The Thornton 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.
Mostly sunny
Partly cloudy; warmer
Mostly sunny
Mostly sunny
Partly cloudy
Mostly sunny
Showers likely
−8° on Nov. 17, 1959, the coldest reading for this week of the year since 1942. Next is −5° in 1986.
Thornton’s center stands about 1,200 ft above sea level. Within three miles the land runs from 1,160 ft to 1,260 ft, a difference of 100 ft: gently rolling country.
Under the town center the state geologic map shows Famennian formations (shale, siltstone), from the Late Devonian, 359 to 374 million years old.
The section below crosses 3 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 9 mi of the line.
Shale, siltstone. Age: Late Devonian, 359 to 374 million years old. About 8 mi of the line.
Dolostone, limestone, siltstone. Age: Early Mississippian, 347 to 359 million years old. About 7 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 Thornton reaches the Gulf of America by way of Bailey Creek, the West Fork Cedar, the Cedar River, the Iowa and the Mississippi: about 1,690 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 Thornton; river miles along the main stem, rounded.
Thornton 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 Thornton 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, 15 miles north-northeast. NOAA, through RCC-ACIS.
