The causes air to flow from areas of relatively high air pressure to areas of relatively low air pressure.

The answer that fits the blank would be the term PRESSURE GRADIENT. The pressure gradient measures which direction and the changes of the rate of the pressure, therefore causing the air to flow from places of relatively increased pressure to places of relatively low air pressure.

Migration people get up and go cause they feel it is best for them 

Answeris associated with sunny,dry weather;