A weather pattern shared by “Fog-locked Beijing”, “Sauna days” and “Pollen allergies”: Why is the weather so stuffy in autumn?

The recent fog in Beijing can be said to be one of the symbolic weather conditions for the North China Plain entering autumn. The descending air currents at high altitudes caused clear skies in the middle and upper parts, and the moisture brought by the easterly wind from the Bohai Sea at low altitudes produced fog under the cooling of the clear nights in early autumn. During the day, the "clear sky, rising temperatures, and light wind" are exactly the meteorological conditions that are conducive to the increase of pollen concentrations, and the high humidity creates a "small sauna day".

Unless otherwise specified, the pictures in this article are screenshots from the Central Meteorological Observatory's business platform.

1. Autumn fog in Beijing does not indicate severe pollution

During the past two days, fog or light mist with visibility of about 1 km appeared in most areas of Beijing at night and in the early morning, and dense fog with visibility below 500 meters appeared in some areas. Although it is definitely an exaggeration to say that "fog locked the capital", it is enough to make people uncomfortable and also posed a certain threat to road traffic safety.

At 08:00 on September 5, Taotao Fengyun was photographed facing south at the Central Meteorological Observatory.

This fog is not unique to Beijing. In fact, the entire North China Plain is covered with light or heavy fog:

Minimum visibility from 08:00 on September 4 to 08:00 on September 6, 2023 (red - heavy fog; purple - dense fog)

Are you reminded of the severe smog weather in previous years? Don't worry, the pollution in the North China Plain has been much lighter now. The smog is mainly caused by high humidity and the pollution is relatively light. Monitoring data from the Ministry of Ecology and Environment shows that the air quality is generally good, with the heaviest pollution in Beijing's main urban area reaching moderate pollution. The peak PM2.5 concentration on the morning of the 5th was basically below 200.

PM2.5 monitoring data from the Ministry of Ecology and Environment at 08:00 on September 5, 2023

Let's choose the most polluted point, the "Temple of Heaven", and look at the time series: the peak was just over 200 from 6 to 8 in the morning yesterday. The temperature dropped rapidly after rising at sunrise, and the lowest point was 53 at around 19:00 in the evening, which was close to an excellent level. It slowly rose again after nightfall, but was only 120 at 8 this morning, a significant drop from yesterday.

PM2.5 monitoring by the Ministry of Ecology and Environment: Temple of Heaven 08:00 on September 4, 2023 - 08:00 on September 6

This change is typical of clear weather: the ground heats up and the air rises, pollutants diffuse upward and dilute, and then compress back to the ground at night. This process can be clearly seen from the observations of the lidar cloud height meter:

Around noon, it weakens significantly and the sun can be seen clearly, as shown in the picture below.

Beijing Observatory's weather phenomenon instrument observation image at 12:00 on September 5

This is an observation image of a meteorological observation instrument called "Weather Phenomenon". In fact, it is a wide-angle photography of the zenith. With it, forecasters can see the real-time sky conditions across the country without leaving home. I like it very much:

Distribution of weather instruments in China (Atmospheric Observation Center of China Meteorological Administration)

2. The meteorological factors that cause the current foggy weather in Beijing also cause "sauna days" and "rising pollen concentrations"

To put it simply: northwest airflow prevails at high altitudes in Beijing, and there is sinking air flow in the vertical direction, which leads to sunny days in the middle and high altitudes; the sinking air flow is the bottom (south) of an anticyclonic (wind direction turns clockwise) high-pressure system in the middle and low altitudes, while at the bottom level the easterly wind brings water vapor from the Bohai Sea.

High-altitude wind field (300hPa, about 9,000-10,000 meters above sea level)

Medium and low altitude (700hPa isobaric surface, about 3 kilometers above sea level)

Near-ground (10 meters) wind field

Weather logic: The significant easterly wind from the Bohai Sea at low altitude transports water vapor to Beijing and Tianjin. The humid air cools at night and condenses into fog on the PM2.5 (the clearer the weather, the stronger the ground radiation cooling, this kind of fog is called radiation fog). After sunrise, the temperature rises and the fog droplets evaporate. The fog weakens and visibility increases significantly, but mild pollution leads to mild haze. Although it is clear, the sky is not transparent.

On sunny afternoons the sun gets hotter, but the humidity remains high in the low altitude - thus a "sauna day" appears.

Temperature (white) and relative humidity (black) at 13:00 on September 6, 2023

With a relative humidity of 60+ and a temperature of 30+, it feels like a small sauna (but still inferior to the south in midsummer).

The four factors of breeze, clear sky, high humidity and high temperature have caused the Beijing-Tianjin-Hebei region to be divided into autumn fog and small sauna weather during the past two days. In fact, breeze, clear sky and high temperature are also meteorological conditions that are conducive to the increase of pollen concentration. According to the pollen broadcast information released by "Beijing Meteorological Bureau" on the 5th, the number of pollen particles in the urban area on September 4 was at a very high level (501 to 800 particles per thousand square millimeters), reminding people with allergies to take relevant protective measures.