The table below lists the statistical behaviour of CSO tau over the period 1989 to 2002 inclusive. For each tau bin (first column under each month heading) the fraction of nights for which tau lay within that bin is listed (“raw”) and then the cumulative fraction is given in the final column (“cml”). The current UKIRT definition of “dry” is <=0.09. Taking for example the month of May, 59% of the time would have tau of this value or below and thus would be suitable for programmes allocated “dry” time by the TAG.
All bins are “centred” – so, for example, the 0.05 bin covers a tau range 0.04-0.06. The only exception is the final (0.19) bin, which contains all points above 0.18.
Our thanks to Remo Tilanus (JCMT) for supplying the dataset. For a comparison between photometric time and water vapour, click here.