-stage evaporative system can extend the useful range because the first stage cools without adding moisture. It does not abolish the limit, and equipment costs considerably more. For most homes the better hedge is a standard cooler plus AC for the humid weeks. Compare options in direct vs indirect evaporative cooling.