96 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



Drawing highly irregular is()l)ars, in order to satisfy slight dif- 

 ferences in pressure distrilnition, is a connnon fault of the begiiuier. 

 On the other hand, after considerable experience, he is inclined to 

 round out or smooth out the isobars too much in an effort to get sym- 

 metrical lines. When irregularities actually exist in the pressure 

 distribution, they are attended by corres})onding irregularities or 

 discontinuities in Avind direction and in the distribution of other 

 elements — a fact which should be kept in mind. 



It is desirable that observations to be entered on any single map be 

 made as nearly as possible at the same moment of time and on suc- 

 cessive maps, at uniform intervals. It is then possible, by compar- 

 ing maps, to learn what changes have taken place in weather condi- 

 tions over a given area within known periods of time and to draw 

 therefrom inferences as to the character of subsequent changes and 

 the time when tliey will occur. 



USE OF THE WEATHER MAP 



The subjects of map analysis and weather forecasting, and their 

 use as an aid to navigation, do not come within the scope of 

 this publication, which is confined in the main to weather observa- 

 tions. However, the Weathei- Bureau is desirous of assisting the 

 mariner in analyzing the comj^leted weather map and in drawing- 

 inferences as to the weather of the future. Meteorologists at port 

 stations of the Bureau will be glad to confer with ships' officers 

 regarding any features of the work. Publications on this subject 

 that are available for distribution will l)e su])[)lied to cooperating 

 officers on recpiest. 



PART VIII. GLOSSARY 



A. TECHNICAL TERMS 



Absolute tcnipcrature. — The temperature of the centigrade tliermometer, 

 increased by 273°, more properly called the temperature on the absolute or 

 thermodynamic scale. On the Fahrenheit scale the absolute zero is approxi- 

 mately 450° below the Fahrenheit zero. 



Actiiwmctci: — An instrument for measuring the intensity of radiation re- 

 ceived from the sun. 



Adiabatic. — The word applied in the science of thermodynamics to a process 

 during which no heat is communicated to or withdrawn from the body or 

 system concerned. Adiabatic changes of atmospheric temperature are those 

 that occur only in consequence of compression or expansion accompanying 

 an increase or a decrease of atmospheric pressure. Such changes are also 

 described as dynamic heating and cooling. 



Advectioii. — The process of transfer liy horizontal motion, particularly ap- 

 plied to the transfer of heat by horizontal motion of the air. The transfer of 

 heat from low to high latitudes is the most obvious example of advection. 



Adrection fog. — Fog resulting from the transfer of warm, humid air over 

 a cold surface, especially a cold ocean surface, or (comparatively rarely) from 

 the transport of air that is relatively very cold over an ocean surface that is 

 relatively very warm. 



Aerolof/ii. — The study of the free atmosphere throughout its vertical extent, 

 as distinguished frtmi investigations confined to the layer of the atmosphere 

 juljacent to the earth's surface. Aerological investigations are made directly 

 with pilot balloons, sounding l)alloons and by means of airplanes. They are 

 also made indirectly by visual observations from the ground. Included in 

 the latter are observations of clouds, meteor trails, the aurora, etc. 



Afterfflow. — The glow in the western sky after sunset. 



I 



