Tricks of the trade

Daily rainfall charts are famously unreadable.

Too often the static overwhelms any signal that may be there.

The above chart shows the seven-day running daily rain total for south Florida.  Seven-day running is not to be confused with weekly because we are plotting a point for every day, instead of every week.  That’s important for seeing a more precise pattern of when the rain is falling (in the current year) and also comparing to the long-term seven-day running mean and median values.  

I avoided that in the chart above by using a 7-day running total.

Thus each day — as represented by the solid blue line — is actually a total of that particular day plus the six that came before. There are two advantages to this approach: (1) it smooths out the line and (2) it allows for ready comparison to the long-term 7-day running mean and median rainfall totals.

The median line is the most telling:

Can you see what the median line does from October and late April?  It drops to zero.  That doesn’t mean we do not get any dry season rain.  Rather, the expected value on any given day is zero.  The mean on the other hand is skewed by the chancy winter rain events.  Compare that to the summer when the 7-day running median jumps up to between one and two inches per week.  Yes we still have rainless days in the summer, but unlike the winter it is those (not the rainy days) that are the norm.

Rain of course is just half the story from late March to May.

Evapotranspiration plays an increasingly dominant role.

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