Marine Boundary Layer Effects

Over warm water, such as the Caribbean and other tropi- cal seas, evaporation inver- sions may create ducts that are

useful in the microwave region between 3.3 and 24 GHz. This

Fig 21.29—Land-breeze convection along a coast after sunset creates a temperature inversion over the land.

inversion depends on a sharp drop in water-vapor content rather than on an increase in temperature to create ducting conditions. Air just above the surface of water at least 30°C is saturated because of evaporation. Humidity drops significantly within 3 to 10 m (10 to 30 ft) altitude, creating a very shallow but stable duct. Losses due to water vapor absorption may be intolerable at the highest ducting frequen- cies, but breezes may raise the effective height of the inversion and open the duct to longer wavelengths. Stations must be set up right on the beaches to ensure being inside an evaporation inversion.