It's getting cold, and the sky refers to the weather and climate. Gradually means gradually, gradually. Cold means cold.
The heart should be warm, and the heart is the human heart. Warmth means that people should keep a positive and warm state of mind, and not affect their mood because of the cold weather.
Autumn rain refers to the weather in late autumn, and raindrops are freezing cold, but we should not be chilling because of such cold weather.
Extended data:
Symbolic significance of autumn:
People often decorate "autumn" with "gold", which often gives people the expectation of a bumper harvest. But in the eyes of China ancients, it seems that there are few such joys, only bleak autumn wind, withered vegetation and a cold scene. Therefore, when they need to express their sadness, they often turn to the calm image of "autumn".
For example:
In the evening, I bid farewell to a guest on Xunyang River. Maple leaves and mature rushes rustle in autumn. (Bai Juyi's Pipa Trip)
Since ancient times, the most sad thing for sentimental people is to leave, not to mention the bleak autumn, how can they stand the sadness. Who knows where I am when I am awake tonight? Yang Liuan, Xiao Feng and the waning moon. (Liu Yong's "Yulinling")
The above two cases are all based on various bleak and cold autumn scenery to contrast and render the feelings of parting, vividly depicting two parting pictures of Qiu Jiang.
Baidu Encyclopedia-Autumn