". . . the fog rose up thick from the hollars"
"If fog clears from the top down,
expect some rain.
If fog clears from the bottom up,
the weather will be fair."
Fog at sunrise imposes a new reality on the landscape, slowly refreshing a familiar view, detail by detail, as the temperatures equalize above and below the bank, and the white veil dissolves into sunshine. This photo provides a hint of John Ordway's view from K'useyneiskit near the east end of the ridge on that late-June morning in 1806. The "hollar" on the south side of the ridge was the Papoose Creek drainage; to the north were the sources of Cayuse Creek and Howard Creek.
In this photo, it is the Clearwater Canyon that is abrim with fog. Nearly two thousand feet beneath the bank, the Middle Fork of the Clearwater River flows swiftly northwest (upper right) toward the Snake River. To the southeast (upper left) are the grain fields of Nez Perce and Camas prairies.
The day after Meriwether Lewis left Pittsburgh on August 31, 1803, he observed the typically seasonal morning fog in the Ohio River valley, and bent his methodical mind to the challenge of explaining it. The fog, he reasoned,
appears to owe it's orrigin to the difference of temperature between the air and water the latter at this seson being much warmer than the former; the water being heated by the summer's sun dose not undergo so rapid a change from the absence of the sun as the air dose consequently when the air becomes most cool which is about sunrise the fogg is thickest and appears to rise from the face of the water like the steem from boiling water.1
Lewis confirmed his analysis with further observations. On the morning of September 4 he reported:
Morning foggy, obliged to wait. Thermometer at 63°— temperature of the river-water 73° being a difference of ten degrees, but yesterday there was a difference of twelve degrees, so that the water must have changed it's temperature 2d[egrees] in twenty four hours, coalder; at 1/4 past 8 the murcury rose in the open air to 68° the fogg dispeared and we set out; the difference therefore of 5° in temperature between the warter and air is not sufficient to produce the appearance of fogg—
On the sixteenth his progress was again delayed by thick fog until eight o'clock. At sunrise the temperature of the air was 54°, of the water, 72°.
If the sight from "Thirteen Mile Camp" on June 29, 1806, prompted Lewis to study this atmospheric phenomenon again, he left us no evidence of it. At least it didn't impede their progress this morning, as it did so often on the rivers they navigated.
1. Lewis may have consulted Owen's Dictionary of Arts and Sciences (1764), a four-volume encyclopedia that he had brought along, from which he could have gained the basic definition for the time:
"If the vapours, which are raised plentifully from the earth and waters, either by the solar or subterraneous heat, do, at their first entrance into the atmosphere, meet with cold enough to condense them to a considerable degree, their specific gravity is, by that means, encreased; and so they will be stopped from ascending, and return back, either in form of dew, or drizzling rain; or remain suspended some time in the form of a fog. Vapours may be seen on the high grounds as well as the low, but more especially about marshy places: they are easily dissipated by the wind, as also by the heat of the sun: they continue longest in the lowest grounds, because these places contain most moisture, and are least exposed to the action of the wind."
The understanding of the earth's atmosphere and the development of the science of meteorology increased steadily during the early 19th century. In 1803, for example, the British meteorologist Luke Howard introduced the names of the basic cloud formations—cirrus, cumulus, stratus, nimbus, and their combinations. Lewis probably was unaware of Howard's contribution, for he never used any of those terms in reporting the weather.
Funded in part by a grant from the Idaho Governor's Lewis and Clark Trail Committee