Can you really see the stars more clearly when the weather is cold?

? Detective Conan mentioned in his little knowledge, "If the weather is cold and the sky is clear, the stars can be seen more clearly." . But the cold sky doesn't have to be clearer, and neither do the stars. From the star-watching point of view, in addition to sunny weather, there is another important factor that affects the brightness of stars, and that is the transparency of the air.

Air transparency is related to many factors. Besides fog (air pollution), another factor is humidity, or dryness. In short, the lower the temperature, the lower the moisture content in the air, which is why the air in the north feels dry in autumn and winter. Relatively speaking, because the temperature in the south is not as cold as in the north, the humidity in the air will be higher.

Humidity will affect the transparency of the atmosphere. During the day, the higher the transparency, the bluer the blue sky you see-that's why the blue sky in the north is always blue in autumn and winter, which is bluer than that in summer or in the south. At night, the difference is that the stars look like how bright. Because the more transparent the atmosphere is, the less starlight is scattered when passing through the air with the same thickness, and the more starlight reaches our eyes or telescopes. ? Another factor will also affect our naked eye's perception of the brightness of stars, that is, the difference between stars and sky background. When the air is particularly transparent, the light scattered by the atmosphere from the ground is less, in other words, the sky background looks darker. In this way, the bright stars match the darker background, and our eyes can see the stars more clearly. However, humidity is not only affected by temperature. Other factors, such as the environment (whether by the lake or by the sea) and meteorological conditions (such as just raining or snowing), will affect the atmospheric humidity. In addition, atmospheric transparency is not only affected by humidity, but also by air pollution. Therefore, if we say "very cold", the sky is not necessarily clearer, and the stars are not necessarily clearer. But for autumn and winter in the north, the situation is often just the opposite-if you can see the stars clearly, it is mainly because cold air and strong winds blow away clouds and pollutants in the air, and the temperature is usually very low.