Do evaporative coolers work in humid climates?

This is the first question to answer about evaporative cooling, and getting it wrong is the most common expensive mistake in the category. A swamp cooler is not a cheaper air conditioner that happens to work a bit less well in humidity. In humid air it does not do the job at all.

Evaporation takes energy. When water changes from liquid to vapor it absorbs heat from its surroundings, and that heat has to come from somewhere — in this case, from the air being pulled through the wet pad. Air goes in warm and dry, comes out cooler and damper. The heat has not left the building; it has been converted into water vapor carried in the air.

This is the same reason sweating cools you, and the same reason stepping out of a pool feels cold.

The limit follows directly. Air can only hold so much water vapor, and how much it is already holding determines how much more it can absorb. Relative humidity is exactly that measure: the percentage of the maximum the air is currently carrying.

• At 15% relative humidity the air has enormous capacity for more moisture. Evaporation is rapid, a lot of heat is absorbed, and the temperature drop is substantial.
• At 50% evaporation is slower, the drop is modest, and you are adding humidity to air that is already halfway loaded.
• At 80% there is almost no capacity left. Very little water evaporates, very little heat is absorbed, and what you mostly get is a fan blowing damp air at you.

And there is a second, nastier effect at the top of that range. Your own body cools itself by evaporating sweat, using the same physics. Add moisture to already-humid air and you make it harder for your own sweat to evaporate. A swamp cooler in a humid room can genuinely make a person feel warmer while the thermometer reads a degree or two lower.

Do not use the humidity figure you remember, and do not use the one on your phone at 8am. Both will mislead you.

1. Look up your local relative humidity at 3–5pm on a hot day. Any weather app shows it. The afternoon figure is the one that matters, because that is when you would be running the cooler.
2. Check it across several hot days, not one. A single dry afternoon in a humid climate is not a pattern.
3. Read it against the table above. Under 40%: buy one. Over 60%: do not.

Why morning humidity misleads you: relative humidity falls as air warms, even when the actual amount of water in the air has not changed at all. Warmer air can hold more, so the same water is a smaller percentage of the maximum. A humid climate can read 85% at dawn and 55% at 3pm with identical moisture content. That afternoon 55% still means the air is carrying a great deal of water — it is genuinely marginal for evaporative cooling, and the morning number would have told you it was hopeless while the afternoon number oversells it slightly.

As a first filter, before you check your own numbers:

• Works well: the desert Southwest and interior West — Arizona, Nevada, New Mexico, inland California, Utah, Colorado, west Texas, eastern Oregon and Washington. Summer afternoon humidity in these areas is routinely under 25%.
• Marginal, and seasonal: the high plains and parts of the Mountain West. Often good in June and poor once monsoon moisture arrives in July and August — which is precisely when you want it most.
• Does not work: the Southeast, the Gulf Coast, the Mid-Atlantic, the Northeast, the Midwest, the Pacific Northwest west of the Cascades. Summer afternoon humidity is commonly 60–80%.

The monsoon case deserves emphasis, because it catches people in Phoenix and Tucson who bought on a dry June and found the machine useless in a humid August. If you are in a monsoon region, expect a few weeks a year where it stops working and have another plan.

Dry air is necessary but not sufficient. An evaporative cooler continuously adds moisture to the space it is in, so in a closed room the humidity climbs until evaporation slows and the cooling stops. The machine effectively defeats itself over an hour or two.

It needs a route out: an open window, an open door, a garage door up. Airflow through the space rather than around it. That single requirement is why these units suit garages and patios so well, and sealed bedrooms so badly.

It is also why the phrase “ventless air conditioner” is doubly misleading when applied to these: they are not air conditioners, and they specifically need ventilation. If you found this page looking for a windowless-room solution, read this instead.

Then this is genuinely excellent value: real cooling at roughly a fifth of the power of refrigeration. The picks, ranked on airflow per watt — including the finding that every coverage claim in the category is just CFM divided by 1.64 — are the next page to read. And the direct cost comparison against an air conditioner is here.