Is Naples humidity why exteriors get dirty?

The National Weather Service defines the two terms separately, and the difference explains why a humidity percentage alone does not tell you how much water is in the air around a house.

Relative humidity is, in NWS's definition, “a dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated.” Because that second amount depends on temperature, NWS adds that “relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present.” The same percentage can describe very different amounts of water at different temperatures.

Dew point is a temperature, not a ratio. NWS defines it as “the temperature to which air must be cooled in order to reach saturation” and says “a higher dew point indicates more moisture present in the air.” Separately, the NWS Miami office writes that the dew-point temperature is “the best measure of moisture levels in Florida.”

A percentage describes the air. Dew is water sitting on the surface itself.

NWS defines dew as “moisture that has condensed on objects near the ground, whose temperatures have fallen below the dewpoint temperature.” What sets it off is the temperature of the object, not a reading of the air around it. GAF's current bulletin on algae staining on shingled roofs ties that moisture to growth: “Blue-green algae requires moisture and proper temperatures to grow. Dew is the primary moisture source.”

GAF does not say how often dew forms on a given roof, in which months, or how long it stays, and no source read measures that for Naples.

None of the NOAA or NWS material read for this answer gives a relative-humidity normal for Naples, so the regional picture comes from what NWS Tampa Bay and NWS Miami do publish.

NWS Tampa Bay writes that the rainy season “is characterized by warm, humid conditions with frequent showers and thunderstorms,” typically with “persistent dew point values above 70°F.” NWS Miami's study of Miami International Airport records puts southeast Florida summer dew points in the lower to middle 70s; the office's drier winter season is described under the dry season.

Those are two regional offices describing West Central and Southwest Florida and southeast Florida, not readings from a Naples station. The glossary defines dew point and the rainy season on their own pages.

A good share of what dirties a Naples exterior has nothing to do with Naples humidity.

The PGT, CGI and WinDoor care guide names salt among the deposits its screens collect and salt spray deposits on its aluminum frames; the salt air answer goes through what the sources say salt does to each material. UF/IFAS Gardening Solutions describes pollen as dust-like grains, some “seen as blankets of yellow and green on cars and other surfaces,” and says wind-pollinated plants mass-produce theirs; the pollen season has its own glossary entry.

Irrigation puts water on a house directly, and UF/IFAS writes that over-watering with reclaimed water is a common problem. Sprinkler water on a wall or a driveway is water on the surface, not humidity in the air; what that water carries is covered in whether reclaimed irrigation water stains exteriors.

The organisms are a separate subject. ARMA writes that the black streaks on many roofs, often mistaken as mold or mildew, are actually algae, and why everything in Naples grows mildew sorts out the rest. The Naples rainfall and temperature normals are set out in the Naples climate section of the roof cleaning page, and whether all three exterior cleanings can share one visit is answered separately.