Mold Season by State: The Moisture Calendar

It depends on where you live, because there are two mold seasons. Across the South and along the Gulf the hard stretch is the warm months, when outdoor air carries enough water to wet any surface colder than itself; Florida tops the country at a mean dew point of 73.4°F in August. Across the northern tier the hard stretch is winter, when moisture generated inside the building condenses on cold surfaces; North Dakota averages 12.1°F in January and its outdoor air warmed to 70F indoors sits near 8% relative humidity. Most states run some of both. Pick your state above for its own calendar.

Because relative humidity changes when the air moves and dew point does not. Relative humidity says how close air is to saturation at its own temperature, so the same air reads 90% at dawn and 45% at noon without a molecule of water going anywhere. Dew point says how much water is actually in the air, and it is the temperature at which a surface starts collecting it. When the question is whether a basement wall or a window will get wet, dew point is the number that answers it. The National Weather Service makes the same argument on its own dew point page, which is quoted on every state page here.

Both, in different places and for different reasons. In a warm humid state the summer problem is outdoor moisture pushing in. In a cold state the winter problem is indoor moisture condensing on cold surfaces, and it happens while the outdoor air is at its driest of the year. The two have opposite answers to a question as simple as whether to open a window.

EPA has answered this directly: "Molds come in many colors including white. "Black mold" is not a species or specific kind of mold, and neither is "toxic mold." Sometimes the news media use the terms "toxic mold" and "black mold" to refer to molds that may produce mycotoxins or for a specific mold, Stachybotrys chartarum. Molds that produce mycotoxins are often referred to as toxigenic fungi." EPA also writes: "No one can tell whether a mold is producing mycotoxins just by looking at it." Every statement about mold itself on this site is EPA's, and for any health question the sources are EPA, CDC, your state or local health department and your own clinician.

EPA's figure, quoted rather than paraphrased: "Keep indoor humidity low. If possible, keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity. Relative humidity can be measured with a moisture or humidity meter, a small, inexpensive ($10-$50) instrument available at many hardware stores." That is national and it does not change from state to state. What changes is how hard the outdoor climate makes it to hold, which is the entire subject of this site.

EPA has written about this directly: "Is sampling for mold needed? In most cases, if visible mold growth is present, sampling is unnecessary. Since no EPA or other federal limits have been set for mold or mold spores, sampling cannot be used to check a building’s compliance with federal mold standards." EPA adds separately: "Please note that this document presents recommendations. EPA does not regulate mold or mold spores in indoor air" Your state or local health department is the source for anything beyond that.

By mean dew point in the warmest month, Florida is the heaviest in the country at 73.4°F and Nevada the lightest, peaking at 39.4°F. That gap is larger than it looks: dew point is a temperature, but the water content it stands for roughly doubles for every 20F you climb. It is also only the warm-season half of the story, and the cold-season column of the table above reorders the map completely.

NOAA NCEI's 1991-2020 U.S. Climate Normals, hourly product, which publishes an official normal dew point and temperature for every hour of a normal year at 138 stations we use across the fifty states. We average those hours into calendar months, then average a state's stations equally, and we name the stations on every state page. The dew point bands are the National Weather Service's, quoted. The figures are observed normals rather than a model or a forecast.

No. This is outdoor climate: it describes the moisture load a building in your state has to work against, which tells you which months to expect trouble in and which mechanism to expect it from. Your drainage, grading, insulation, ventilation and leaks decide the outcome inside, and none of those is in a climate record. EPA puts the useful part in one sentence: "The key to mold control is moisture control."