224 ROCHESTER ACADEMY OF SCIENCE, [April I4, 



-12° for Rochester ; and St. Vincent, which is, however, somewhat 

 further north, with a maximum of 103° and a minimum of -54°. The 

 Atlantic ocean is also near enough to occasionally modify to some 

 extent both winter and summer temperatures. 



[ J^an?/ Waves. 



As a rule warm spells are of home manufacture, meaning by this 

 that they are generated over the interior of this country and not 

 imported from beyond its borders. The conditions for their produc- 

 tion are a quiescent state of the lower air, one in which there is no 

 movement to mix the lower with the upper masses. They appear on 

 the western face of a high pressure where the tendency of the force of 

 the prevailing pressure is to bring the lower winds up from the South, 

 and are usually located in the eastern half of a large and nearly force- 

 less low whose center is somewhere over the West and mo\'ing 

 slowly. Given these conditions and the production of warmth is 

 certain at some place in the debatable land between the high and the 

 low. One such warm spell at Rochester was the direct result of a 

 cold wave that came in over the Northeast and held the lower air 

 almost absolutely quiescent under its southwest quadrant, much as if 

 the air were anchored in position. The heat of the sun was sufficient 

 to heat up a thin layer near the ground to a high temperature 

 in the course of about three days. Usually warm waves originate 

 west of the Mississippi in the country where the rays of the sun 

 reach the earth without being interfered with by cloudiness and 

 gradually drift eastward frequendy gaining while journeying. 



Cold Waves. 

 With cold waves the facts are entirely different. No cold wave 

 ever originates over this section. They are all importations. At 

 long intervals cold appears to be originated over the Western plains, 

 generally in Wyoming on the eastern slope of the Rocky mountains, 

 but these spells are always of minor importance and seldom reach the 

 dignity of what would be called real cold. Cold waves of importance 

 appear to all originate in that portion of the continent north of the 

 United States and east of the northern range of mountains. Their 

 generation is conceived to proceed from the slow accumulation of a body 

 of dry air over the region mentioned wherein the temperature is 

 lowered principally by radiation from the surface of the earth. That 

 portion of the country is covered with ice and snow during the winter 

 and radiation proceeds uninterruptedly. The air is dry because its 

 moisture is removed in passing over the mountains to the westward by 



