Finned tube heat exchangers are widely used in refrigeration systems. These heat exchangers are prone to frost accumulation under humid conditions. Frost accumulation can severely reduce the heat transfer capacity of the device by building a thermal barrier between the air and the fins and by reducing the air flow through the heat exchanger. This study numerically investigates the effect of air humidity level on the cooling capacity drop of a finned tube heat exchanger under frosting conditions. Simulations are conducted using the transient, two-dimensional Eulerian-Eulerian multiphase solver in ANSYS Fluent. Frost accumulation is modelled through a mass source term implemented via user-defined functions. Frosting simulations were conducted at 60% and 80% relative humidity levels. Numerical predictions on the heat transfer rate are in close agreement with the experimental data gathered in the closed-loop frosting tunnel at Özyeğin University. The results show that the heat transfer rate at the early phase of the frosting increases with the increasing relative humidity level. However, as frosting progresses the enhanced frost accumulation at the case with the higher humidity level inhibits the heat transfer to the surface more strongly than it does with the lower humidity level.