When winter is running out of steam but spring is not yet willing to come, this results in a series of temperature highs and lows known in Japan as 三寒四温 (sankan shion), an alternation of three cold (三寒) and four warm (四温) days. Meteorologically speaking, this phenomenon is much more pronounced on the Asian continent, where it occurs in the middle of winter rather than toward its dirty end. That the expression is originally an import from the mainland may also explain why, on closer inspection, there seems to be something odd about the two temperature words it contains.
In order to understand this, we need to appreciate that Japanese is quite "sensational" when it comes to temperatures. For instance, where in English something is just "cold," in Japanese it's either 寒い (samui) or 冷たい (tsumetai). The choice between the two is based on a number of conditions, including whether one specific part of the body is concerned or the body as a whole, and whether something is tactile or nontactile. Accordingly, when it's cold outside, it's 外が寒い (soto ga samui), but the cold hands that result from this condition are described as 手が冷たい (te ga tsumetai).
Samui is prototypically used when talking about oneself, whereas other people have to be described as 寒そう (samusō, "looking as though they feel cold"). The logic behind this is that since feeling cold is a subjective state of mind, one cannot know for sure — or so Japanese grammar insists — if another person is really cold or they just look like they are freezing to death while in fact being quite warm on the inside. In comparison to this, tsumetai is much more objective. It describes low-temperature liquids, dishes, limbs and other tangible things, no matter if they belong to the speaker or to others.