With the upcoming trend in electric aviation a good model that describes how the solar spectrum depends on altitude and solar zenith angle can ease the design process significantly. Knowing what solar spectrum to expect makes it possible to design the solar system of an aircraft more efficiently as the optimal incoming energy is included in the weight estimation. However, not only solar powered aircraft bene t from this model. Solar systems in rural mountain areas can use this model to predict more precisely the energy yield of solar systems and their return on investment. In this thesis a model for the prediction of the solar spectrum for various altitudes and solar angles is proposed. This model, that uses the physics of Rayleigh scattering and light absorption to determine the solar spectrum, is found to be accurate for clear skies without clouds and low turbidity. This is because when clear skies are assumed the majority of light interaction can be described by Rayleigh scattering and light absorption. To obtain reliable spectra at cloudy days one needs to include Mie and non-selective scattering to the model. To prove that this model can ease the design process of solar powered aircraft a case study has been performed that focuses on the design of a two-manned solar powered aircraft capable of crossing the globe. This aircraft, also known as a Very High Endurance Solar Powered Aircraft (VHESPA), has been subject to a new weight prediction model. This new weight prediction model has been based on the model proposed by Noth that uses reference solar powered aircraft together with battery and propulsion technology research to come up with a first order weight estimation of solar powered aircraft. To con rm the correctness of this model the designed aircraft has undergone a simulation that reproduces the incoming solar power as well as the power required for continuous flight and the power choices that need to be made throughout the day. It has been found that an aircraft designed using this new weight estimation model gives a reliable first order solar powered aircraft design.