Don’t forget that the main feature of autumn and winter weather is fog! Don’t just rush to travel as early as possible

There are actually two main types of weather in China during autumn and winter: cold air weather and pre-cold air weather. The most harmful pre-cold air weather is fog, which can last all day long, but often only in the late night or early morning. If you want to travel, it is best to wait until sunrise rather than "get out early".

1. Two major trends in my country's autumn and winter weather: cold air and pre-cold air

Most of China has a monsoon climate, and the autumn and winter seasons are dominated by the winter monsoon (dry cold air from the continent). Cold air always moves eastward and southward in waves: a small wave every one or two days, a large wave every three or four days, and a strong wave every seven or eight days, without interruption. Therefore, the autumn and winter weather in my country is actually quite simple: when the cold air comes, it is one type of weather, and when the cold air does not come, it is another type of weather.

1. Cold air: source, movement and impact - strong winds and cooling

The main principle behind the generation of large-scale cold air on Earth is the loss of heat balance: the Earth's surface continuously exports heat to the cold outer space, but lacks sufficient sunlight income to balance it, so the temperature drops and cold air is produced in the lower atmosphere.

In addition, the fact that high latitudes receive less sunlight than low latitudes and that continents spend more heat than oceans also affects the production of cold air. The polar regions have the lowest sunlight income, and Siberia, as the largest continental hinterland, has the highest expenditure, so the cold air that affects my country in autumn and winter comes from these two places. Polar cold air basically has to pass through Siberia to reach my country, so it is enough to say that Siberian cold air is present, and it is manifested as Siberian high pressure in the air pressure field.

Where does the cold air go after it is generated? It goes to where the air becomes less. In warm places, the air will rise to the sky, so the air in the lower layer will become less, so the cold air will rush there to replenish it.

The Siberian cold air can move towards the warm areas in three directions: east, south and west. However, the powerful westerly winds in the mid- and high-latitudes prevent it from moving west. When moving eastward and southward, the earth's rotation effect will make it turn right (so the cold air is northwest wind in the north of my country and northeast wind in the south). In addition, when moving southward, it will be blocked by the Qinghai-Tibet Plateau. These reasons jointly determine the main theme of China's cold air activities.

Cold weather conditions affecting China in autumn and winter

2. Cold front: return, warming, calm and stable - record-breaking "high temperature", haze

A cold air mass (cold high pressure) moves eastward and southward and then enters the Northwest Pacific. The right turn will cause the cold air to return. Before the return, it will be heated by the relatively warm underlying surface, causing it to change in nature when it returns. Compared with the new cold air in the future, it is warm air. The return not only causes warming, but also makes the wind smaller - small horizontal winds are "calm".

If the weather is clear at this time, the ground will radiate and cool more drastically at night, forming an inversion layer near the ground (several hundred meters above sea level), meaning the ground is colder than the sky. As the temperature drops as the air cools, the inversion layer will prevent the air on the ground from moving upwards, which is like adding a lid - no vertical flow is "stable".

Inversion layer diagram: vertical observation of temperature and humidity at Beijing Station at 08:00 on October 28

The "quiet and stable" situation is what we often call "poor conditions for air pollution diffusion", so man-made or natural atmospheric suspended particulate matter will accumulate in the low altitude near the ground, causing haze in a dry environment and conducive to the formation of fog in a wet environment.

This situation will continue as long as the next cold air arrives, so the warming, fog and haze weather under calm and stable conditions can be regarded as "pre-cold air" weather.

The weather situation of the cold front that affects China in autumn and winter

The rain and snow weather accompanied by cold air is between the cold air weather and the pre-cold air weather situation, that is, it can occur before the arrival of the ground cold air or after the arrival of the cold air. The details are quite complicated and I will not go into details here.

2. Continued sunshine and warming created record-breaking "high temperatures"

If the cold front is not strong, the "front cold front" weather situation will last for a long time, and there will be continuous southerly winds to bring warmth. If it is sunny and sunny, the temperature will rise. This is the case recently. The long-term "front cold front" situation has led to record-breaking "high temperatures" in North China, Huanghuai and other places - the daily maximum temperature in a large area is above 30℃, breaking the historical record for the same period.

The highest temperature in the past week

The highest temperature in the past week broke the historical extreme value in the same period

3. Avoid heavy fog and travel early - Beijing-Tianjin-Hebei and Jiangsu

The same will continue for the next five days. Except for a small amount of cold air that moves eastward from northern Xinjiang through Mongolia and then across the northern part of Northeast my country, most of the middle and eastern regions of China are not affected by any significant cold air and are in a "front cold air" situation. In the past half month, if it were 10 years ago, there would undoubtedly be a large-scale, long-term heavy pollution weather in North China and the Huanghuai region.

However, times have changed. Thanks to the effectiveness of pollution control, fog and haze are much lighter in the same weather conditions as in the past. This lightness is not only lighter in degree, but also shorter in duration. The disastrous low visibility caused by fog and haze mainly occurs in the coldest period from the second half of the night to the early morning, which is also the most "calm" period.

The data below gives the PM2.5 concentration at the time when pollution was relatively heaviest (early morning of the 25th) against the background of a significant warming and stability this week. The peak was around 200, and most other periods were excellent or good, which can be said to be far better than that year.

PM2.5 concentration at 0:00 on October 25 (left) and PM2.5 concentration at Tianjin Yongyang West Road Station from October 1 to 28 (right) (Air quality monitoring by the Ministry of Ecology and Environment)

In terms of fog, the heaviest is in Jiangsu. Due to the backflow that brings water vapor from the ocean, combined with Jiangsu's large plain terrain and strong radiation cooling, heavy fog is very likely to occur from the second half of the night to before sunrise.

The lowest visibility in the past week (red fog, purple dense fog, deep purple strong dense fog)

The situation in the next three days will be very similar to that of the past few days, and the haze situation is expected to be similar. The heaviest areas along the North China and Huanghuai plains and along the Taihang and Yanshan Mountains will be the heaviest, with moderate to heavy haze.

28-day forecast 30-day ambient air quality forecast (Ministry of Ecology and Environment)

Overall, the visibility caused by haze weather will not be too low. Low visibility is mainly caused by heavy fog, with the focus on the Beijing-Tianjin-Hebei region and Jiangsu:

The next three days of calm and stable situation

The fog is heaviest a few hours before sunrise and significantly weakens or dissipates in the morning, so vehicles in the above two areas are advised not to go on the highway during this period. The probability of heavy fog is extremely high, and there is a possibility of encountering fatal fog that almost loses visibility.

This situation will be the mainstream at least until November 2, after which the cold air will become increasingly stronger.