134 Part II. Chapter 2. 
location of the center of extreme heat will be found generally on the northern 
and western outskirts of the area of high pressure. The longevity of the hot 
wave, which is so detrimental to vegetation, seems to depend upon a slack- 
ing up in the general circulation. Aside from the direct heating from the 
sun, dynamic heating and lack of radiation have an important influence in 
maintaining the temperature at an abnormal figure. Hot waves according to 
Professor BIGELOW come from two sources. They originate in a mass of 
heated air on the Rocky Mountain plains and gradually move eastward from 
thence; on the other hand, a high area settling over the South Atlantic and 
Gulf states has the effect of charging the stagnant air with heat, as if the 
eastward circulation in the upper air was suspended for quite long intervals 
of time). 
The cold wave is remarkably well developed in the winter storms of 
the central and eastern parts of North America. When supplied by an ara 
of strong high pressure in the northwest, it sweeps down from the cold plans 
of farther Canada and brings with it the low temperature of that bleak region. 
Its movement, obliquely towards the cyclonic center that it follows, is a 
celerated by the winter high pressure characteristic of its source in the con- 
tinental center and it is nowhere impeded by transverse mountain ranges. Near 
the track of the cyclone, the cold wave arrives suddenly and in almost füly- 
developed strength displacing the antecedent warm spell in a few hours nd 
causing an abrupt fall of temperature. This fall of temperature in the spring = 
is especially destructive to all kinds of vegetation. The deforestation of Michi- 
gan is said to have given more ready access to cold waves, hence the peach in 
crop has nearly disappeared from that state. E: 
SE 
The advent of spring in northern latitudes is dependent on the relative 
distribution of the cyclonic and anticyclonic areas. The advent of spring is 
due to the heat that causes the snows to disappear and external manifestations 
of life to appear in the plant. The temperature of 6°C., or 42.8°F., is that | 
at which the protoplasm of most plants becomes inactive. The advent of 
spring is considered as taking place at the advent of an isotherm one degree 2 
higher 43.8° F. (6.6° C.). The progress of the mean isotherm of 43° F. (6.1°C.) 
may therefore be considered to represent the average advent of spring. — 
Along the Gulf. of Mexico there is no advent of spring wich we find in the 
north, for on February ıst, the isotherm in question is found crossing the 
United States from the vicinity of Cape Hatteras on the east to the north of 
El Paso, then going westward and reaching the Pacific coast at some distance 
north of San Francisco. In the interval between February ıst and March ıst, 
however, it begins a rapid advance, and by April ıst, it passes over central : 
New Jersey nearly westward to the vicinity of Denver, here it makes a bend 
southward in crossing the mountains and passes abruptly northward to the 
ı) Burrows, ALvin T.: Hot Waves: Conditions which produce them, and their effect On 
Agriculture. Yearbook United States Department Agriculture 1900: 334. 
