It is shown that the parameters of a contrail of a large civil aircraft (distance to the contrail and the time of its formation, contrail cross size, ice content, water content, cross optical thickness) depend strongly on the exhaust/atmosphere pressure ratio (parameter of jet nonisobarisity) and the distribution law of jet parameters at an exhaust nozzle of an aircraft engine. The initial part of the pressure leveling-off in the nonisobaric jet is described by full Navier-Stokes equations and calculated by the method of large particles. The far field, including the contrail, is described by the parabolized equations like Prandtl boundary layer equations and calculated by the finite-difference algorithm. The coefficients of radiation scattering and absorption by liquid water droplets and ice crystals are calculated using the Mie theory with allowance for the particle size distribution. It is shown that a clearing channel can be created by a laser pulse of relatively low power.