This kō is the calendar's own bookend to 虹蔵不見 (#58, "rainbows hide, seen no more") in early winter — the same character pair, 虹, marking both when rainbows stop appearing and, six months later, when they start again. What's actually changing is humidity: spring air finally holds enough moisture after rain for sunlight to refract through it consistently, which is why rainbows genuinely do become more common from here through early summer, not just more noticed.
虹 alone reads as a summer season word in classical haiku convention, tied to the muggy air of the coming rainy season — but 初虹 (hatsuniji), "the first rainbow," specifically belongs to spring, this kō's own event rather than the general phenomenon.
Nearly translucent and barely an inch or two long, traditionally served alive in a small bowl of water or briefly blanched — a delicacy defined by its fragility.
Sweeter and less acidic than most winter citrus, holding through its final good weeks now.
Grown widely in Hokkaidō and Nagano's cooler climates, reaching its peak tenderness in this exact window before the summer heat toughens it.
Wisteria's hanging clusters of purple blossom have been prized in Japanese gardens for over a thousand years specifically for the way they're grown — trained flat over a trellis so the racemes can drop straight down, turning an entire arbor into a kind of slow, deliberate curtain rather than a shrub.
Spring is when daytime low tides recede furthest, exposing tidal flats that are underwater the rest of the year — the physical reason shiohigari clusters into this window rather than any other season. It doubles as good timing by taste as well as access: asari clams are fattening up ahead of their spring spawn and are at their best right now, before the flavor thins out through early summer.
虹's own construction carries an old belief inside it, the same one already noted at its winter counterpart (#58): 虫, the serpent/insect radical, paired with 工, to pierce — ancient China pictured a rainbow as a two-headed dragon-serpent driven through the sky, drinking from a river at each end, rather than an effect of light at all.
The "seven colors" most people in Japan grow up with is a cultural convention, not a fixed fact of the light itself — the same spectrum is commonly described as six colors in parts of the US and UK, five in Germany, and with far fewer named bands in some cultures outside the temperate latitudes.
This kō's rainbows appear in the brief window when the sun breaks through low in the sky just as a passing shower moves off, not in the rain itself.
Your back is always to the sun and the rain is always ahead of you when a rainbow appears — a small, reliable piece of physics anyone can check outdoors for themselves.
The best digging window is short — roughly two hours either side of the lowest point of a daytime spring tide — so timing matters more than the day itself.
April showers this time of year tend to be brief and are often followed directly by clearing skies — exactly the conditions a rainbow needs.
Every rainbow is one continuous smear of light bending through raindrops — there's no natural seam between one band of color and the next. Where the lines get drawn is inherited, not observed: seven bands across Japan and much of the English-speaking world (a count that traces back to Isaac Newton, who added indigo specifically to make the total match the seven notes of a musical scale), six with no indigo in parts of the US and UK, five in Germany, sometimes as few as two among cultures further from these latitudes. Nobody involved is looking at a different rainbow — only carving up the same light differently. What in your own life might be one continuous thing you've been taught to see as several separate ones, or several things you've been taught to see as one?