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A what? A sling psychrometer is a difficult word for a very simple tool. It dates back to the 1700s and is actually still used today! If it ain’t broke, don’t fix it.

A Sling Psychrometer is a simple device used to measure relative humidity in the atmosphere. It consists of a handle with two thermometers:

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• A regular thermometer – Measures the actual air temperature.
• Wet Bulb Thermometer – Has a cloth wick soaked in water around the bulb; as water evaporates, it cools the thermometer.

To use it is very simple. The psychrometer is swung around in the air (hence the "sling" part), allowing evaporation to occur. The difference between the dry bulb and wet bulb temperatures is then used to determine relative humidity using a psychrometric chart.

While sling psychrometers are still used in some settings, like education, fieldwork, and HVAC applications, modern digital hygrometers and electronic humidity sensors have largely replaced them due to their accuracy and ease of use. However, they remain valuable as a low-tech, reliable backup in cases where electronic instruments might fail or have other issues.

But for centuries, meteorologists, sailors, and even early aviators relied on it to gauge humidity levels before digital sensors became mainstream. But you still might find someone in forestry or a field meteorologist who has one with them - just in case they need a backup.

J​ennifer Gray is a weather and climate writer for weather.com. She has been covering some of the world's biggest weather and climate stories for the last two decades.