A general requirement in ground-based γ-ray astronomy is a moonless night with good atmospheric transparency since the atmosphere is an integral part of every atmospheric Cherenkov telescope and its sensitivity considerably depends on variable atmospheric conditions. To fulfillment the above condition we need a clear atmosphere which is not always a case. If clouds are present in the atmosphere, they have a considerable impact on the atmospheric transparency. However, from the HEGRA and MAGIC experiences, we know that sometimes is possible to operate the detector with considerably high efficiency in the presence of high dense cloud layers. Measurements of atmospheric transmission with MAGIC LIDAR at La Palma show that up to 30% of measurements made in conditions like those mentioned above can be used by applying proper corrections. In this context, one of the aims of this thesis has been to study clouds impact on the atmospheric transparency, in order to show there is a possibility to prolongate duty cycle of CTA-N with observations during cloudy nights. In order to examine the influence of cloud altitude and optical depth on CTA-N performance, transmittance simulations for the atmosphere with clouds were made. The clouds simulations were done with MODTRAN computer program, while the atmospheric models produced by MODTRAN were used as inputs for Monte Carlo simulations with sim_telarray. Computed simulations can be used as a base for effective adaptive scheduling observations in the future.