How to convert daily total rain, recorded at daily intervals, to daily average rain? I am seeking some assistance in how to take into account difference in trials periods when calculating average rainfall. I am investigating the effect of rain on plant disease severity. My trials have been conducted in different years, and the length of the trial varied in different year. I have been asked to use average rain instead of total rain to take into account difference in trial periods. Our weather station recorded total rain per day (at daily intervals), I need to calculate average rain per day. To achieve this, I have been asked to divide total rain in the trial period by number of days rain occurred (rainy days). Is it the right way to calculate average rain per day? Or we are misleading the results by calculating total rain per day instead (maybe in an incorrect way)? Please see blue column to see what I'm talking about. Thanks very much.


If we have (as in your example) within a particular year (2006)  41 rainy days and 156.3 cm of total rain, the average is indeed 3.81cm per rainy day. That said, your intuition is correct, this is not the whole story. It makes sense to also have the average rainfall per trial day, i.e. 2.27 cm per trial day. These two statistics help us capture different aspects of the rainfall's impact. The first one gives us an idea of how much it rained "overall" and the second indicator of how "strong" (temporally localised) the rainfall was.
For example, in a 20-day period if it rains every second day by 3cm that that means we have 30/20 = 1.5 cm of average rain per day and 30/10 = 3 cm per rainy day. For the same plant and period if we had only 2 days of 15 cm rainfall each, we would still have 30/20=1.5 cm of average rain per day but 30/2=15 cm per rainy day, which obviously will have a much different impact on the plant's well-being.
So in effect, neither you nor your collaborators were "right" or "wrong", just aimed to capture different aspects of the rainfall's pattern.
