(Sources: Official Journal and Notice Paper No 33 of Saturday, March 18, 1876, page 6)
State of the atmosphere in January 1876
The month of January was characterized by a not very intense, but very persistent cold. Indeed, in Neuchâtel, with an average temperature of -2.08 degrees (C) which is not much lower than the normal temperature of this month, there were only five days, the 2, 3, 4, 22 and 23 where the temperature has risen above zero. The coldest day was January 7th, with an average temperature of -7.6 degrees and an absolute minimum of -8.5 degrees.
In Chaumont, on the other hand, the average temperature of the whole month was even lower than in Neuchâtel, a rare thing. In January we had the phenomenon of the reversal of the ordinary law of the diminution of temperature with height, which we witness every winter.
Indeed, during 12 days, it was considerably hotter in Chaumont than in Neuchâtel; on January 26 the difference in favor of Chaumont was 7.5 degrees, because we had in Neuchâtel that day -2.5 degrees and in Chaumont + 5.0 degrees; and the maximum of the day was at Chaumont + 9.0 degrees, while at Neuchâtel, at the hottest time we still had -0.6 degrees, which makes a difference of 10.6 degrees, and as normally Chaumont is about 4 degrees colder as Neuchâtel, the anomaly was therefore 14 to 15 degrees.
As always, this phenomenon was accompanied by a complete calm of the atmosphere, which is a condition of a considerable atmospheric pressure, and fog spread over the whole Swiss plain, at a height varying from 100m to 300m while the sun was shining on the heights. But the fog below and the sun on the mountain are not the causes, but rather the consequence of this abnormal distribution of temperature; for during two days it was warmer up, even with the overcast sky, and especially the high temperature persisted on the mountain also during the nights which, at that time, lasted fifteen hours. As a striking example, we will cite the case of the 28th of January, where in the evening at 9 o'clock the thermometer indicated + 1.2 degrees and in the morning at 7 o'clock the 29 + 1.3 degrees.
The opinion which attributes to the lake and its evaporation the long and sad fogs that bury us at this time, is equally erroneous; for the fog extended over the whole plain between the Alps and the Jura; and the phenomenon of the reversal of temperature was found along the whole northern slope of the Alps as far as Austria.
The barometric pressure was fairly constant throughout the month and considerably above average except for a few days. At the beginning of the month a little snow and rain fell; but the total amount of water dropped is only 14mm for Neuchâtel and 16.7mm for Chaumont.