The Duncan plate. Relief from Copernicus GLO-30; rivers and lakes from USGS NHD; watersheds from the USGS Watershed Boundary Dataset; roads and city limits from Census TIGER 2024.
Sunny
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Partly cloudy
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Showers likely
−8° on Nov. 17, 1959, the coldest reading for this week of the year since 1942. Next is −5° in 1986.
Duncan’s center stands about 1,240 ft above sea level. Within three miles the land runs from 1,200 ft to 1,280 ft, a difference of 80 ft: gently rolling country. The highest named summit within 15 miles is Pilot Knob, about 1,440 ft, 13 miles northeast.
Under the town center the state geologic map shows Lime Creek Formation (shale, limestone, dolostone), from the Late Devonian, 374 to 384 million years old.
The section below crosses 5 rock units mapped at the surface.
Heights every 0.1 mile from the Copernicus GLO-90 surface model; vertical exaggeration about 64×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Shale, limestone, dolostone. Age: Late Devonian, 374 to 384 million years old. About 17 mi of the line.
Sandstone, shale. Age: Early Cretaceous to Late Cretaceous, 94 to 113 million years old. About 2 mi of the line.
Limestone, dolostone. Age: Middle Devonian to Late Devonian, 374 to 388 million years old. About 2 mi of the line.
Shale, siltstone. Age: Late Devonian, 359 to 374 million years old. About 1 mi of the line.
Dolostone, limestone, siltstone. Age: Early Mississippian, 347 to 359 million years old. About 1 mi of the line.
From the USGS State Geologic Map Compilation (Horton and others, 2017), the states’ geologic maps joined at a common scale.
Heights from Copernicus GLO-90 (a surface model: it includes buildings and tree canopy); summits from USGS GNIS; rivers from USGS NHDPlus HR; rock from the USGS State Geologic Map Compilation.
A raindrop that lands in Duncan reaches the Gulf of America by way of the West Branch Iowa, the Iowa and the Mississippi: about 1,750 river miles.
1,410 miles of the trip are on the Mississippi.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Duncan; river miles along the main stem, rounded.
Duncan has a hot-summer humid continental climate. Summer afternoons average 83° in July; winter nights fall to about 7° in January. A normal year brings about 44 inches of snow; with the rain, that comes to about 36 inches of water.
On November 14 Duncan normally reaches 44° and falls to 25°. The record high is 71°, in 2025; the record low −7°, in 1959. The most rain on this date, 0.30 in, fell in 1947.
Normals 1991–2020 and records since 1942 at Mason City Municipal Airport, 20 miles east. NOAA, through RCC-ACIS.
