The usual picture of the development of temperature and humidity boundary layers in a land air mass that moves offshore is shown to be very wrong under one type of Foehn condition in southern California, and it is probable that similar conditions can prevail in widespread areas around the globe, notably the Mediterranean Sea and the monsoonal regions of the Near East and Southeast Asia. A formalism is developed for analyzing the modification that seems to represent the observations satisfactorily, and graphical solutions for radio and optical ducting are given. It is shown that offshore modification can lead to elevated layers rather than to surface based layers, and the height of the layer base is theoretically predicted. Values of evaporation and heat flux into such an air mass are calculated, and the distance offshore at which dew point depression becomes zero is predicted. A method for measuring the downward heat flux in elevated inversion layers is described and results are given.