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No. In humid air an evaporative cooler barely cools, and it adds moisture to the room while failing to. The Department of Energy's own figures make the size of the effect clear: at 100°F with 10% relative humidity a good cooler delivers supply air at about 73°F — a 27-degree drop. At 95°F with 50% relative humidity, the same machine reaches only 84°F — an 11-degree drop. At 100% relative humidity, evaporative cooling "cannot occur" at all. If you live on the Gulf Coast, in Florida, or anywhere with heavy summer air, do not buy one.
We never buy, test or physically handle the products we rank; recommendations come from aggregated owner reviews and expert consensus, described in how we rank. More at the heating & cooling hub.
Why does humidity stop it working?
An evaporative cooler has no refrigerant and no compressor. It pulls air through a wet pad, some of the water evaporates, and evaporation absorbs heat — so the air leaves cooler and wetter than it arrived. That is the entire mechanism, and it depends completely on the air having room to accept more water vapor.
Humid air does not have that room. The closer air is to saturation, the less water can evaporate into it, so less heat gets absorbed and less cooling happens. The theoretical floor for the supply air temperature is the wet-bulb temperature of the incoming air — the lowest temperature you can reach by evaporating water into it. In dry desert air the wet-bulb sits far below the actual air temperature, which is why the drop is dramatic. In humid air the wet-bulb sits close to the air temperature, so there is almost nothing to gain. As DOE's Building America Solution Center puts it, when relative humidity reaches 100% the wet-bulb equals the dry-bulb temperature and "evaporative cooling cannot occur."
And here is the part that makes it actively counterproductive rather than merely disappointing: the moisture the machine does add stays in your room. In dry air that is a bonus. In humid air you are paying electricity to make a room damper without making it cooler.
How much does the drop actually shrink?
| Outdoor condition | Supply air temperature | Effective drop | Where this is typical |
|---|---|---|---|
| 100°F, 10% RH | ~73°F | ~27°F | Phoenix, Las Vegas, Tucson in June |
| 95°F, 20% RH | ~74–77°F | ~18–21°F | Albuquerque, Denver, Salt Lake City |
| 95°F, 35% RH | ~79–82°F | ~13–16°F | Boise, Reno, Spokane in a wet spell |
| 95°F, 50% RH | ~84°F | ~11°F | Kansas City, Dallas, St. Louis |
| 90°F, 70% RH | ~84–86°F | ~4–6°F, plus added moisture | Houston, Orlando, Atlanta, Charleston |
| Any temperature, 100% RH | No change | None | Fog, heavy rain, a saturated room |
The 100°F/10% and 95°F/50% rows are figures published by the DOE source above. The intermediate rows are interpolated from the same psychrometric relationship, and the direction of travel is what matters more than the exact digits: halve the air's dryness and you roughly halve the cooling.
Notice also that the bottom two rows deliver a drop small enough to be within the range of what a plain fan achieves by moving air over your skin — without putting a single drop of water into the room.
What is the actual cutoff?
DOE guidance sets the threshold on wet-bulb temperature, not relative humidity: "areas with wet-bulb temperatures of 70°F or lower are appropriate for evaporative cooling," and coolers can be used where coincidental wet-bulb temperatures run "as high as 74°F but high humidity will reduce performance."
Wet-bulb temperature is not something most people have to hand, so here are two practical proxies:
- Summer dew point. Widely reported in weather data and a good indicator. Average summer dew points in the low 50s°F or below suggest evaporative cooling will work well. Dew points in the 60s make it marginal. Dew points in the 70s, which are routine on the Gulf Coast, rule it out.
- Relative humidity at the hottest part of the day. Below about 30% is excellent, 30% to 50% is useful, and above 50% to 60% the machine is mostly a humidifier with a fan attached.
One nuance worth knowing: relative humidity falls as air warms, so the same outdoor air that reads 80% at 7am can read 40% at 3pm. That is why it is the afternoon reading, not the morning one, that tells you whether a cooler will earn its keep.
Where in the US does this rule out an evaporative cooler?
| Region | Verdict |
|---|---|
| Arizona, Nevada, southern California, west Texas, southern New Mexico | Excellent — this is the technology's home |
| Colorado, Utah, Idaho, Wyoming, northern New Mexico | Excellent |
| Eastern Washington and Oregon, inland California valleys | Very good |
| Western Kansas, Nebraska, the Dakotas, Montana | Good, with weaker humid spells |
| Oklahoma, north and central Texas, Missouri | Marginal — fine on dry days, poor on muggy ones |
| Upper Midwest, Ohio Valley, Appalachians, Northeast | Not recommended for summer humidity |
| Southeast, Gulf Coast, Florida, coastal Carolinas, east Texas | No — the machine will add humidity and not cool |
Texas is the instructive case, because it appears in three rows. El Paso, Midland and Lubbock have dry summer air where an evaporative cooler is genuinely good. Houston, Beaumont and Galveston do not. Same state, opposite answers — which is why "will it work here" is a question about your local dew point rather than about your state.
The second failure: a closed room becomes a humid room
There is a version of this problem that catches out buyers even in dry climates, and it is worth flagging because it produces the same symptom. An evaporative cooler pushes humid air into a space continuously, and that air has to leave. In a closed room, the humidity climbs until the air inside approaches saturation — and then the cooler stops working, for exactly the reason described above. You have created a humid climate inside one bedroom.
DOE guidance calls for about 2 square feet of unrestricted air relief per 1,000 CFM delivered, and 20 to 40 air changes per hour through the space. For a 3,100 CFM unit that is over 6 square feet of open window — roughly a 2.5-foot square, ideally on the far side of the room. A cracked window is not enough. If your reason for buying was to cool a bedroom with the door and windows shut, an evaporative cooler is disqualified by its own operating requirement regardless of where you live.
The humidity you add is not harmless
Running an evaporative cooler in air that cannot absorb the moisture has consequences beyond disappointment. Sustained high indoor humidity condenses on cool surfaces — window glass, exterior wall corners, cold water pipes, the back of furniture against an outside wall — and persistent moisture on a surface is what mold needs to establish. The EPA's guidance on mold growth is consistent on the point that controlling moisture is the control measure; there is no separate trick that lets you keep the damp and avoid the growth.
It also feels worse. How hot a room seems depends on humidity as well as temperature, because the higher the humidity the less readily sweat evaporates from your skin. That is why a 4-degree drop bought with a large humidity increase can leave a room feeling less comfortable than it was before you switched the machine on — and why so many one-star reviews on these products come from humid states describing exactly that.
What to use instead in a humid climate
- An air conditioner. It removes moisture rather than adding it, which is half of what makes a humid room comfortable. A window unit or a mini-split is more efficient than a portable; among portables, dual-hose beats single-hose. See our portable AC picks and portable versus window AC.
- A dehumidifier, if the room is clammy rather than hot. Dropping indoor humidity from 70% to 50% changes how a room feels more than dropping the temperature two degrees does.
- Fans. Air moving over your skin speeds evaporation of your own sweat, and that is the one form of evaporative cooling that still works in humidity — because your body supplies the water instead of a machine putting it into the room. See tower fans and box fans.
- Shading and night flushing. Free, and often the largest single lever available. Our cooling without AC guide covers the sequence.
The honest bottom line
Evaporative coolers are excellent machines in the wrong-looking package: cheap to buy, roughly a tenth of the electricity of an air conditioner, and capable of a 20-degree-plus drop in supply air. All of that is true in dry air, and none of it is true in humid air. The performance does not degrade gracefully — it collapses, and the machine keeps adding moisture the whole time.
If your summer afternoons are dry, buy one; our evaporative cooler picks and swamp cooler picks rank the field. If your summer air is heavy, buy an air conditioner and a fan instead. We would rather tell you that than sell you a machine your climate has already ruled out.
More from this hub: heating & cooling · evaporative cooler vs air conditioner · how much water does a swamp cooler use · best portable evaporative coolers · how we rank.