What wet-bulb temperature is too high for a swamp cooler?

Once the outdoor wet bulb passes about 72°F, even rigid media cannot deliver air much below 75°F, and indoor humidity climbs. Above about 75°F wet bulb, evaporative cooling alone cannot keep a home comfortable.

Why does Phoenix rate only marginal when swamp coolers are so common there?

The index uses a hot design day, and Phoenix's hottest design conditions are shaped by monsoon moisture. In May and June, when wet bulbs are often in the low to mid 60s, Phoenix coolers perform excellently; during monsoon spells they struggle.

Is the index the same as the summer average?

No. It uses approximate 1% design conditions, the kind of hot day exceeded only about 88 hours a year. Most summer hours are cooler and drier, so typical performance is better than the table shows.

My city is not listed. How do I check it?

Look up a typical hot afternoon temperature and relative humidity for your area and enter them in the cooling temperature calculator, which computes the wet bulb and expected supply air temperature.

Does elevation change the results?

Slightly. At high elevation the lower air pressure makes evaporation a little more effective for the same temperature and humidity, so mountain-city results are, if anything, conservative.

Swamp Cooler Climate Index: 50 U.S. Cities Rated

Whether a swamp cooler can keep you comfortable is not a matter of opinion or marketing. It is set by one number you can look up: the wet-bulb temperature of your outdoor air. We took approximate summer design conditions for 50 U.S. cities, ran them through the standard evaporative cooling equation for both aspen and rigid media pads, and rated each city. The result is a practical map of where evaporative cooling makes sense, and where it does not.

How to read the index

Each row starts with two climate figures. The design dry bulb is the outdoor air temperature on a hot day that is exceeded only about 1% of the hours in a year (roughly 88 hours). The coincident wet bulb is the average wet-bulb temperature that occurs at the same time. The gap between them, the wet-bulb depression, is the most cooling an evaporative cooler could ever deliver at those conditions.

Real pads never reach 100% saturation, so we then calculated the supply air temperature leaving the cooler at two efficiencies:

Supply air temp = Dry bulb − Efficiency × (Dry bulb − Wet bulb)

70% represents clean, properly wetted aspen pads in a typical residential cooler.
88% represents 8-inch rigid media in good condition.

Use the region filter to compare places with similar climates. Ratings are explained in the next section.

Show region

Approximate 1% summer design conditions and calculated supply air temperature, sorted coolest first. Temperatures in °F.
City	Design dry bulb	Coincident wet bulb	Wet-bulb depression	Supply air, aspen (70%)	Supply air, rigid (88%)	Rating	Notes
Flagstaff, AZ	85	56	29	65	59	Excellent	High elevation, short cooling season
Santa Fe, NM	88	57	31	66	61	Excellent	High elevation
Cheyenne, WY	86	58	28	66	61	Excellent
Colorado Springs, CO	88	58	30	67	62	Excellent
Bend, OR	90	58	32	68	62	Excellent	Large day to night swing
Casper, WY	91	58	33	68	62	Excellent
Denver, CO	91	59	32	69	63	Excellent
Albuquerque, NM	94	60	34	70	64	Excellent	Monsoon humidity July to August
Reno, NV	95	60	35	70	64	Excellent	Large day to night swing
Spokane, WA	92	61	31	70	65	Excellent
Grand Junction, CO	96	61	35	72	65	Excellent
Billings, MT	93	62	31	71	66	Excellent
Pueblo, CO	95	62	33	72	66	Excellent
Salt Lake City, UT	96	62	34	72	66	Excellent
Boise, ID	96	63	33	73	67	Excellent
Los Angeles, CA	84	65	19	71	67	Good	Marine layer, mild summers
Las Cruces, NM	99	63	36	74	67	Excellent	Monsoon humidity July to September
Yakima, WA	93	64	29	73	67	Excellent
San Diego, CA	84	66	18	71	68	Good	Marine layer, mild summers
El Paso, TX	100	64	36	75	68	Excellent	Monsoon humidity July to September
Rapid City, SD	93	65	28	73	68	Excellent
Amarillo, TX	96	66	30	75	70	Excellent
Tucson, AZ	104	65	39	77	70	Excellent	Monsoon humidity July to September
Medford, OR	98	66	32	76	70	Excellent
St. George, UT	104	66	38	77	71	Good
Lubbock, TX	97	67	30	76	71	Good
Las Vegas, NV	106	66	40	78	71	Good	Brief monsoon spells
Midland, TX	99	67	32	77	71	Good
Riverside, CA	99	68	31	77	72	Good
Redding, CA	104	68	36	79	72	Good
Sacramento, CA	100	69	31	78	73	Good	Delta breeze evenings
San Bernardino, CA	100	69	31	78	73	Good
Fresno, CA	101	70	31	79	74	Good
Bakersfield, CA	102	70	32	80	74	Good
Phoenix, AZ	110	70	40	82	75	Marginal	Monsoon humidity July to September
Honolulu, HI	89	73	16	78	75	Marginal	Trade winds
San Antonio, TX	98	73	25	80	76	Marginal	Humid Gulf air
Chicago, IL	91	74	17	79	76	Marginal
Wichita, KS	100	73	27	81	76	Marginal	Humid Gulf air
Yuma, AZ	111	72	39	84	77	Marginal	Gulf moisture surges in late summer
Oklahoma City, OK	98	74	24	81	77	Marginal	Humid Gulf air
Austin, TX	98	74	24	81	77	Marginal	Humid Gulf air
Dallas, TX	100	74	26	82	77	Marginal	Humid Gulf air
Omaha, NE	93	75	18	80	77	Poor
Atlanta, GA	93	75	18	80	77	Poor
Palm Springs, CA	111	73	38	84	78	Marginal	Occasional monsoon surges
Kansas City, MO	95	76	19	82	78	Poor
Miami, FL	91	77	14	81	79	Poor
Houston, TX	96	77	19	83	79	Poor
New Orleans, LA	94	78	16	83	80	Poor

How the ratings work

Ratings start from the rigid-media supply air temperature, because that is what a well-specified modern installation achieves. Two adjustments then account for how often a city sees worse days than the design point.

Rating rules used in the index
Rating	Rigid-media supply air	What it feels like indoors
Excellent	70°F or below	Whole-house comfort in the mid 70s or cooler, even on the hottest days
Good	70 to 74°F	Comfortable most of the summer; a few warm, sticky afternoons
Marginal	74 to 78°F	Real relief from the heat, but warm and humid on design days
Poor	Above 78°F	Cannot hold comfortable conditions; adds humidity to an already humid house

Wet-bulb cap. A city whose coincident wet bulb is 72°F or higher is rated no better than marginal, and 75°F or higher is rated poor. High wet bulbs mean muggy air that pushes indoor relative humidity toward uncomfortable levels, regardless of the temperature drop.
Small-depression cap. A city with a wet-bulb depression under 20°F is rated no better than good. Mild coastal climates can show cool supply temperatures on paper, but there is little headroom when a humid day arrives.

What the data shows

The high, dry interior is the sweet spot

Every city in the Great Basin, the interior Pacific Northwest and the Rocky Mountain and High Plains region rates excellent. Denver, Salt Lake City, Boise, Reno, Spokane and Cheyenne all have design-day wet bulbs at or below 63°F, so rigid media delivers supply air in the low to mid 60s even at 91 to 96°F outside. In these places a properly sized evaporative cooler is not a compromise. It can match or beat central air for comfort, at a fraction of the running cost, and the dry air feels fresh rather than clammy.

The low desert paradox

Phoenix, Yuma and Palm Springs rate only marginal, which surprises people who know how many rooftop coolers those cities have. The explanation is timing. Their hottest design conditions coincide with the North American monsoon, when Gulf of California and Gulf of Mexico moisture pushes wet bulbs into the 70s. In May and June, a 108°F Phoenix afternoon at 8% humidity has a wet bulb near 66°F, and rigid media would deliver roughly 71°F air. That is why many low-desert homes run a cooler in early summer and switch to refrigerated air from about mid July. Our monsoon season guide covers how to stretch the cooler season.

Worked example: Phoenix in June versus August

June afternoon: 108°F and 8% relative humidity gives a wet bulb of about 66°F. Depression is 42°F, and rigid media at 88% delivers 108 − 0.88 × 42 = 71°F air.

August monsoon afternoon: 104°F and 30% relative humidity gives a wet bulb of about 79°F. Depression is 25°F, and the same cooler delivers 104 − 0.88 × 25 = 82°F air. Same machine, same house, an 11°F worse result.

Tucson, El Paso and Albuquerque beat Phoenix

Higher elevation and slightly lower design wet bulbs push Tucson, El Paso, Las Cruces and Albuquerque into excellent territory. Albuquerque in particular, at 94°F design dry bulb and about 60°F wet bulb, gets supply air in the mid 60s with rigid media.

California's Central Valley is good, not great

Fresno, Bakersfield, Sacramento and the Inland Empire all rate good. Their wet bulbs sit around 68 to 70°F on design days, so a rigid-media cooler delivers air in the low 70s. That keeps a house comfortable most of the summer, with the caveat that well-sized airflow matters more here than in Denver. Use our sizing calculator with the hot-dry setting.

West Texas versus the rest of Texas

Lubbock, Amarillo and Midland, on the dry side of the dry line, rate good to excellent. Dallas, Austin and San Antonio, a few hundred miles east, rate marginal because Gulf moisture lifts their wet bulbs to 73 or 74°F. Houston rates poor. Texas shows how quickly evaporative cooling stops working as you move toward a moisture source.

The humid East and Gulf Coast

Kansas City, Omaha, Atlanta, Houston, New Orleans and Miami rate poor. With design wet bulbs of 75 to 78°F, the best possible supply air is close to 80°F and heavily humidified. In these climates, evaporative cooling belongs in garages, workshops and outdoor spaces with lots of airflow, not in sealed homes. See do swamp coolers work in humidity for the details.

Aspen versus rigid media by city

The two supply-air columns show what upgrading media buys you. The gap is 18% of the wet-bulb depression, so it grows with dry heat. In Phoenix on a design day the difference is about 7°F (82°F versus 75°F). In Denver it is about 6°F (69°F versus 63°F). In Houston it is barely 3°F, because there is so little depression to work with. If you live in a city rated good or marginal, rigid media is often what moves a cooler from disappointing to acceptable. Read aspen vs rigid media before your next pad change.

Methodology and limitations

Climate inputs. Design dry-bulb and mean coincident wet-bulb values are approximate 1% cooling design conditions, rounded to whole degrees, consistent with the climatic design tables published in the ASHRAE Handbook of Fundamentals. Published values differ slightly between editions and weather stations, so treat each figure as accurate to about ±1 to 2°F.
Efficiency assumptions. 70% and 88% are mid-range figures for clean aspen and 8-inch rigid media. Dirty, scaled or dry-striped pads perform worse; 12-inch rigid media performs a little better.
Supply air is not room air. Air warms by roughly 4 to 10°F as it absorbs heat from people, appliances, walls and sun before it leaves the house. The cooling temperature calculator shows this range for any conditions you enter.
Design days are not typical days. Most cooling-season hours are milder and drier than the 1% point, so real-world performance is usually better than shown.
Local variation. Irrigation, nearby water, elevation and urban heat can shift conditions in your neighborhood. For local hourly data, the National Weather Service and NOAA NCEI publish observations you can plug into our calculator.

You may cite or reuse this index with a link to this page. If you spot a value that differs from your local design data, please let us know.

Swamp Cooler Climate Index: How Well Evaporative Cooling Works in 50 U.S. Cities