Abstract: Apparatus and method for determining and tracking dew point temperatures. The dew point is measured by lowering the temperature of a condensing surface that is associated with a heat flow sensor. After condensation occurs the heat flux across the condensing surface is maintained at a preselected value by adjusting the temperature of the condensing surface. As long as a positive heat flow is maintained into the condensing surface, the temperature of the condensing surface is maintained at a slightly lower temperature than the saturation temperature which is by definition the dew point. The dew point temperature calculation is based on the relationship that when condensate forms on the condensing surface, the temperature of the condensing surface approaches the dew point temperature as the heat flow rate across the condensing surface approaches zero, i.e., at adiabatic conditions.