From a meteorological perspective, how did Zhuge Liang succeed in "borrowing" the east wind?

Produced by: Science Popularization China

Author: Meteorological Science Team

Producer: China Science Expo

Everyone is familiar with the story of Zhuge Liang "borrowing" the east wind during the Battle of Chibi. However, this east wind was certainly not "borrowed" by Zhuge Liang. It can only be said that his understanding of the weather patterns in the Chibi area was much deeper and more specific than that of Cao Cao.

In the Peking Opera "Gathering of Heroes", Cao Cao sang the line "I only said that east wind is rare in November", which clearly shows that he regretted his misjudgment of the weather during the Battle of Red Cliffs, which led to his heavy losses.

Today we will try to do some analysis from the perspective of meteorology and weather patterns and discuss it with you.

The frontal cyclone that helped

From the Battle of Red Cliff, the strong southeast wind blew over the Yangtze River, to Cao's army's defeat at Huarong Road

There was another downpour, and judging from the weather conditions, it looked very much like a frontal cyclone.

Frontal cyclones are most common in spring and less common in autumn in my country. It is a deep-developed low-pressure system with low pressure in the center and high pressure around it. The air flows from the periphery to the center, rotating in a counterclockwise direction. Therefore, the southeast wind blows in the front of the cyclone (i.e. the east), and the northwest wind blows in the back of the cyclone (the west) (as shown in the left picture). Convergent rising air currents prevail inside the cyclone, which can cause large areas of rainfall. Therefore, when the southeast wind blows continuously, it often indicates that the weather will get worse. The weather proverbs that "Southeast wind and rain are ancestors, northwest wind is nothing" and "East wind and rain, west wind clear" have certain practical significance.

Most coastal areas in my country have more southeast winds in summer and more northwest winds in winter. This prevailing wind direction that changes with the seasons is called monsoon. With the change of this prevailing wind direction, there are obviously different weather and climate. When the winter monsoon prevails, the air flows from the northwest continent. The air at this time is cold and dry, with little precipitation; when the summer monsoon prevails, the air flows from the southeast ocean. The air at this time is warm and humid, with more precipitation.

Wind (Image source: Veer Gallery)

2. Monsoons that prefer the southeastern region of my country

The most obvious monsoon phenomenon in the world is in the eastern and southern parts of Asia, so the southeastern part of my country has become a strong monsoon area. The wind directions here are almost opposite in winter and summer. For example, in Shanghai, the northerly wind (including northwesterly wind, northerly wind and northeasterly wind) is 62% in January, that is, 62 times out of 100 wind directions are northerly winds, and the southerly wind is 57% in July; in Guangzhou, the northerly wind is 55% in January and the southerly wind is 52% in July. The different weather and climate characteristics caused by the winter and summer monsoons are mainly dry in winter and rainy in summer.

Why is the monsoon particularly obvious in the southeastern region of my country? This starts with the reasons for the formation of the monsoon. We know that the temperature of the sea and land is different in winter and summer. The ocean is warmer than the land in winter, and the ocean is colder than the land in summer. The temperature is different, and the air pressure is also different. In winter, the temperature on the continent is lower and the air pressure is higher, while the temperature on the ocean is higher and the air pressure is lower, while in summer, it is the opposite. Since there is a difference in air pressure between the sea and the land, the air will flow from the place with high air pressure to the place with low air pressure, thus forming different wind directions between the sea and the land in winter and summer, from land to sea in winter and from sea to land in summer.

When a place is affected by a moving closed high pressure center, the wind direction changes clockwise. That is, when the cold high pressure begins to move to the sea, the southeast wind prevailing behind the high pressure will temporarily control the middle and lower reaches of the Yangtze River. Since the cold high pressure moves quickly southward in winter, the last cold air that follows the southward invasion will arrive soon; therefore, the southeast wind lasts for a short time and people often ignore it.

Zhuge Liang lived in Nanyang, not far from Chibi, and he knew the signs before the southeast wind appeared, so he made an accurate medium-term weather forecast. But in order to fool Zhou Yu, he set up an altar to worship the gods and "borrow the east wind."

3. Pay attention everywhere, you can also read the wind and know the weather

“East wind brings moisture, west wind dries, south wind warms, north wind chills.” This proverb is popular in the middle and lower reaches of the Yangtze River, and it means that different winds bring different weather conditions, such as cold, warm, dry or wet.

The middle and lower reaches of the Yangtze River are adjacent to the ocean in the east and the continent in the west. The wind here blows from east to west and from south to north, and is responsible for exchanging cold and heat and transporting water vapor. The east wind is humid, the south wind is warm, and the warm and humid southeast wind provides abundant water vapor conditions for the formation of clouds and rain. As long as there is a chance of rising, clouds will condense and rain will fall. Therefore, there is a saying that "to know how far the rain is, just look at the southeast wind" and "Southeast wind during the day, wet clothes at night."

In the winter half of the year, the cold air is stronger than the warm air, and the northwest wind often pushes the frontal cloud and rain belt to the southern ocean. At this time, the middle and lower reaches of the Yangtze River are under the control of a single cold air, and the weather is clear, just as the proverb says, "After autumn, the fields in the northwest are dry", "In spring, the northwest is scorching; in winter, the northwest will definitely become sunny." If the southeast wind blows at this time, but it does not blow for long, this is "the south wind blows to the end, and the north wind returns the favor", indicating that the frontal cloud and rain belt affects the local area, and the sky will become cloudy, "If you want to know how far the rain is, just look at the southeast wind."

In this way, it seems that Zhuge Liang's prediction in November, the beginning of winter, based on the weather changes in the middle and lower reaches of the Yangtze River at that time, that there would be strong southeast winds, and his further inference that the weather would deteriorate, was in line with the law of weather evolution.

However, with global warming, the monsoon pattern in Asia is changing. Scientists are also studying the mechanisms and reasons behind the changes and look forward to the results being revealed soon.