dry cool air with high barometric pressure areas and are called 

 "highs." Where these air masses meet, a "front" is formed, and the 

 rapidity with which this front moves through an area depends on the 

 temperature and pressure gradient on either side of the front. 



An understanding of frontal systems, with their associated wind, 

 temperature and humidity, is one of the keys to understanding when 

 birds migrate. You must not only watch the fronts in your area but 

 the progress of nearby air masses as well because the birds migrating 

 through your area have started their journey to the north or south of 

 you depending on the season. The weather conditions at point of 

 departure will dictate if and when birds will be passing through your 

 area in the near future. 



During fall migration, the best passage of migrants usually occurs 

 2 days after a cold front has gone through. That is, the low has passed 

 and it is being followed by a high characterized by dropping 

 temperatures, a rising barometer, and clearing skies. The 24 hours 

 just after a low has passed are not always conducive to a good passage 

 of birds because winds are often too strong and turbulent in the 

 trough between the two air masses. Hochbaum (1955) correlated 

 mass movements of ducks through the prairies with weather systems 



Figure 12. A hypothetical weather system that could be ideal for mass migrations of 

 waterfowl in the fall. The strong southerly flow of air created by counter-clockwise 

 winds about the lows and the clockwise rotation of air about the highs, aids the rapid 

 movement of waterfowl from their breeding grounds in the Canadian prairies to 

 wintering areas in southern United States. 



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