Abstract: Apparatus for determining the relative humidity of an atmosphere by measuring the dew point of the atmosphere. The dew point is measured by lowering the temperature of a condensing or sensing surface associated with a heat flow sensor. When condensation begins to occur this is detected by the heat flow sensor as a change in the rate of heat flow through the condensing surface. In one embodiment a pair of heat flow sensors are employed, one maintained at a slightly lower temperature than the other. The sensors are connected in series opposition so that output signals due to background or convective heat transfer are cancelled, any change in the combined output signal of the two sensors then indicating heat flow resulting from condensation. The temperature of the cooler face will be the dew point temperature. Two arrangements of sensors are disclosed.