dry season

Count on lots of sun and then more sun during Florida’s dry half, and some cooler days, too. | Florida’s cycle Weather | Wet season | Dry season | Endless summer | Waiting for fall | Coolish winter | Spring drought | Hydrologic holidays

Intro - Dry season paradox?

Winter rains are the exception, not the rule

By Robert V. Sobczak

Believe it or not,

It rains during the winter in south Florida.

South Florida skies are reliably sunny in winter and spring

Even more quizzical, some of those individual rainfall events can be quite large, gulley washers even. The big difference with dry season rains is that they simply don’t add up over a monthly scale. One two inch rain event (however impressive) plus 29 zeros add up to a whopping two inches of rain. Compare that to the typically 7-9 inch totals of the core summer wet season months of June, July, August and September.

The water table reliably and steadily drops during the dry season, starting in November and lasting into May, as a result of the lack of steady daily rains. The caveat is when a big frontal storms pass through. Unlike summer storms that tend to be more local in nature, a winter cold front can dump water across the entire southern peninsula.

Another factor is the cooler temperatures. Winter storms don’t lose as much of their water to evapotranspiration back into the sky. The result is that every drop of winter rain counts as two, and also has a longer “staying power” on the landscape.

That changes in the latter half of the dry season somewhere around the vernal equinox when daylight hours start to grow, the cypress trees green out and air temperatures start to rise. Without rains in March, April and May, the swamp nosedives into a deep drought.

Cycles of flood and drought

How deep and how long will the spring drought last?

Usually not too far into May and rarely into June. It only takes a few weeks (sometimes less) for the summer rains to lift south Florida out of drought.

Isummary: Yes, dry seasons are wet, and that’s not paradoxical. Just don’t count on them too much.

Recent blog posts

Tale of two dry seasons

Well, we knew it couldn’t last …

Both the rainy pace of the start of the dry season,

Comparison of the dry season’s two halves

And the prodigious pool it dropped on the swamp.

The water has receded towards the domes for much of the Preserve.

And don’t look now:

We’re about to enter into the heart of the dry season.

Bar chart showing calendar year and water year rainfall accumulations, in inches

Read more

How to: Weather a dry spell

Lawns may be drying up in town,

But the swamp is doing just fine thank you.

Whoa, can you see how
September and March of
Water Year 20202 were
unusually dry?

The reason:

Fifteen inches of rain over the past two months.

The swamp sponge helps retain that rain,

And also weather a dry spell or two.

Great job Swamp!

dry season

History of “dry season” rain

To be sure,

The dry season is not over yet.

The chart above shows dry season rainfall in Big Cypress Nat’l Preserve from 1970 to present.  Red bars indicated “dry” dry seasons and blue bars indicate “wet” dry seasons.  Can you see how the past few dry seasons have been either “wet” or “dry?”  This year’s dry season total to date is just over 6 inches.

We still have another two months.

So it’s too early to predict a deep spring drought.

But now that the green out has begun,

A good 3-4 inches of dry season rain may be in store …

To boost us back into the long-term norm of around 11 inches of winter rain.

Days, months and seasons (of rain)

In the dry season,

Sometimes a single day of rain can make the month.

Monthly rainfall in Big Cypress National Preserve

And the single month can sometimes make the dry season, too.

Last year that happened in January.

As for this dry season,

No rain in March could send us into deep drought.

As usual, it’s a wait and see.

No(vember) rain

Normally we can count on …

About 43 inches of wet season rain.

A look at wet season rain totals
in Big Cypress Nat’l Preserve
over the years

By wet season, I mean the six-month span from May through October.

Compare that to the dry season half where the average is only twelve.

The only good news is with the cooler temperatures …

Evapotranspiration is finally starting to slow down.