Approved for public release; distribution in unlimited. / This is a study of abnormal refractive layer occurrence over the Amazon region and possible effects on radar and ESM systems, ground or airborne based. Climatologic data from three stations in that region are analyzed using computations from the Global Tropospheric Experiment (GTE), soundings and satellite imagery. The GTE data provide monthly occurrences and seasonality of atmospheric ducts and superrefractive layers. Further, individual soundings from the March-June 2003 period and the Advanced Refractive Effects Prediction Systems (AREPS) 2.1 software are used in a case study that analyzed these layers and, in addition, subrefractive and multiple layers. Selected soundings were used in simulations to explain the effects of different types of abnormal layers on the electromagnetic propagation. Although abnormal layers did not affect ground systems, airborne ones were. For radar, a region with low or no detection is created when an abnormal layer refracts the electromagnetic energy upwards or downwards. Some combinations of multiple layers may cause effects even stronger. It is concluded that knowledge of the abnormal layers occurrence is important for operations in the Amazon region. Further, airborne radar platforms should measure local refractive conditions, if possible. A comprehensive study in time and space is recommended to provide forecasting. / Lieutenant Colonel, Brazilian Air Force