Why did meteorologists in the past searched for hair of red-haired women?

Air humidity affects people's health and well-being, food shelf life, and a successful harvest, among other things.

It was in ancient China that people first started noticing the way humidity affects their environment. Over 3000 years ago, on the shores of the Yellow River, scholars of the Shang Dynasty's royal palace began to weigh bits of charcoal and handfuls of soil. They did that right after drying the material, and then again after letting it sit in the humid open air. They inscribed the results of their experiments on tortoise shells. The objects changed mass, indicating humidity fluctuations in the surface air. This knowledge was a great help to merchants, builders, and all agricultural workers.

Man of many talents Later, Europeans also started to note the importance of humidity. The first engineer that tried to create a hygrometer (an instrument that measures air humidity) was Leonardo da Vinci, of course.

Man of many talents

Later, Europeans also started to note the importance of humidity. The first engineer that tried to create a hygrometer (an instrument that measures air humidity) was Leonardo da Vinci, of course.

Da Vinci's hygrometer drawing. Source: Codex Atlanticus: https://codex-atlanticus.it/#/Detail?detail=30

In late 15th century, he conducted experiments to study the compression and expansion of organic matter of different weight at different humidity levels. Finally, he came up with the idea of using a scale with two pans balanced against each other. He put a piece of waterproof beeswax on one pan and a permeable cotton ball on the other. When the humidity level changed, the cotton ball absorbed moisture from the air and grew heavier, tipping the balance. But such a hygrometer seriously lacked precision.

Cherchez la femme rousse

A proper breakthrough in humidity measurement was only made 300 years after da Vinci's discovery. In 1783 in Switzerland, Horace Bénédict de Saussure invented a hygrometer that used... a human hair.

What makes hair useful? It's hygroscopic. In other words, it can absorb moisture and extend when the air gets more humid. In a more dry environment, hair shrinks back to its previous state. That happens because of keratin, which is the protein prevalent in the chemical composition of hair. The superpower of keratin is that it absorbs moisture from the environment. When the air is humid, hair begins to absorb the moisture, becomes heavier, changes structure, gets wavy and curly, and extends.

So what did de Saussure come up with? He created a hair tension hygrometer. Basically, he took a human hair, attached a weight to it to create tension, and connected it to a scale with a pointer. Depending on the humidity, the hair would expand or contract, and you could see the changes on the scale.

Saussure's hair hygrometer. Source: Museo Galileo  https://catalogue.museogalileo.it/gallery/SaussureHairHygrometer.html

But not all hair is equally hygroscopic. Red hair is the most hygroscopic, because it's mostly made of pheomelanin, which doesn't really protect the hair from damage, makes it highly porous, and causes it to absorb moisture quickly. The type of red hair that's the most sensitive to humidity changes is women's hair. Genetically, it's the least dense type of hair.

Of course, de Saussure and his successors didn't know such things 200 years ago. But experiments have shown that women's red hair is best suited to the purpose.

Meteorologists have been following this rule for dozens of years: if you want to create a high-quality hygrometer, cherchez la femme rousse (French for "look for a red-haired woman").

A less poetic approach

Further on, meteorology developed in more prosaic ways. Horsehair, baleen, catgut (cattle intestines), and synthetic fiber began to be used in tension hygrometers. By the way, hair hygrometers were produced in the USSR right until its collapse, and people often used them at home. At the same time, scientists were developing condensation hygrometers. Those were used to measure relative humidity by determining the dew point, that is, the temperature at which the vapour in the atmosphere condenses into dew.

Then there were psychrometers, or two mercury thermometers with a wet and a dry bulb. Their temperature difference was used to measure the relative air humidity. Psychrometer readings provided far more accurate information.

Many other types of hygrometers were invented in the 19th–20th centuries. Of course, it all culminated in the electronic hygrometer which is the most popular type of hygrometer today.

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