Chapter 14 — AIR MASSES AND FRONTS 



As an air mass expands and slowly moves 

 out of its source region, it travels along a 

 certain path. The surface over which this path 

 takes the air mass after leaving its source 

 modifies the air mass. The type of trajectory, 

 whether cyclonic or anticyclonic, also has a 

 bearing on its modification. The time the air 

 mass has been out of its source region will 

 determine to a great extent the characteristics 

 of the air mass when a thermodynamic classi- 

 fication is attempted. 



Surface Conditions 



The first modifying factor on an air mass 

 as it leaves its source region is the type and 

 condition of the surface over which it travels. 

 Here, the factors of surface temperature, 

 moisture, and topography must be considered. 



TEMPERATURE. — The temperature of the 

 surface relative to that of the air mass will 

 modify not only the temperature, but its 

 stability as well. For example, if the air mass 

 is warm and moves over a colder surface, 

 such as tropical air moving over colder water, 

 the cold surface cools the lower layers of the 

 air mass and its stability is increased. This 

 stability will extend to the upper layers in 

 time, and condensation in the form of fog or 

 low stratus normally occurs. 



If the air mass moves over a surface that 

 is warmer, such as polar continental air moving 

 out from the continent in winter over warmer 

 water, the warm water heats the lower layers 

 of the air mass, increasing instability and 

 consequently spreading to higher layers. 



The changes in stability of the air mass 

 give valuable indications of the cloud types 

 that will form, as well as the type of precipi- 

 tation. Also, the increase or decrease in 

 stability gives further indications of the lower 

 layer turbulence and visibility. 



MOISTURE. — The air mass may be modified 

 in its moisture content by the addition of 

 moisture by evaporation or by the removal of 

 moisture by condensation and precipitation. If 

 the air mass is moving over continental regions, 

 the existence of unfrozen bodies of water can 

 greatly modify the air mass and, in the case 

 of an air mass moving from a continent to an 

 ocean, the modification can be considerable. In 



general, dependent upon the temperature of the 

 two surfaces, the movement over a water surface 

 will increase the moisture content of the lower 

 layers, and the relative temperature of the 

 surface. 



For example, the passage of cold air over 

 a warm water surface will decrease the stability 

 of the air with resultant vertical currents. The 

 passage of warm moist air over a cold surface 

 increases the stability and could result in fog 

 as the air is cooled. 



TOPOGRAPHY. — The effect of topography 

 is evident primarily in the regions of mountains. 

 The air mass is modified on the windward side 

 by the removal of moisture through precipitation 

 with a decrease in stability; and as the air 

 descends on the other side of the mountain, the 

 stability increases as the air becomes warmer 

 and drier. 



Trajectory 



After an air mass has left its source region, 

 the trajectory it follows, whether cyclonic or 

 anticyclonic, has a great effect on its stability. 

 If the air follows a cyclonic trajectory, its 

 stability in the upper levels is decreased; this 

 instability is a reflection of the cyclonic relative 

 vorticity. The stability of the lower layers is 

 not greatly affected by this process. On the 

 other hand, if the trajectory is anticyclonic, 

 its stability in the upper levels is increased 

 as a result of subsidence associated with anti- 

 cyclonic relative vorticity. 



Age 



Although the age of an air mass in itself 

 cannot modify the air mass, it will determine, 

 to a great extent, the amount of modification 

 that takes place. For example, an air mass 

 that has recently moved from its source region 

 will not have had time to become modified 

 significantly. However, an air mass which has 

 moved into a new region and stagnated for some 

 time, and is now old, will be found to have 

 lost many of its original characteristics. 



Summary 



In table 14-1 the two modifying influences 

 are classified thermal and mechanical. The table 



315 



