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.
A borderline case receptive to both tactile and nontactile coldness is 風 (kaze, wind). When it’s 冷たい風 (tsumetai kaze), the focus is on the wind as a low-temperature object. In the case of 寒い風 (samui kaze), the perspective shifts to the freezing person on the receiving end of said wind.
The tactile/nontactile divide applies in just the same way when the lexical thermometer moves up to higher degrees, were it not for one more or less trivial point: The difference is only in writing. Because in spoken language, there is just one word each for atatakai (warm) and atsui (hot). However, when putting it on paper, one has to indicate whether the sensation is nontactile, corresponding to samui, or if it is of the tactile, tsumetai type. In the former case the terms are written 暖かい (atatakai) and 暑い (atsui), whereas in the latter case 温かい (atatakai) and 熱い (atsui) are to be used.
As in many other languages, temperature expressions provide favorable material for metaphors. Starting with the colder regions, an example is 懐が寒い (futokoro ga samui), where “bosom is cold” expresses the emptiness of one’s inside pocket, or, in plain language, being broke. Another case in point is 背筋が寒くなる (sesuji ga samuku naru), with samui describing a chill running down one’s spine. A similar sensation can be expressed when samui is used to communicate one’s state of mind after a cheesy joke.
Metaphors with tsumetai mostly relate to emotional indifference and lack of sympathy, in a similar way as English “cold.” Thus it is appropriate to refer to someone who doesn’t care about other people as 冷たい人 (tsumetai hito, a cold-hearted person).
Both atatakai and atsui have a couple of metaphoric extensions, too. Atatakai is used to express emotional warmth, like in 暖かいお言葉 (atatakai o-kotoba, “your kind words”). Atsui stands for excitement and other “hot” feelings, as in 熱い論戦 (atsui ronsen, a heated debate). One difference between warm and hot is that atatakai metaphors commonly use both kanji options, whereas atsui always goes for the tactile variant, 熱い.
Two temperature expressions that complete the list are 涼しい (suzushii, cool) and ぬるい (nurui, lukewarm). The former is on the nontactile side of the sensory divide, the latter can be used only for tangibles. And whereas suzushii includes some sort of positive appraisal that it’s not as hot as expected, like a welcome breeze of 涼しい空気 (suzushii kūki, cool air), nurui contains a pinch of disappointment about something being less hot than desired, most prototypically ぬるいお風呂 (nurui o-furo, a lukewarm bath).
Coming back to the phrase from the opening, the thing about 三寒四温 is that it mixes the two levels of sensation. Whereas 寒 refers to nontactile cold, the warmth of 温 is of the touchable type. As both characters are supposed to describe outside temperature, which for all we know is nontactile, one might well wonder whether 三寒四暖, read sankan shidan, wouldn’t be a more appropriate term. But to be honest, either type of warmth is fine with me as long as it makes the winter go away.