Is it not because of the cold that we exhale white air in winter? There is no white air in Antarctica!

The temperature has dropped continuously in recent days, and most parts of the country have entered the "quick freeze" mode. Some southern cities have also seen snow. Friends in the south can finally watch the snow without "cloud" online!

Image source: Editor's circle of friends

But when the weather gets cold, white air will come out when you yawn, talk, or even breathe. White air seems to have become a symbol of winter and cold. Have you ever wondered why we exhale white air in winter but not in summer? Is it because the colder it is, the more white air we exhale?

01

There is no white air in Antarctica

If we want to know the relationship between exhaling white air and cold, we can go to the coldest place on earth - Antarctica to find the answer. Although we cannot go to Antarctica in person, we can get a glimpse of clues from film and television works. For example, in the TV series "Antarctica", the protagonists do not seem to exhale white air. Is this TV series not filmed in Antarctica? This is not true. During the Antarctic scientific expedition, the scientific expedition team members often cannot exhale white air. Why is this?

Image source: Stills from "Antarctica" This starts with the cause of the white gas. The essence of white gas is droplets condensed from water vapor . Our body temperature is around 37 degrees. The temperature in winter is very different from the temperature of the gas we exhale. The exhaled gas contains a lot of water vapor. The water vapor cools and condenses instantly to form small droplets, which form what we call "white gas". However, the condensation of water vapor requires condensation nuclei . Condensation nuclei are various small solid particles. Water vapor adsorbed on small particles can condense into liquid water faster. The air in Antarctica is very clean and there are few condensation nuclei, so it is difficult to form droplets immediately . If the water vapor does not condense quickly, it will diffuse into the surrounding air. Low-density water vapor is more difficult to condense, so the "white gas" cannot be seen.

02

The three forms of water

We can take a closer look at the phase diagram of water . Each point in the phase diagram represents the state of water when it exists stably at the corresponding temperature and pressure. The partial pressure of the water vapor we exhale is generally between 0.6kPa and 101kPa. When the temperature drops, the phase point of the water vapor will move to the left and become liquid water .

However, if there are no condensation nuclei in the air, the water vapor will temporarily enter a "supercooled" state. If condensation nuclei suddenly appear at this time, the water vapor will quickly liquefy to form a large number of droplets. However, if the water vapor diffuses before the condensation nuclei appear and the pressure drops below 0.6 kPa, it will no longer condense into liquid water . Even if the temperature continues to drop, the water vapor will directly condense into solid ice, and no white gas will appear.

Phase diagram of water

Image source: Voice of Chinese Academy of Sciences

If the pressure of water vapor in the air is lower than 0.6kPa , the water vapor will directly condense to form ice crystals . Winters in the north are generally dry, so if the temperature outside is very low, the water vapor in the house will directly condense when it hits the low-temperature windows, forming beautiful ice flowers.

The snow in winter is also formed by the direct condensation of water vapor in nature. If you zoom in, you will find that the snowflakes are beautiful hexagons. This is the structure naturally formed when water vapor condenses, and it is a masterpiece of low-pressure water vapor.

Image source: Photo Network

If the pressure of water vapor in the air is higher than 0.6kPa , the water vapor will form droplets when it cools down . If it is in the high altitude, it is the common cloud we see; in the low altitude, it is what we call fog. In other words, the white air we exhale is essentially the same thing as clouds and fog.

Generally, we see fog in the morning because the temperature is lower at night. If there is more water vapor in the air, it will condense into fog. After the sun comes out, the temperature gradually rises, and the fog formed at night will slowly dissipate and evaporate into water vapor again.

Image source: Photo Network

03

Advantages and disadvantages of condensation nuclei

Knowing the principle of condensation nuclei, we can use condensation nuclei to produce droplets faster. Sowing appropriate condensation nuclei or other catalysts in the clouds can make the droplets in the clouds grow faster and fall down. This is artificial rainfall . The process of artificial rainfall is actually quite complicated, but one of the important factors is to increase the condensation nuclei . However, it should be noted that this method of increasing condensation nuclei only accelerates the process of clouds turning into rain . If the cloud itself does not have the potential to form raindrops, then adding more condensation nuclei will be futile.

Image source: Photo Network

Although condensation nuclei can serve us, they can also cause troubles if they are produced unintentionally. Car exhaust and factory exhaust contain a large number of condensation nuclei, which will form what we call smog. In fact, fog and haze are two different things. Fog is small droplets in the air , including water and other liquids in the air, while haze is solid particles in the air .

Generally speaking, small particles stay in the air longer and are more likely to be inhaled by the human body . In order to control air pollution, the country has formulated standards for various pollutants. For example, PM2.5 refers to fine particles in the air with a diameter of less than 2.5 microns, and PM10 refers to particles in the air with a diameter of less than 10 microns, which means that PM10 includes PM2.5.

Image source: Photo Network

These particles are the "byproducts" of the development process. In contrast, the fact that you can't exhale white air in Antarctica is the result of excellent air quality. In recent years, with effective governance, air quality has gradually improved. I believe that air pollution is just the pain of the development process. In the cold winter, if one day you can't exhale the same white air as usual, that is the best feedback from nature after pollution control!

Author: Jiang Chang, PhD, Institute of Physics, Chinese Academy of Sciences

Editor: Tinker Bell

Title image source: sohu.com