Dew point

The National Weather Service gives dew point as a temperature: the temperature to which air must be cooled to reach saturation, assuming air pressure and moisture content stay constant. A higher dew point, NWS adds, indicates more moisture present in the air.

What happens once air is cooled past that point has its own NWS name. Condensation is, in general, the physical process by which a vapor becomes a liquid or a solid, the opposite of evaporation. Dew point marks the temperature; condensation is the change.

Follow an ordinary evening through that definition. As the air near the ground cools after sunset, surfaces cool with it, and once a surface, a car hood, a railing, a pane of glass, falls below the dew-point temperature, moisture condenses on it.

NWS defines dew as moisture that has condensed on objects near the ground whose temperatures have fallen below the dewpoint temperature. It did not fall as rain; it formed in place on the surface that cooled. Fog of that kind on the outside of a window is answered on its own terms at whether foggy windows can be cleaned, and dew's place in which roof slope grows algae belongs to why roof algae grows on the north side.

NWS Miami calls the dew-point temperature the best measure of moisture levels in Florida and says that during the summer period it remains in the lower to middle 70s. NWS Tampa Bay, whose pages cover West Central and Southwest Florida, says the start of the rainy season is typically accompanied by an increase in humidity with persistent dew point values above 70 F.

NWS Miami's study of 42 years of Miami records put the median onset of the summer season at May 21 and the median end at October 17: medians from southeast Florida records, not dates for any one year or for the Gulf coast. The season NWS Tampa Bay describes has its own entry at rainy season. What Naples humidity has to do with a dirty exterior is argued at whether humidity is why exteriors get dirty, and a yellow cast on glass in spring is a separate question, at why windows turn yellow in spring.