Russia; thus the isotherm of 39° runs from St Petersburg to Orenburg, and that of 35° from Tornea in Finland to Uralsk.
And S.; thus Odessa and Konigsberg are situated on the same winter isotherm of 28°; St Petersburg, Orel and the mouth of the Ural river on about 20°; and Mezen and Ufa on 9°.
The summer isotherm of 68° F., which in Europe passes through Cracow and Kaluga, traverses Omsk, Krasnoyarsk and Irkutsk, whence it turns north to Yakutsk, and then south again to Vladivostok.
The isotherm of - 22° F., which touches the north point of Novaya Zemlya, passes in Siberia through Turukhansk (at the confluence of the Lena and the Lower Tunguzka) and descends as low as 55° N.
Both the midwinter isotherm of Montreal and the midsummer one of Washington, D.C., pass through the state.
As a result, the isotherms of July are strongly convex poleward as they cross the United States, the isotherm of 70 Sweeping up to the northern boundary in the north-west, and the heat equator leaping to the overheated deserts of the south-west, where the July mean is over 90.
The July isotherm of 80° crosses the state a little N.
Of the 80° isotherm the average temperature for July is between 80° and 85°.
The January isotherm of 15° F.
Again, the July isotherm of 75° crosses the middle section of the Caspian, nearly coinciding with the January isotherm of 25°, while that of 80° skirts the southern shore of the sea, nearly coinciding with the January curve of 40°, so that the mean annual range over the northern section of the sea is 60° and over the southern section 40°.
The same effect is well shown by the linguiform isotherms. In January, for example, the isotherm of 14°, after skirting the north coast of the Scandinavian peninsula, turns southward along the Keel, crossing the upper part of the district of the great northern lakes.
The south-eastern corner is crossed by an annual isotherm of 60°, the north-western by one of 50°; and although in the former region sometimes not a day in the year may show an average temperature below freezing-point, at Jefferson City there are occasionally two months of freezing weather, and at Rockport three.