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Valley fog: why low fields fill with fog on calm fall mornings
When: September to November
On clear, calm fall nights the ground radiates its heat to space, cool air slides downhill, and by dawn the low fields and river meadows hold pools of fog that the sun erases within hours.
Why this time of year
Fall is the prime season: nights are long enough to cool the ground hard, the air still carries summer's moisture, and the National Weather Service calls fog formed this way most prevalent in fall and winter.
How to notice it
- Check tonight's forecast: clear skies and calm wind are the recipe.
- Be at a view over low ground around sunrise; the Sudbury River meadows and any big open field work.
- Notice the fog's shape: it fills the hollows and stops partway up the slopes, because the coldest air drained downhill like water.
- Watch it go: the fog thins and lifts quickly once the sun is up, so the show is over by mid-morning.
Try this with kids: Where does the fog stop on the hillside? That line is where the cold air pooled.
radiation fog forms when the ground cools the air above it to its dew point on a clear, calm night
Safety
- Fog this thick can cut driving visibility to near zero; if you drive to a viewpoint, go slow with headlights on.
Sources
- forecast.weather.govRadiation fog: a fog that forms when outgoing longwave radiation cools the near-surface air below its dew point temperature.
- weather.govFog forms overnight as the air near the ground cools and stabilizes to saturation; favored areas are sheltered valleys with little wind and locations near bodies of water; it is most prevalent during fall and winter and goes away the next day under the sun's rays.
- weather.govThis fog forms at night under clear skies with calm winds when the day's heat radiates into space; with a moist layer near the ground, humidity reaches 100 percent and fog forms; ridgetop air cools, becomes dense and heavy, and drains to valley floors; it tends to dissipate very quickly once the sun comes up.
In the guide for September, October, November