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7 things you don't know about humidity

7 things you don't know about humidity

18 May 2020


1. The humidity of 100% doesn't mean it's going to rain.

Humidity is a physical quantity used to describe the amount of water vapor in the air.

There are many ways to express air humidity, of which the most commonly used is the "relative humidity" in meteorology.

So what is relative humidity?

Let's think of air as a sponge. It can hold at most a certain amount of water, say 1 liter.

Then the relative humidity refers to the ratio of the actual water content of the sponge (that is, absolute humidity) to the maximum amount of water that can be held.

If the sponge does not drink water, the relative humidity is zero; if the sponge drinks 500 milliliters of water, the relative humidity is 50%.

The maximum amount of water vapor the atmosphere can hold depends mainly on the temperature.

A rise in temperature is equivalent to making the sponge bigger to hold more water.

Therefore, when you increase the size of the sponge without adding more water, the relative humidity decreases.

If you continue to add water after the sponge is full, the extra water vapor will precipitate.

But that doesn't mean it's going to rain.

Because the relative humidity is usually measured on the ground, it does not accurately reflect the humidity in the upper air.

Even when the humidity in the upper air reaches saturation and the extra water vapor condenses into small water droplets, these small water droplets will not immediately drop into rain as they do in a water-filled sponge.

Because they are light and small (most of them are only 0.01 to 0.02 mm in diameter), they are supported by rising air to form floating clouds.

Only when small water droplets keep absorbing water and getting fat that the air can't hold them, will they drop and turn into drizzle one after another.

So, 100% humidity doesn't mean it's going to rain.

2. The higher the dew point, the more uncomfortable it is.

However, when the humidity reaches 100%, there is bound to be dew.

In the case of constant water vapor content, the temperature decreases, which is equivalent to a smaller sponge, and finally makes the water vapor in the air reach a state of saturation.

As a result, the excess water vapor precipitated and turned into dew.

The ambient temperature at which water vapor in the air condenses to form dew is called the dew point.

For example, the dew point is 18℃, which means that the external temperature must drop below 18℃before the water vapor in the air can reach saturation, forming sparkling droplets on the grass and leaves.

Dew point is a way to measure absolute humidity.

For example, if the dew point of a place is 12℃, the absolute humidity of the air here is the amount of saturated water vapor at 12℃.

Thus it can be seen that the higher the dew point, the more water vapor in the air.

When there is a lot of water vapor, the temperature or relative humidity is relatively high. after all, the bigger the sponge is, the more water it can hold, and the fuller the sponge is, the more water there is under clothing.

On the contrary, the lower the dew point, the less water vapor in the air.

There are also two cases of less water vapor, one is the low temperature, that is, the sponge is small, the water capacity is not large; the other is the low relative humidity, that is, the sponge is big but drinks less water.

Although the weather forecast does not mention the dew point, it is actually an important indicator of whether the body feels comfortable or not.

Together with wind, sunshine and other factors, it affects our somatosensory temperature (that is, the air temperature that the human body really feels).

People usually feel uncomfortable when the dew point is high.

Because as mentioned earlier, the temperature is generally higher at the high dew point, which makes people sweat.

High dew point is sometimes accompanied by high relative humidity, which hinders sweat volatilization, causing discomfort and even illness due to high body temperature.

When the dew point is low, the temperature or relative humidity will be relatively low, both of which can make the body dissipate heat effectively, so it is more comfortable.

The water vapor in the air mainly comes from the evaporation of oceans and lakes.

The warmer the water is, the faster it evaporates and the more water vapor is in the air, so most of the earth's high dew point areas are close to warm waters, such as the Persian Gulf, the Red Sea and the Gulf of Aden, the three highest dew points in the world.

The Persian Gulf is the region with the highest water temperature on earth, and its summer water temperature can reach 3033℃.

As a result, the highest dew point on record in the world-35℃(3 p.m., July 8, 2003) appeared in Zahran on the west coast of the Persian Gulf.

3. Cities stink more in summer.

Have you noticed that the city is more smelly in summer, and there will be a bad smell in the air from time to time?

Why is that?

In addition to the fact that rubbish in summer is more likely to rot, scientists have also found that people's sense of smell is more sensitive in humid environments.

This is because high temperatures and high humidity make more water molecules floating in the air and move faster.

A large number of active water molecules carry more smelly particles into our noses, so cities are more smelly in summer.

This may also explain why wet dogs smell worse.

There are many microbes such as bacteria and yeasts in the dog's hair, which produce some foul-smelling compounds.

After the dog is wet, these smelly substances are brought into our noses by evaporated water molecules, making the dog smell worse.

The stench emitted by garbage can also be smelled in winter, but cold, dry air limits the distance the odor particles can travel, greatly reducing their killing range.

4. The air is dry and singing is easy to be out of tune.

Our vocal cords are made up of a pair of left and right symmetrical mucous membranes.

When making a sound, the air flow from the trachea and lungs continues to impact the vocal cords, causing the vocal cords to vibrate and produce sound.

The vocal cords control the tone of our speech or singing by controlling the airflow.

Interestingly, it is difficult not to sing out of tune in a dry environment.

In fact, the researchers speculate that it is humidity that gives the language a rich tone.

After counting more than 3700 languages around the world, they found that languages with complex tones-such as Cantonese, Vietnamese and many African languages-were more likely to appear in humid parts of the earth.

Research shows that the vast majority of languages with complex tones are found in tropical regions of Southeast Asia and Africa, with a few in humid areas of North America, the Amazon and New Guinea.

Rather than multi-tone languages, such as various European languages, including English, they tend to appear in drier places-the cold north or dry deserts.

The reason for this interesting pattern is that the humidity of the air affects the elasticity of the vocal cords.

There is a fixed proportion of water and polysaccharides in the mucous layer on the surface of the vocal cords to keep the mucus layer soft and elastic, which is the key to sound production.

Inhaling dry air dehydrates the vocal cords, causing the mucus layer to become more viscous and less elastic, making it difficult to produce complex tones.

5. Hair can measure humidity.

If you have long hair, you may not have to bother the weather forecast to tell you the humidity of the air.

Because hair is sensitive to humidity, straight hair will bend when the air is wet, while curly hair will be more curly.

Swiss physicist Southall also discovered this interesting phenomenon, and then he used hair to make the world's first hair hygrometer.

Southall fastened one end of a 25.4-centimeter-long bunch of hair to a screw, while the other end passed through a pulley to connect to a weight.

After absorbing water and moistening, the hair becomes shorter, driving the weight upward.

Southall calculates the humidity of the air based on the distance the weight moves.

Why does the hair get shorter after absorbing water?

The main component of hair is a protein called keratin.

As we all know, proteins are made up of amino acids, which "hold hands" and form long peptide chains.

As shown in the figure above, through disulfide or hydrogen bonds, the peptide chain can form a helical structure and further fold.

The disulfide bond is stable, it is not affected by humidity, and it can be almost permanent as long as you don't perm your hair.

This gives our hair strength and toughness.

On the other hand, the hydrogen bond is relatively weak, it is very sensitive to humidity and can be interrupted and reconstructed at any time.

When it is moist, there are more water molecules in the air, which means that in addition to hydrogen bonds between adjacent amino acids, more hydrogen bonds are formed between amino acids located in different parts of the peptide chain and between different peptide chains. This causes the peptide chain to fold and bend.

When the humidity decreases, many hydrogen bonds are broken and the peptide chains are re-stretched and lengthened.

Its macroscopic performance is that the hair becomes shorter as the humidity increases.

If you think of the hair as a spring, blowing dry is equivalent to straightening the spring and making it longer, while when it is wet, a large number of hydrogen bonds form to further bend, fold, or even wind the spring, making the hair shorter.

Although the hair hygrometer is so rough that you can even do it yourself at home, it was not until the 1960s that the hair hygrometer was replaced by an electronic hygrometer.

6. It is dangerous to exercise when it is hot and humid.

Even professional athletes must adjust their exercise according to changes in humidity.

Baseball is a sport that is very affected by humidity.

If the player cannot adjust the batting strength according to the field, a home run occurs for every 20% increase in relative humidity (after the batter hits the ball and successfully returns to home run around the base before the ball is caught by the opposing player, it is a home run.

The most direct way is to hit the ball directly outside the home run wall 90 miles and 100 meters away, so baseball is as far away as possible.

The probability will be reduced by about 25%.

When the humidity on the field is high, the baseball will absorb water and gain weight, resulting in a decrease in hitting speed, and the ball will suffer more resistance in the moist air, and under the combined influence of the two, the distance of the baseball will be shorter.

For every 1 meter reduction in the distance the ball flies, the probability of hitting a home run is reduced by about 7.5%.

So, at high altitudes, players are more likely to hit home runs because the humidity is lower and the ball flies farther.

In fact, strenuous exercise in a high humidity environment is very dangerous.

Because when we exercise in a hot environment, the body cools down by sweating.

If the air is dry, sweat can evaporate quickly and take away heat from the body surface, effectively lowering body temperature.

But if the humidity in the environment is so high that it is difficult for the sweat to evaporate, the body will have to sweat more to try to lower the body temperature.

Sweating can lead to dehydration, and if left untreated, people can easily get heatstroke.

Therefore, when it is hot and humid, it is best to reduce the time for outdoor sports and replenish water in time after exercise.

7. Humidity tells moths where there is more nectar.

The hot and humid environment we hate is a paradise for insects.

Bugs are more likely to get dehydrated than large animals because they have a larger relative surface area (the ratio of body surface area to volume), which means more water is lost from the body surface.

And we know that most insects are very small, high humidity will increase their survival rate, so most insects are very sensitive to humidity, always like a wet paradise.

Among them, moths are the most, which can detect humidity changes as small as 4%.

In addition, the moth can also find its food-nectar-by detecting changes in humidity.

Nectar has transpiration, and the relative humidity above the newly blooming flowers is about 4% higher than that of the environment.

The humidity difference then gradually decreases until the nectar is exhausted about half an hour later.

In other words, half an hour later, the flowers may still be in full bloom, but the nectar is gone.

Moths can enjoy nectar only if they find flowers that are open for no more than half an hour.

So a keen sense of humidity changes can help moths quickly determine which flowers have more nectar.


Related products: digital humidor hygrometer

 
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