48 The Ottawa Naturalist [June 



THE WEATHER. 



By Otto Klotz. 



The continued cool weather during the past spring together 

 with a heavy snow-fall on the 4th May, and some snow on 28th 

 of the same month here in Ottawa, has not passed without com- 

 ment by old and young, and by the Press. 



The question naturally is asked Why is there such a con- 

 tinuance of low temperature ? 



If the meteorologist is able to tell us what the weather is 

 going to be tomorrow, why not that of a month or several 

 months hence? Let us tr}^ and get a clear idea of how the daily 

 weather forecasts are made. Weather is essentially a matter of 

 the circulation of the atmosphere, and the circulation in turn is 

 due to difference of pressure, the movement of the air being to- 

 wards those parts of the earth's surface where the pressure is 

 least. The instrument used for measuring the pressure or 

 weighing the air is the barometer, and it is the principal one 

 used in obtaining data from which to make the forecasts. 

 Over the continent from the Yukon to the Gulf of Mexico are 

 distributed stations which daily report telegraphically at the 

 same time either to Toronto or to Washington the state of the 

 atmosphere, that is, the pressure, temperature, direction and 

 velocity of wind, so that the probability officer has the equiva- 

 lent of an instantaneous photograph of the weather conditions. 



Entering the data for the respective places on a map pre- 

 pared for the purpose, and joining places having the same pres- 

 sure, or more strictly speaking, the places for which the pres- 

 sures are given, serve for drawing the lines along which the 

 pressure is expressed in inches and tenths of an inch. Such a 

 line is called an isobar, and the interval between anv two iso- 

 bars represents a difference of a tenth of an inch. If the iso- 

 bars are crowded together, it is something like having a steep 

 roof; the water running down it faster than on a flatter roof, 

 similarly it is with the air, it is a matter of gradient. When 

 the daily map has thus been filled with isobars immediately on 

 receipt of the data, the officer sees at a glance how the great 

 atmospheric swirl is moving, moving of course from where the 

 pressure is great to where it is less, or technically speaking 

 from an area of "high" barometer to an area of "low" baro- 

 meter. The area of "high" barometer we may describe as one 

 where the air is piled up and flowing towards the valley of the 

 "low". Now when this "high" is pouring down, and our 



