The South is Drier Than the North? Big Data Reveals Where is the Driest Place in my country

In early December this year, most of the south remained dry, 80% drier than usual. Fujian, Guangdong, and Guangxi hardly saw a drop of rain, and the average minimum relative humidity in many places was less than 30%, which was drier than the north. Why is the south so dry recently? Is winter the driest season of the year? Where is the driest place in my country? China Weather Network has answered this question by reviewing a set of big data.

The south was drier than usual in early December, and 80% drier than the north

In early December, the weather was sunny and dry in most parts of the south, especially in southern Jiangnan and southern China, where there was almost no rain. Light to moderate meteorological drought occurred in eastern Guangxi and most parts of Guangdong, and the relative humidity of the air in the afternoon in many places was abnormally lower than usual.

Among the provincial capital cities, such as Guangzhou, Nanning, Changsha, Nanchang, Wuhan, Hefei and Nanjing, the average minimum humidity in early December hovered around 30%. Especially in Nanning and Hefei, the minimum daily relative humidity was less than 25% for 6 consecutive days (December 1 to 6), which is very rare in history. Among them, Hefei has the most consecutive dry days in the past 30 years, and Nanning can also rank among the top five in the history of December observation.

Relative humidity distribution map of the country at 14:00 on December 1

The dryness of southern cities even surpassed that of northern cities. For example, on December 1, the minimum relative humidity in Lanzhou reached 20%, Yinchuan 23%, Hohhot 34%, while the minimum relative humidity in Hefei and Wuhan was only 19%, which was abnormally dry.

Shi Yan, an analyst at China Weather Net, said the recent dryness in southern China is mainly due to the fact that most areas have been sunny with few clouds and little rainfall since December; at the same time, the dry and cold air from the north has blown away the little moisture in most parts of the south (including the coastal areas of Fujian and Guangdong), making the air even drier.

The Central Meteorological Observatory predicts that there will be a large-scale rainy weather process in the south from the 15th to the 16th. After the 20th, there may be another precipitation process in the southeastern coastal areas, and the heaviest rainfall since December may occur. At that time, the dry situation will be significantly alleviated.

National dryness index ranking released: Spring is better than winter

Do you think this is the driest season? Actually, it is not. According to the national station's monthly average minimum relative humidity data for the whole year (1991-2020), the top three months are April, March, and May. April is the driest, with an average minimum relative humidity of less than 40%, while December, where we are now, ranks sixth in the whole year. Why is spring the driest season of the year?

Shi Yan explained that the minimum relative humidity depends on the absolute humidity on the one hand and the air temperature on the other. Absolute humidity is the actual water vapor content in the air, generally expressed as "specific humidity", while relative humidity is the ratio of the actual water vapor content in the air to the theoretical maximum water vapor content in the air, generally expressed as a percentage, which changes with temperature changes. When the temperature rises, the relative humidity decreases. In April, the temperature in most parts of my country rises, and dry and warm air masses often control most of the central and eastern parts. As the temperature rises in the afternoon and there is wind, the humidity often drops rapidly. Therefore, spring is the driest season in my country throughout the year.

From the China Weather Network's ranking of the driest cities in winter, we can see that the top ten are all northern cities. Lhasa's climate belongs to the plateau temperate climate zone. Due to its high altitude, low precipitation in winter and long average sunshine time, even if precipitation occurs, it is easy to evaporate, so it is very dry. Xining, Yinchuan and Lanzhou in the northwest are deep inland and belong to the temperate continental climate. There is little rain and snow in autumn and winter, the water vapor conditions are poor, and they are more susceptible to cold and dry cold air in winter. The humidity is low, so it is also very dry, and it is obvious that they are on the list.

Beijing, as a representative of typical dry cities in North China, also made it into the top three. Beijing has more than half of dry days throughout the year, with spring being the driest, with an average minimum relative humidity of 23%; followed by winter, with an average minimum relative humidity of 25%. Data shows that Beijing has an average annual minimum relative humidity of less than 30% for 202 days (1991-2020). In December, there are 6 months with an average annual minimum relative humidity below 30%. Even on April 6, 2000, and October 28 and 29, 1999, the daily minimum relative humidity was 0, making it a truly dry city.

Why is Beijing so dry? Shi Yan said that Beijing has a temperate monsoon climate. In winter, it is affected by the northerly winds from the high latitudes inland, and polar continental air masses prevail, which are cold and dry. In spring, Beijing has strong winds, strong evaporation, and an even drier climate.

Kunming is the driest southern city in winter, with an average minimum relative humidity of 35% in winter. This is because Kunming has distinct dry and wet seasons. The dry season in Kunming is from November to April of the following year. During this period, there is little precipitation and high evaporation, so it is easy to get dry.

China Weather Network reminds that friends in both the south and the north should humidify and hydrate in winter, especially in the north where there is heating indoors, which makes the air even drier. Humidifiers should be turned on, and moisturizing sprays, facial masks, lipsticks, hand creams, water cups and other equipment should be prepared. At the same time, all regions should pay attention to fire safety and be alert to forest fires.