292 Wisconsin Academy of Sciences, Arts and Letters. 
ture some 6° below the monthly mean, while south and south¬ 
east winds are 7° warmer than the mean. If, then, thru any 
cause, such as the passage of a cyclonic center nearby, the 
southerly wind of average winter temperature is, within a 
short time replaced by an average northwest wind the thermo¬ 
graph registers a fall of 13°. However, it is not necessary to 
look to the Red River Valley for air of low temperature. 
Directly above us is air of extreme coldness. At a height of 
two miles, or even less, during the winter we may find air 20° 
colder than at the earth’s surface. Any effort to force 
this air vertically downward, however, results in warming it 
so much by compression that at the ground it would be warmer 
than the air already there. The extreme dryness of this upper 
air, increasing when forced downward, is of importance in 
cooling the lower atmosphere, as in the case of the anti-cyclonic 
center. This dryness of the air is very favorable for cloudless 
skies, permitting rapid radiation of heat from the earth’s sur¬ 
face, and in the winter, when the days are short and the nights 
long, is often attended by extreme cold. The intense winter 
cold of the Red River Valley and of the Canadian provinces of 
Manitoba and Saskatchewan is produced in this manner. 
Cloudiness in winter is favorable for the retention of heat 
at or near the surface of the earth, Dr. Fassig has found 
that at Baltimore the average temperature of cloudy winter 
days is some 5° higher than that of clear days at that season. 
A cloudy day, preventing the sun’s rays from reaching the 
ground, followed by a cloudless night, permitting the escape of 
terrestrial radiation, would naturally be favorable for lowering 
the average lower air temperature in winter. 
A cyclonic storm, central over or to the northward of south¬ 
ern Wisconsin, unites the above mentioned conditions favorable 
for a rapid fall in temperature at Madison. As the storm center 
moves eastward, warm, cloudy winds from the south are soon re¬ 
placed by cold west to northwest winds with clearing weather. 
If the central pressure of the storm is sufficiently low, and if 
there is, as often occurs, a high pressure area accompanied by 
cold weather in the Dakotas, Montana or western Canada, these 
