Why does algae grow faster in Naples than up north?

Naples sits on the Gulf, so it is tempting to read algae discoloration as a Gulf Coast problem that simply runs hotter here than everywhere else. The Asphalt Roofing Manufacturers Association's own bulletin does not draw that line.

ARMA writes that “Algae discoloration of roofs is most widespread in the Gulf States and along the Northwest and Eastern Seaboards.” The same bulletin adds that it is not “confined exclusively to these regions,” and that algae growth “occurs to varying degrees in all regions of the U.S. and southern regions of Canada, especially those subjected to warm, humid conditions.”

In other words, a roof in the Pacific Northwest or on the Eastern Seaboard sits in the same top group in ARMA's sentence as a Gulf Coast roof. The same bulletin describes the discoloration as usually brown to black, and notes it may be mistaken for fungus growth, soot, dirt, moss or tree droppings.

What GAF and CertainTeed publish about blue-green algae is a description of the conditions it needs, never a stopwatch comparing one city against another.

GAF's technical bulletin on the organism says “Blue-green algae requires moisture and proper temperatures to grow. Dew is the primary moisture source,” and separately that it “grows across the U.S. and Canada” and is “well adapted to extreme conditions.” None of those sentences puts a number on how fast it grows in one place versus another, and none names the temperatures it calls proper.

CertainTeed's page on algae-resistant shingles is just as plain about the conditions without a rate attached: “hot, humid conditions promote algae growth,” and “shaded areas serve as excellent breeding grounds.” Read together, the two makers describe what algae needs, not how quickly it takes hold once it has it.

Of the conditions the makers name, NOAA's thirty-year normals for Naples speak to two, temperature and rain. The figures quoted here do not record humidity, dew or the dew point.

Naples Municipal Airport's coolest normal month, January, averages 65.3 F, and no month in the normals averages below that. The rainy season months of June through September bring 7.41, 7.77, 8.89 and 8.60 inches of normal rain, against 1.32 to 1.68 inches in each month from November through February. The Naples climate normals on the roof cleaning page set out all twelve months, and the twelve months of Naples normals on the seasons page add the normal highs and lows.

ARMA's bulletin on prevention says “Coastal or humid climates create the most conducive conditions for algae growth.” Naples sits on the Gulf coast. None of these documents sets a temperature below which algae stops growing, so the normals show warmth in every month without showing a rate.

What the sources support is a comparison of conditions, not of speed, and even that comparison has limits.

GAF describes an organism well adapted to extreme conditions; no document read says a northern winter stops it, or that a Naples winter keeps it growing at any pace. The monthly normals record rain, so the dry season figures for November through February cannot show how much dew a roof collects overnight, and dew is what GAF names as the primary moisture source. ARMA's algae and moss page adds that by the time algae is visible, it has likely been present for several months or longer, which is about visibility, not speed.

A wet winter on top of a normally dry season is its own question, and so is whether growth on a roof means a dirty roof makes a house hotter. Why pavers turn black in summer is answered on its own page as well.