A simulation study shows the impact of the weather chronology on the thermal performance of a typical office room without internal and solar heat gains but with a typical air change during the day. Consequently, the mean room temperature is identical to the mean ambient air temperature since the room is in thermal balance with its environment. The weather is simulated for periodic steady state conditions whereby a mean summer day (Ta, m=19.4 °C) and a warm summer day (Ta, m=26,3 °C) alternate in certain time intervals:

— 2 average days followed by 1 warm day,

— 10 average days followed by 5 warm days and

— 40 average days followed by 20 warm days.

As in each simulation the average period is twice longer than the warm period, the mean outdoor and indoor temperature in each simulation is 21.7 °C, but Fig. 2 shows that the temperature distribution differs: While short-term variations are highly attenuated, the thermal inertia is not large enough to mitigate variations with a longer cycle period. Thus, the room cannot compensate for the temperature variations at very long cycle periods and the room temperature is coupled more directly to the ambient air temperature. Due to this close correlation, the slope of the regression line increases with the duration of the cycle period.

frequency [1/d] ; 60 days

The sketches on the right side show the cycle periods for each simulation run and the corresponding Fourier series for 60 days. Consequential, the daily amplitude is identical.

The low frequencies (= small numerical value) have a stronger impact (= higher amplitudes) when the chronology is characterised by longer cycle periods.

Fig. 3 shows the regression lines for the three office buildings for the summer 2002 and 2003. Additionally, the mean room and ambient air temperature in the summers of 2002 and 2003 are marked by grey dots. The stability index R2 is 0.6 for Pollmeier and 0.7 for Fraunhofer ISE and Lamparter. Though the room temperatures are higher in summer 2003, the excess temperature is smaller than in summer 2002. For better clarity, the line ambient = room air temperature is plotted.

In other words: On the one hand, the hourly room temperatures are not only higher but also exceed the comfort criteria more often in the summer of 2003 than in the summer of 2002.

On the other hand, the mean room temperatures increase less than expected.