5 GO Principles of Weather Forecasting. 



In drinking and in sucking animals take advantage of 

 the air pressure to perform these operations, which would 

 be impossible without the pressure, and difficult where the 

 pressure is small. 



Even in eating, animals with lips and cheeks take ad- 

 vantage of air pressure to force the food from between the 

 teeth after it has been masticated, and a man would make 

 awkward work eating for the first time in a vacuum. 



In the common suction pump and the siphon air pres- 

 sure is an essential factor, as it is in the low pressure steam 

 engine. 



All of the machines invented for milking cows develop 

 a vacuum and depend upon atmospheric pressure to force 

 the milk from the udder. 



713. Temperature of the Atmosphere. The air is warmed 

 in three ways : first, and chiefly, by contact with the earth's 

 surface and with solid objects upon it, this heating giving 

 rise to ascending currents of warm and descending ones of 

 cold air. Second, by dark heat radiations outward, which 

 are absorbed by the atmosphere as water absorbs light. 

 Third, by absorption of the direct rays from the sun on 

 their way to the earth's surface. 



When air descends from a higher to a lower level the 

 pressure upon it becomes greater and its volume is reduced. 

 This reduction of volume causes it to have a higher tem- 

 perature, and so if the air rises it expands, and this expan- 

 sion results in lowering tin temperature. A rise or fall 

 of 100 feet causes a change of temperature of .55 F. in 

 dry air. 



If dry air crosses a mountain range and falls 2,000 feet 

 its temperature is raised 11 F. 



When moisture is condensed or frozen in the atmos- 

 phere the air temperature is raised by the heat generated 

 during condensation. So, too, if water is evaporated in 

 the air, or snow melts, the temperature falls. This is why 

 the weather is always warmer in winter when it snows, and 

 cooler after showers. 



