How to Run a Swamp Cooler During Monsoon Season
Most summers bring a stretch when the swamp cooler that kept the house comfortable in June can't keep up. Usually the cooler is fine: monsoon moisture has raised the humidity, and an evaporative cooler needs dry air to work. This guide covers how to spot that in a forecast and what to change until the air dries out.
Updated
The short version
- The National Weather Service treats June 15 through September 30 as monsoon season in the Southwest.
- Humidity limits how far evaporation can cool air, so the same cooler delivers warmer air on muggy days.
- Watch the dew point in your forecast: the higher it is, the warmer the air your cooler puts out.
- Open more windows, move more air, cool the house early, switch to fan-only when the pads stop helping, and keep the water clean.
When monsoon season is
Until 2008, the National Weather Service declared the monsoon's start when the air got humid enough: three days in a row with an average dew point of at least 55°F in Phoenix, or 54°F in Tucson. Since then it has used fixed dates, starting June 15 (University of Arizona CLIMAS), with the season tracked through September 30 (NWS Tucson). The Tucson office still charts dew point through the season.
Why your cooler feels weaker
A swamp cooler can only cool air down toward its wet-bulb temperature, the lowest temperature evaporation can reach. Dry air has a wet bulb far below the thermometer reading, so there's plenty of room to cool. When monsoon moisture arrives, the wet bulb climbs and the cooler's output climbs with it (PNNL).
You can see it in the chart. On a 100°F afternoon with a dry 30°F dew point, a cooler with good pads delivers about 68°F air. Raise the dew point to 55°F and that becomes 76°F. At a muggy 70°F dew point it's 83°F, which feels warm in a house that's been cooking all afternoon.
Check the dew point in the forecast
Most weather apps list the dew point next to the temperature. Unlike relative humidity, which swings through the day as the temperature changes, the dew point tells you how much moisture is actually in the air. Use this table to estimate what your cooler will deliver this afternoon.
| Dew point | On a 92°F afternoon | On a 100°F afternoon |
|---|---|---|
| 40°F | 67°F | 70°F |
| 50°F | 71°F | 74°F |
| 55°F | 72°F | 76°F |
| 60°F | 75°F | 78°F |
| 65°F | 77°F | 80°F |
| 70°F | 79°F | 83°F |
At dew points of 50°F or lower, a cooler with good pads delivers air in the low 70s or cooler. Above about 65°F it puts out air in the high 70s or low 80s, enough to take the edge off a hot afternoon.
What to do until it dries out
- 1Open more windows, in the rooms you're using. A cooler needs somewhere for air to leave: at least 2 square feet of opening for every 1,000 CFM of cooler. Air flows toward the openings, so open windows in the rooms you want cool and close the doors to rooms you don't (PNNL). When it's humid, err toward more opening so moist air leaves instead of building up inside.
- 2Run the fan on high. When the cooler can't drop the temperature much, moving more air past you still makes the room feel cooler.
- 3Cool the house early. At the same dew point, cooler outdoor air means cooler air from the pads. Running hard in the morning gets the house down before the afternoon heat, when the cooler has less to work with.
- 4Switch to fan only when the pads stop helping. After a storm, or on a mild evening, the outdoor air can be cooler than the house. The vent or fan-only setting brings it in like a whole-house fan without adding moisture (PNNL).
- 5Shut it off for dust storms. A cooler pulls outdoor air straight through the pads and into the house. Turn it off and close up until the dust settles, then check the pads.
- 6Keep the water clean. Drain the pan if the cooler will sit idle for more than a few days. Standing water can breed mosquitoes and bacteria, including Legionella (PNNL).
All of this is easier when the cooler is on a thermostat instead of a dial. A smart thermostat can follow a schedule, start cooling before you get home and switch to fan-only from your phone. Our guide to smart thermostats for swamp coolers explains how the wiring works.
When a cooler isn't enough
If dew points often sit above 65°F for days at a time where you live, a standard cooler will struggle through much of the summer. Two options are worth a look. A two-stage (indirect/direct) cooler pre-cools air through a heat exchanger before the wet pads, reaching 95% effectiveness or better with less added moisture (PNNL). Or keep the cooler for the dry months and use refrigerated air for the muggiest weeks; our swamp cooler vs. air conditioner guide compares the two.