As the saying goes, the south wind warms the north wind, and the east wind is wet and the west wind is dry. The term "southerly sky" that people often say is not a specific term in meteorology. But literally, it can be understood that this resurgence phenomenon is related to the warmer south wind.
Southerly days mostly appear in spring or rainy season every year. After beginning of spring every year, indoor moisture regain will occur in some areas, which makes clothes and furniture damp and moldy, which is often called "southerly day".
The cause of the south wind
The reason for the high indoor moisture regain in southerly days is that the temperature of indoor walls and floors decreases day by day after winter, and the indoor temperature rise rate is slower than that of outdoor after spring. When the outdoor air suddenly rises sharply and is very humid, it will condense into small water droplets on the cold ground and walls when it is blown indoors, and even form a flowing water layer. If it is cloudy in the morning, the weather is too warm and there is a strong south wind, there will be a resurgence that day.
In these areas, in March and April every year, cold air affects from time to time, resulting in low surface temperature of buildings and so on. After the cold air passes, the warm and humid airflow from the ocean quickly "bounces", the temperature rises rapidly, and the air humidity increases. When the surface of a cold object meets the warm and humid airflow, water droplets will be generated on the floor, walls, etc.