CIBSE TM33 vs Vista Solar Data — Why the Difference?
The raw weather data quoted in CIBSE Part J which forms the TRY weather data set shows data as hourly information. It would perhaps be right to think of the data at 'Hour Ten' to have actually been recorded at 10 o'clock. This is how it was recorded for some sections of the data - Dry Bulb Temperature for example is the correct temperature recorded at the (GMT) time of 10 o'clock.
For solar radiation however the value shown at 'Hour Ten' is actually the solar radiation averaged over the period between Hour Nine and Hour Ten. So because this is an average over that hour it is really the value for 9.30 (i.e. the time average of this period). OK so this means when IES want to use the data from CIBSE if we used the data as shown in the TRY and in TM33 we are using temperatures recorded at 10 o'clock but with radiation figures for 9.30. Here you can see that if we did this in IES we would be using the wrong data. The solar radiation would be wrong by half an hour. So within IES we take the raw CIBSE Part J TRY data and we 'correct' the radiation figures to make sure we use the correct values at 10 o'clock. In essence what this means is that we take the CIBSE TRY raw data at 9.30 and at 10.30 and perform an average on it. GLOBAL(10.00) = (GLOBAL(9.30)+GLOBAL(10.30))/2. The value you see in Vista is the result of this manipulation.
The manipulation is usually called 'solar correction'.
