Why do we have a proverb called' Leng Xue is cold before frost' tomorrow? What is the scientific significance?

Frost is an ice crystal formed by condensation of water vapor in the ground air at low temperature. In order to condense the water vapor in the air, the temperature must be very low, that is, at least below 0 degrees Celsius, so the temperature should be very low before the frost forms.

Another reason is that the frosting time is about 1-4 in the morning, and the human body and mind are at a low tide, and the food on the eve of the human body has been consumed almost. At this time, the human body's heat production function is relatively low, and it is more afraid of cold, which will make people feel colder.

And at this time, the water vapor in the air is saturated, and the wet and cold environment will make people feel colder.

The process of water vapor condensation is exothermic, that is, a lot of heat is released in this process, which will alleviate the cold of the air, which may be the reason why it is not too cold after frost.

Cold after snow: It is exothermic when snow solidifies from liquid to solid, and because it is cloudy when it snows, the clouds can reflect a lot of ground long-wave radiation, even visible light, so they can keep warm, so it is not too cold before snow, but the temperature has dropped to about zero.

After the snow, because the ground is controlled by cold air mass, the weather is fine and there is often snow on the ground. During the day, snow can reflect the heat radiation from the sun. The melting of snow is a process from solid to liquid, that is, it absorbs a lot of heat.

At night, because the weather is fine and the temperature is low, there is little water vapor in the air (water vapor can absorb long-wave radiation more), and there is no reflection of clouds in the sky. Long-wave radiation from the ground can pass through the atmosphere unimpeded and be dissipated into outer space, so the temperature will drop sharply.

In another case, after snow absorbs the waste heat from the ground, it will reduce the temperature to about zero. At this time,

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The cold air continues to push forward, further lowering the temperature. Therefore, it is cold after snow.

If it's still cloudy a few days after it snows, it won't be so cold without strong cold air.

Snow melts and absorbs heat, so it is cold after snow.