From the fog, look at the weather

Produced by: Science Popularization China

Author: Meteorological Science Team

Producer: China Science Expo

Fog is ever-changing and complex. Some fog phenomena may be commonplace to us, and there are many natural laws in them. But at the same time, fog is closely related to future weather changes. By observing fog, we can roughly predict whether it will be sunny or rainy next. So what are the specific knowledge involved?

1. Why is there often fog in winter mornings?

There is a limit to the amount of water vapor that the air can hold. When the maximum limit is reached, it is called water vapor saturation. The higher the temperature, the more water vapor the air can hold. For example, in 1 cubic meter of air, at a temperature of 4°C, the maximum amount of water vapor that can be held is 6.36 grams. At a temperature of 20°C, 1 cubic meter of air can contain a maximum of 17.30 grams of water vapor.

If the water vapor in the air exceeds the saturated water vapor under certain temperature conditions, the excess water vapor will condense and become small water droplets or ice crystals. If at 4°C, 1 cubic meter of air contains 7.36 grams of water vapor, then the excess 1 gram of water vapor will condense into water droplets. Therefore, when the water vapor in the air exceeds the saturated amount, it will condense into water droplets, which is mainly caused by the decrease in temperature. The loss of heat from the ground will cause the ground temperature to drop, and at the same time it will affect the air layer close to the ground, causing the air temperature to drop as well. If the air layer close to the ground is quite humid, then when it cools to a certain degree, part of the water vapor in the air will condense and become many small water droplets, suspended in the air layer near the ground. When there are too many small water droplets in the air layer near the ground, which obstructs people's sight, fog is formed.

Fog and clouds are mainly caused by a drop in temperature, so fog can actually be said to be clouds close to the ground.

The temperature is generally higher during the day, and the air can hold more water vapor. However, at night, the temperature drops, and the air can hold less water vapor. If there is more water vapor in the air at that time, some of the water vapor will condense into fog. Especially in winter, due to the long nights and more chances of sunny days with light winds, the ground dissipates heat more quickly than in summer, and the temperature close to the ground drops sharply, so that the water vapor in the air layer near the ground is easily saturated in the second half of the night and morning and condenses into small water droplets, which float in the air near the ground to form fog. Therefore, there is often fog on clear mornings in winter. This kind of fog is called "radiation fog".

2. Why is it usually sunny when there is fog in the morning?

Sometimes when you get up in the morning, you will see a thick fog. When you open the doors and windows, it will float in like light smoke. But it won't be long before you can vaguely see the scenery outside the window. Finally, the fog will dissipate and the sun will shine brightly. As the saying goes, "If the ground is covered with fog in the morning, you can still dry the rice." This is exactly what it means.

There was fog in the morning and the atmosphere was humid, but it was a sunny day. What was the reason?

During the day, the sun shines on the ground, and the ground accumulates a lot of heat. Due to the evaporation of water, the higher temperature of the air can also accommodate more water vapor, so there is more water vapor in the air. After the sun sets, the heat begins to dissipate into the air, and the temperature of the air close to the ground also decreases. The better the weather, the fewer clouds in the sky, and the heat of the ground is not hindered by anything, so it dissipates faster and the air humidity drops lower. In the second half of the night and the morning, the temperature of the ground air has dropped very low. At this time, even indoors, we can easily feel that it is much cooler in the first half of the night. After the temperature of the air close to the ground drops, the water vapor in the air exceeds the saturation state, and the excess water vapor condenses into tiny water droplets and distributes in the low altitude. This is what meteorology calls "radiation fog". This fog usually occurs near the center of high pressure, and near the center of high pressure, it is often sunny. So when this kind of fog appears, even if the fog is thick in the morning, as long as the sun comes out and evaporates the fog, the day will mostly be sunny.

3. Why is it possible to rain if the fog does not dissipate?

Fog that does not dissipate during the day is mostly related to the passage of the front. Before the warm front passes, frontal fog often appears. Above this fog, there are thick rain clouds. The rain that falls from the bottom of the rain clouds evaporates below the bottom of the clouds and condenses again near the ground. This is the reason for the formation of frontal fog. Since there are thick rain clouds on the top of this fog, sunlight cannot penetrate in large quantities, and the conditions for the formation of fog continue to exist, so this fog will certainly not dissipate. Soon, as the rain clouds become thicker and thicker, the water vapor below the cloud bottom becomes more and more abundant. The raindrops cannot evaporate in the space under the clouds, but fall directly, and it rains at this time.

When a warm front passes through, if there is a mixture of cold and warm air, frontal fog can be caused. In the warm area after the warm front passes, warm-area fog will be produced again because the warm and humid air flows over the cold ground. This fog is very humid and is often combined with drizzle. Behind it, there is often a cold front moving southward, causing cold front precipitation. In coastal areas, sea fog sometimes invades the land at night. If this fog does not dissipate by morning or mid-morning, it will turn into layered clouds and rain will fall. Therefore, the saying "when the fog dissipates, it will rain" is correct.

The relationship between fog and weather is so close that we can know the weather changes by observing the fog. However, the above explanation is only for the general situation. The relationship between fog and weather is not so simple, and there are many complex contents, so it cannot be simply applied mechanically. In other words, to accurately know the weather by observing the fog, we must make multi-faceted observations, analyses, and comprehensive judgments.