Intuitively one decomposes climate variables such as solar irradiation and ambient temperature into their deterministic and stochastic components. The deterministic component comprises a sum of contributions at various frequencies, thereby defining the climate of the location. The stochastic component comprises the fluctuations about this component, giving the day-to-day weather variations. Boland (Solar Energy 60(6) (1997) 359) shows that on a day-to-day basis both components must be included in building thermal performance simulations to give sensible results. It is shown in this paper that on an hour-to-hour basis the stochastic component may be disregarded in building thermal, solar process heat and photovoltaic system performance without significantly affecting the results, Since the hourly stochastic component of the solar irradiation is very difficult to model, this result greatly simplifies the construction of synthetic climate data sets.