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Originally Posted by Steam Man

OK, I have a friend who is a retired physics teacher who, hopefully has sorted this issue out.

Steam or water vapour is only invisible below 100C; if a tight fitting damper successfully limits enough air entering the fire box to dampen the fire within it, then the exhaust gasses from the cylinders are emitted into the chimney. This allows the steam exhausted from the cylinders to cool more quickly, thereby reaching the critical (or visible temperature) of 100C when water vapour becomes visible, or precipitates. Conversely, climbing a hill with the damper open greatly increases the mix of firebox gasses mixing with cylinder gases, thereby raising the temperature well above the critical temperature for water vapour to be visible.

So there, its not called primming anymore !!!

Steve.