The Harmon 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
Mostly sunny; warmer
Partly cloudy
Partly cloudy
Mostly cloudy; cooler
Partly cloudy
Chance of showers
75° on Nov. 13, 1999, the hottest reading for this week of the year since 1913. Next is 73° in 1953.
Harmon’s center stands about 1,710 ft above sea level. Within three miles the land runs from 1,630 ft to 1,920 ft, a difference of 290 ft: rolling country. The highest named summit within 15 miles is Crown Butte, about 2,300 ft, 7 miles west-southwest.
Under the town center the state geologic map shows Cannonball Formation (shale, sandstone), from the Paleocene, 56 to 66 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 32×. The colored band under the surface is the rock the state geologic map shows at the surface, not a picture of what lies beneath.
Claystone, siltstone. Age: Paleocene, 56 to 66 million years old. About 11 mi of the line.
Shale, sandstone. Age: Paleocene, 56 to 66 million years old. About 11 mi of the line.
Siltstone, claystone, sandstone. Age: Paleocene, 56 to 66 million years old. About 3 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 Harmon reaches the Gulf of America by way of Square Butte Creek, the Missouri and the Mississippi: about 2,490 river miles.
Lakes on the way: Lake Oahe, Lake Sharpe, Lake Francis Case and Lewis and Clark Lake.
1,170 miles of the trip are on the Mississippi.
Traced down the USGS NHDPlus V2.1 river network from the Census center point of Harmon; river miles along the main stem, rounded.
Harmon has a warm-summer humid continental climate. Summer afternoons average 83° in July; winter nights fall to about 3° in January. About 18 inches of rain fall in a normal year.
On November 14 Harmon normally reaches 40° and falls to 20°. The record high is 72°, in 2025; the record low −19°, in 1940. The most rain on this date, 1.29 in, fell in 1944.
Normals 1991–2020 and records since 1913 at Mandan Experiment Station, 10 miles south-southeast. NOAA, through RCC-ACIS.
