For automatic target detection in maritime environments using optical imaging systems, it may be critical to have a valid a priori knowledge of the elevation of the border between sea and sky backgrounds. In operational conditions, this border, called the horizon, is predicted to move up and down depending on the refractivity conditions. Predictions of horizon elevation can be made from ray-tracing using bulk estimates of refractivity profiles based on the Monin-Obhukov similarity theory. In this paper, predictions of horizon elevations obtained with IRBLEM, a DRDC Valcartier computer model, are compared with observations made in the North Sea in two different meteorological environments: in Katwijk (The Netherlands, October 1993) and Sylt (Germany, June 1992). Good agreement is shown between observations and model calculations. The expected variation of horizon elevation with changing refractivity conditions is discussed.