Misting cooling potential calculator

How the calculator works

The calculator does three things, in order. First it finds the wet-bulb temperature of your air, the lowest temperature evaporation can reach at that humidity. Second it applies a system efficiency: the share of the gap between air temperature and wet-bulb temperature that a real misting system closes in open air. Third it recalculates humidity for the cooled air (the wet-bulb temperature stays constant during pure evaporative cooling) and compares the National Weather Service heat index before and after.

1. Wet-bulb temperature. Computed with the Stull (2011) empirical formula, accurate to within about 1 °C for humidity between roughly 5 and 99 percent at normal outdoor temperatures.
2. Misted air temperature. Misted temperature = air temperature minus efficiency multiplied by (air temperature minus wet-bulb temperature).
3. New humidity. Found by searching for the relative humidity at the misted temperature that has the same wet-bulb temperature as the original air.
4. Heat index. The NWS Rothfusz regression with its standard low-humidity and high-humidity adjustments.

Where the efficiency numbers come from

Wetted-media evaporative coolers (swamp coolers) routinely reach 70 to 90 percent of the wet-bulb depression because every bit of air is forced through wet pads. Open-air misting has no such enclosure: wind carries cooled air away, some droplets land before evaporating, and people sit at the edge of the plume. The values in the dropdown are our editorial planning assumptions for a system that is laid out correctly. They are estimates, not measured values. A breezy afternoon or a poorly placed line can put you well below them; a fan pushing high-pressure mist directly at a seating area can beat them. See how misting systems work for the physics and nozzle spacing and layout for what "laid out correctly" means.

Reference results for typical summer afternoons

The pattern is the most important thing on this page. In the desert row the air temperature and the feels-like temperature fall together, by roughly 20 °F. In the Gulf coast row the thermometer drops 8 °F but the heat index improves by only about 6 °F, because the air ends up near 80 percent humidity. That is why misting in humid climates depends so heavily on air movement from misting fans.

Using the result to choose a system

• Depression above 25 °F: any pressure class will cool, but only high pressure (800 to 1,000 psi) cools without wetting. See high-pressure misting systems.
• Depression 15 to 25 °F: high pressure is worth the investment; mid pressure works on smaller, sheltered patios.
• Depression 9 to 15 °F: choose fan-assisted misting and expect modest results. Low-pressure kits will mostly get things wet.
• Depression under 9 °F: put the money into shade and fans first.

Once you know misting makes sense, size the hardware with the nozzle and pump sizing calculator and check running costs with the water use and cost calculator.

Limitations to keep in mind

• The result describes air in the core of the mist zone, not the whole yard. A few feet outside the plume, temperatures return to ambient quickly.
• Humidity from a weather station can differ from your patio, especially next to lawns, pools or irrigated beds that already add moisture.
• Radiant heat from sun-baked walls and pavers is not in the model. Shade often matters as much as mist.
• The heat index formula assumes shade and light wind. Direct sun can add up to about 15 °F to how hot it feels.