At 6,288 feet, Mount Washington in New Hampshire experiences an abrupt transition into autumn. As summer warmth fades, September brings freezing fog, sharp temperature drops, and high winds to the summit.
Within about two weeks into the month, supercooled cloud droplets will begin freezing onto peak structures in delicate, feather-like formations of rime ice. Soon after, the season’s first measurable snowfall historically makes its initial appearance across the Presidential Range.
Early signs of autumn on the summit
While lowlands across New England enjoy mild late-summer weather all the way through early September, Mount Washington follows a much faster seasonal timeline across the terrain. Ambient temperatures at 6,288 feet drop sharply as solar radiation diminishes and Canadian air masses push south.
In early-to-mid September, daily highs average in the upper 40s Fahrenheit and lows remain around 37°F, though subfreezing temperatures become increasingly possible as the month progresses. Overnight readings routinely dip below freezing across the exposed alpine ridge.
Wind speeds across the mountain frequently average 30 to 40 miles per hour, producing winter-like wind chills long before autumn officially begins on the calendar.
These shifting atmospheric patterns quickly close the summit’s brief summer window. As moisture-laden cloud layers sweep across the Presidential Range, sub-freezing surface temperatures convert liquid fog into solid ice accretion. This transition marks the start of the observatory’s severe cold-weather operational season.
How supercooled fog forms white rime ice feathers
By mid-September—about two weeks into the month—the summit usually sees its first significant rime ice formations.
Rime ice occurs when supercooled water droplets suspended in fog remain liquid below 32 degrees Fahrenheit due to a lack of ice nuclei. When these droplets hit surfaces at or drop below freezing point, they freeze instantly on contact.
Because fog shrouds the summit roughly 60 percent of the time, this icing process recurs constantly throughout autumn. Supercooled droplets build up against the windward side of weather towers, radio antennas, and rock outcrops on the land.
Over hours of continuous freezing fog blanketing the region, the ice grows outward directly into the prevailing cold regional winds.
These expanding formations create delicate, crystalline shapes that resemble white feathers or icy horns. Rime ice can build up to be several inches thick within just a single night, adding heavy mechanical weight to structures and changing how meteorological instruments operate in their measurements.
Tracking historical September snowfall averages on Mount Washington
Along with rime ice conditions, September historically brings the return of accumulating snow to the peak of Mount Washington, according to the Mount Washington Observatory.
Observatory climatological records show that the median date for the summit’s first measurable snow—defined as 0.1 inches or more thick—lands in mid-to-late September. Under the Mount Washington Observatory’s current 1991–2020 climate stats, September averages 1.2 inches of total snowfall throughout the month, down from older historical averages.
While measurable snow has fallen as early as late August in extreme years, September remains the true threshold between seasons.
Cold fronts that come sweeping down from Canada interact with regional moisture to produce brief snow squalls, coating the boulder fields in early white ice cover.
Snow accumulation is variable, changing each year
September snow accumulation stays highly variable from year to year. Warm air off the ocean can temporarily push freezing levels back above the peak, melting early snowpack within a few hours.
Still, these flurries signal a permanent shift in summit work, according to another report from the Mount Washington Observatory. Observational staff start to move from summer maintenance tasks to active deicing routines, using heated anemometers and manual mallets to clear frozen buildup from meteorological measurement tools. The arrival of snowfall in September and feather-like rime ice confirms that severe winter conditions have returned to the highest peak in the whole of the American Northeast.
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