When standard atmosphere is used in range and altitude flight profile assessment of the unmanned aerial vehicles (UAVs) flight performance, it does not take into account the huge variety of meteorological factors on different routes. Hence, it does not accurately assess the maximum flight range. It is a strategy of the “worst combinations in terms of the probability of meteorological factors” for the entire surface of the Earth at all seasons. The paper proposes a global atmosphere model to be used in flight range assessment, and compares the model with the meteorological data archive. To build the reach line when preparing flight missions for sea-based unmanned aerial vehicles, the study proposes to use the global atmosphere model, which takes into account the meteorological parameters of a given place and month, since the wind, for example, has a particularly strong effect on the flight range and has significant seasonal-latitudinal variability. The global atmosphere model and a simplified mathematical flight model can be also used in assessing the maximum permissible length of the operational section when preparing a flight route.