Look at these "pearls" that only appear in the early morning: How is dew formed?

Produced by: Science Popularization China

Author: Meteorological Science Team

Producer: China Science Expo

In daily life, we often encounter this phenomenon: in winter, when you blow on the window glass, you will find a layer of small water droplets on the glass. This is the result of condensation when water vapor encounters the surface of a colder object.

Dew in the fields is also small water droplets condensed when water vapor in the air hits the colder ground or objects near the ground. In the early morning of warm seasons, people can often see dew on the grass, leaves and crops on the roadside.

If there is more dew on a certain day, we may even wonder if it rained last night and that is why the floor is so wet.

So, is dew formed in the same way as rain? Does it also fall from the sky?

Dew drops on leaves (Photo source: veer Gallery)

1. Formation and disappearance of dew

Dew does not fall from the sky. Its formation causes and processes are the same as frost, except that the temperature when it is formed is above 0°C.

The temperature above 0°C when the air reaches water vapor saturation due to cooling is called the "dew point temperature". In warm seasons, when ground objects are strongly cooled by radiation at night, the temperature of the air in contact with the surface of the object drops, and when it drops to the "dew point", excess water vapor precipitates. Because the temperature is above 0°C at this time, this excess water vapor condenses into water droplets attached to ground objects, which is dew.

Like frost, dew also mostly appears on clear, windless or breezy nights. At the same time, objects that are prone to dew formation are often those with relatively large surface areas, rough surfaces, and poor thermal conductivity. Sometimes, dew is formed in the first half of the night, and the temperature continues to drop in the second half of the night, causing the dew on the object to freeze. This is called frozen dew. Some people classify it as frost, but its formation process is different from that of frost.

Dew usually forms at night. After sunrise, the temperature rises and the dew evaporates and disappears.

Dew often appears during the growing season of crops. It is beneficial to agricultural production. In the summer in northern my country, evaporation is very fast. When there is a lack of rain and drought, the leaves of crops are sometimes dried up by the sun during the day, but when dew appears at night, the leaves return to their original state. This is why people often refer to "rain and dew" together.

Crops exposed to dew on a sunny day (Image source: Veer Gallery)

2. Why is it usually a sunny day when there is dew?

The formation of dew requires certain weather conditions, that is, the atmosphere is relatively stable, the wind is light, the sky is clear and there are few clouds, the heat on the ground can dissipate quickly, and the temperature drops, so when water vapor encounters colder ground or objects, dew will form.

On the contrary, if there are clouds in the sky at night, the ground is like a big quilt covering it. Heat has difficulty escaping into space and passing through this big barrier. After hitting the clouds, part of it is reflected back to the ground, and the other part is absorbed by the clouds. The heat absorbed by the clouds will slowly radiate to the ground later. Therefore, the clouds are like the roof of a greenhouse, which has the function of heat preservation. Therefore, when the sky is full of clouds at night, the temperature near the ground is not easy to drop, and dew is difficult to appear.

Similarly, if there is wind at night, it can make the air above and below exchange, increase the temperature of the air near the ground, and make the water vapor diffuse, so it is difficult to form dew. Therefore, when there is dew, it is usually a sunny day.