The Doppit Almanac · No. 33

Riseit, a free daily thermometer sudoku

Digits rise along every thermometer

What it is

Riseit is thermometer sudoku. The sudoku rules hold, so each row, column and box ends up with every number once, and drawn across the grid are thermometers: a round bulb with a stem trailing away from it. The digits along a thermometer strictly increase from the bulb, step by step to the tip.

That one rule does a lot of work. A long thermometer cannot start high, because it needs room to keep climbing, and it cannot end low. Reading how much a thermometer can rise pins its cells down to a small window, and the sudoku scan finishes the rest.

How it plays

• Fill every cell so each row, column and box holds every number once.
• Along each thermometer the digits strictly increase, starting from the round bulb.
• A thermometer only constrains its own cells. Two can cross the same box.
• Pick a number chip, tap a cell to place it, tap again to clear it.
• Notes pencils small candidates while you narrow a cell down.
• A digit out of order on a thermometer glows red. Stuck? Hint names the next step.

The difficulties

Easy is a six-wide grid where the thermometers alone squeeze cells down to a single number, a quick half-minute solve. Medium keeps that six-wide grid but opens with eight to fourteen numbers instead of twenty-one, so how far each thermometer can rise carries most of the solve. Hard is the classic nine-wide, with twenty to thirty-two numbers down, and it takes at least three thermometer squeezes every time. Expert is nine-wide too but opens with about seventeen numbers, and four times over you settle a cell by reading a thermometer against the numbers its neighbours can still take, not against numbers already placed. A fresh board is dealt every day, for nothing.

Nearby in the almanac: Digit, Rankit and Skyit