Wet Bulb Temperature Explained: The Number That Decides If a Swamp Cooler Works

Wet bulb temperature is the temperature a thermometer reads when its bulb is wrapped in a wet wick and air blows across it. Water evaporating from the wick cools the bulb until the heat coming in from the air balances the heat leaving with the evaporating water. That equilibrium temperature is the wet bulb. It is exactly what a swamp cooler is chasing: the wet pad is a giant wet wick, and the coldest air it can make is the wet bulb temperature of the air coming in.

If you only learn one weather number as a cooler owner, make it this one. It tells you, in a single figure, how much cooling the air can give you.

Dry bulb, wet bulb and dew point: three different temperatures

Dry bulb

The ordinary air temperature on a thermometer shielded from sun and moisture.

Wet bulb

The temperature reached by evaporative cooling to saturation. Always at or below dry bulb.

Dew point

The temperature at which the air's existing moisture would start to condense if cooled with no moisture added. Always at or below wet bulb.

The order is always dry bulb, then wet bulb, then dew point, with all three equal at 100% relative humidity. On a 100 F afternoon at 15% RH, for example, the dry bulb is 100 F, the wet bulb about 67 F and the dew point about 44 F. Dew point tells you how much moisture is in the air; wet bulb tells you how much evaporative cooling is available. Weather forecasters in the Southwest watch dew point to call the monsoon, and it moves the wet bulb along with it.

Why relative humidity alone can fool you

Relative humidity is the percentage of the moisture the air could hold at its current temperature. Because warm air can hold much more water than cool air, the same RH means very different things at different temperatures. Two examples:

• 105 F at 20% RH: wet bulb about 74 F.
• 85 F at 30% RH: wet bulb about 64 F.

The first case has the lower humidity reading but the worse cooling floor. A cooler will make about 79 F air in the first case and about 67 F air in the second (both at 85% efficiency). That is why we size and judge coolers on wet bulb, not RH.

Wet bulb lookup table

Values are rounded from standard psychrometric relationships. At high elevation, wet bulbs run roughly 1 to 2 F lower for the same temperature and RH. For exact values at your conditions, use the cooling temperature calculator.

Using wet bulb to predict cooler output

The gap between dry bulb and wet bulb is the wet bulb depression. A cooler captures a fraction of it set by its pad's saturation efficiency:

The usable rule that falls out of this arithmetic: for comfortable rooms you generally want supply air at or below about 75 F, which with good media requires a wet bulb around 70 F or lower on a hot afternoon.

How to measure wet bulb yourself

You do not need lab equipment. Three practical methods, from most to least hands-on:

1. Sling psychrometer. Two thermometers on a swivel handle, one with a wet cotton wick. Wet the wick with distilled water, whirl it for about 1 to 2 minutes in shade, read the wet bulb, whirl again and read until the number stops falling. These typically cost $25 to $80. Technicians commonly use them to check pad efficiency.
2. Digital psychrometer or thermo-hygrometer. Many HVAC meters calculate wet bulb from temperature and RH directly. Accuracy depends on the RH sensor, often plus or minus 2 to 3% RH.
3. Weather data. Take current temperature and RH or dew point from the National Weather Service or a nearby station and calculate the wet bulb. This is good enough for decisions.

Common mistakes when reading wet bulb

• Using morning humidity. RH is highest near sunrise and lowest in mid afternoon. Judge cooler weather from afternoon conditions, or better, from the dew point, which changes little through a dry day.
• Measuring in the sun. A sling psychrometer or thermometer in direct sun reads high on both bulbs. Always measure in open shade with moving air.
• Measuring near the cooler. Air next to a running cooler, a lawn sprinkler or a pool is more humid than the general outdoor air. Take readings at least 10 to 20 ft away from water sources.
• Dirty or dry wicks. A crusted or partly dry wick reads high. Replace wicks when they stiffen and wet them with distilled water.
• Ignoring elevation. Online calculators that assume sea-level pressure overstate the wet bulb by roughly 1 to 2 F at 5,000 ft. That small error can make a good climate look marginal.

Design wet bulb: the number for sizing and buying

Daily wet bulbs swing, so engineers use design conditions: values exceeded only a small fraction of summer hours, published in the ASHRAE Handbook of Fundamentals. For evaporative cooling, the key figure is the 1% or 0.4% design wet bulb (and the mean coincident dry bulb). A city with a 1% design wet bulb of 64 F will almost never see a day when a good cooler cannot deliver about 70 F air. A city at 76 F will see weeks when it cannot.

Our climate index compiles design dry and wet bulbs and the achievable supply temperatures for 50 US cities. Use it alongside best climates for swamp coolers when deciding whether a cooler fits your region, and the cooler size calculator once you know it does.

Wet bulb across a day and a season

In dry climates the dew point is fairly steady through a day while the dry bulb swings 25 to 35 F. That means the wet bulb rises much less than the temperature during the afternoon, and the depression is largest exactly when you need cooling most. Coolers naturally deliver their biggest temperature drop at 3 to 5 pm.

Seasonally, the picture changes when moist air arrives. In Arizona and New Mexico, the monsoon typically pushes dew points from the 30s and 40s F into the upper 50s and 60s F between early July and mid September, and afternoon wet bulbs climb into the low to mid 70s F. That is the period when coolers struggle; see swamp coolers during monsoon season for tactics.

Wet bulb and human heat stress

Your skin cools itself the same way a pad does, by evaporating sweat, so wet bulb is also a measure of how well your body can shed heat. As the wet bulb rises, sweating becomes less effective. Heat-safety guidance from OSHA and the National Weather Service uses related indices such as heat index and wet bulb globe temperature for work and outdoor activity. A swamp cooler adds airflow, which helps, but in humid heat it cannot lower the wet bulb of the room. If indoor conditions become dangerous during a heat wave, move to air-conditioned space and follow local heat guidance.