In thermography each thermograph is topped by a vertical mast at the mean value or count of a game or go position. The base of the vertical mast lies at the temperature of the game (miai value of the largest local play).
In terms of play, neither player gains, on average, by making a local play at the ambient temperature where the thermograph is a vertical mast. A local play often entails a loss in such a situation. See Colored Mast.