Rain over the last 30, 60, 90 or 180 days divided by the 1991-2020 average for the same calendar days. 60% means 40% less rain than usual.
Koh Samui watch
The app adds up the rain observed on Samui (ERA5 reanalysis, about 5 days behind) over several windows, and compares each with the 1991-2020 ERA5 normal for exactly the same days of the year.
100% = a normal amount.
Below 75% = drier than normal (the water factor starts at Watch).
Above 100% = wetter than normal.
Short windows (30 days) react fast but a single storm can swing them. Long windows (180 days) show the slow deficit that empties reservoirs.
How to read it
Look at both the % and the millimetres: 40% of a dry-season normal of 60 mm is only 36 mm missing, while 60% of a November normal can be over 100 mm missing.