To send charcoal in the middle of the snowstorm
To provide help in one's hour of need
Positive
救灾物资及时送到灾区，真是雪中送炭 | Jiùzāi wùzī jíshí sòng dào zāiqū, zhēn shì xuězhōngsòngtàn
The disaster relief materials' arrival in the stricken region really was help in the hour of need.
[Source]
邻里街坊的帮助犹如雪中送炭，使他非常感动 | Línlǐ jiēfāng de bāngzhù yóurú xuězhōngsòngtàn, shǐ tā fēicháng gǎndòng
The neighbor's help was like charcoal sent in a snowstorm, and the recipient was very moved
[Source]
None
雪里送炭 (xuě lǐ sòng tàn)
Just for clarification purposes, the 炭 (tàn) in this chengyu refers to 木炭 (mùtán), not 煤炭 (méitàn, coal)
《大雪送炭与芥隐》(dà xuě sòng tàn yǔ jiè yǐn), a work by Sòng Dynasty (宋, 960 - 1279) author 范成大 (Fàn Chéng Dà)
This chengyu comes from a line in the poem listed in the Origin section. In the poem, the author references an act of benevolence from the second emperor of the Sòng Dynasty (宋, 960 - 1279). According to historical records, one year there was a massive blizzard, and the air was frigid. The emperor, sitting in his palace, began to worry about how his people were weathering the storm, so he ordered grain and charcoal to be delivered to them. This reportedly allowed them to fill their stomachs, get warm, and last through the worst of the storm.