Abstract: A method and apparatus are provided for measuring quantitatively the amount of liquid and convection heat emitted from a surface by diffusion. When the surface is a derm surface the transepidermal water loss is measurable by a cylindrical shield device in which one open end is engaged with the derm layer and the other open end is exposed to the surroundings and within the cylindrical shield are placed a pair of temperature transducers and a pair of humidity transducers respectively positioned orthogonally to the derm surface at first and second known distances therefrom. The temperature and humidity readings at these two distances from the surface of the derm are processed by a circuit device to calculate the amount of water evaporated per surface unit over a given time interval.