What is Thermo Sudoku?
Thermo sudoku is classic sudoku with one elegant extra: thermometer lines drawn across the grid. Along each thermometer, the digits must strictly increase from the round bulb to the tip — no equal digits, no dips.
The rules
Everything you know still applies: every row, every column and every 3×3 box contains the digits 1 to 9 exactly once. On top of that, each thermometer orders its cells: the bulb holds the smallest digit and every step towards the tip must be larger than the one before. Thermometers can bend, run diagonally and cross box borders, and the digits do not have to be consecutive — 2, 4, 7, 8 climbs perfectly well.
What a thermometer tells you
Thermometers are generous clues even when empty. A cell k steps up a thermometer can hold at least k+1, and a cell with m cells above it can hold at most 9−m — so on a seven-cell thermometer the bulb is 1, 2 or 3 and the tip is 7, 8 or 9 before you have placed anything. Every digit you do place squeezes the whole line: a 5 midway forces everything below it under 5 and everything above it over 5.
A thermometer climbing 2 → 4 → 7 → 8: every step is larger, gaps are fine.
How to get started
- Start with the longest thermometers — the longer the line, the narrower the range of each cell. A nine-cell thermometer would be forced to be exactly 1 through 9.
- Think “small near bulbs, large near tips”: bulbs love 1s and 2s, tips love 8s and 9s, which pairs beautifully with ordinary row and box elimination.
- Write the possible range in each thermometer cell as pencil marks, and shrink both ends every time a digit lands anywhere on the line.
- Where a thermometer crosses a row or box that is nearly complete, the remaining digits of that row often decide the whole line.