Mar 30, 2011

Drought battles Big Rain Day

Big Rain Days get all the attention …
On Monday we had our second of the year,

But what about all those dry season days in between when it doesn’t rain at all:

Don’t they count, too?

Over the long run, Big Rain Days are no match for south Florida's spring drought

Just to refresh your memory, a Big Rain Day (BRD) is any day in which south Florida (as an aggregated whole) averages one inch or more of rain. One inch of rain isn’t all that unusual for any single gage, but as a regional average it’s a pretty rare event:

On average over the past 20 years we’ve averaged around six BRDs per year.


On the opposite end of the spectrum are Florida’s prolific days of pure sun.

They may register “zero” in all the rain gages but on the ground are an invisible indication of water “pointing up.” Evaporation and transpiration drop the water table down.

June and September have the highest monthly occurrence of BRDs,
but forty percent of them fall during the dry season (Nov-Apr)
The calendar chart below highlights these two extremes:
  • Big Rain Days (BRDs) are plotted as blue rain drops. Fay totaled 5.7 inches over a single day but it’s Mitch with 5.3 inches in November 1998 that wins the one day prize.
  • Insignificant Rain Days (IRDs) are plotted as orange suns and include any day which recorded a south Florida wide average of 0.05 inches of rain or less.

Two interesting patterns emerge:
  1. BRDs appear to have a good chance of occurring any time of year, both during the wet season and the dry season (see bar chart above). Why? Despite getting 3/4ths of our rain during the summer wet season, those showers are often localized in nature. In comparison, a winter cold front can spread water across all the basins in one fell swoop.
  2. IRDs rule the dry season skies. Yes, a BRD or lesser rain may aperiodically pass through, often achieving a “dry season’s month” worth of rain in a single day (and momentarily bumping the water up), but it’s the long-run of semi-consecutive IRDs which usually win by late spring: Surface waters have vanished and wildfire is on the prowl.
This chart displays the distribution of south Florida's two meteorologic extremes:
Periodic Big Rain Day versus long-stretches of unending sun
So, to answer the question:

Yes, all those “zeros” of no rain add up to quite a big sum!

2 comments:

The Florida Blogger said...

After a week of rainy days up in the Tampa area, I'm ready for a few sunny days.

Janie said...

I'm surprised s. FL only gets 6 BRD's per year.
Here in Utah, the meteorologists get all excited if we get 1/4 inch of rain at one time!