AEROGRAPHER'S MATE 3 & 2 



at right angles across isobars (lines connecting 

 points of equal barometric pressure) from high 

 to low pressure. From observation we know 

 the wind actually blows parallel to isobars 

 above any frictional level. Therefore, other 

 factors must be affecting the windflow, and one 

 of these factors is the rotation of the earth. A 

 particle at rest on the earth's surface is in 

 equilibrium. If the particle starts to move 

 because of a pressure gradient force, its 

 relative motion is affected by the rotation of 

 the earth. If a mass of air from the Equator 

 moves northward, it is deflected to the right, 

 so that a south wind tends to become a south- 

 westerly wind. 



An air mass moving from the North Pole 

 tends to become a northeasterly wind. This 

 deflection is known as the Coriolis effect and 

 is stated as a law. (See fig. 12-10.) This law 

 states that when a mass of air starts to move 

 over the earth's surface, it is deflected to the 

 right of its path in the Northern Hemisphere 

 and to the left of its path in the Southern 

 Hemisphere. Coriolis effect is dependent upon 

 the latitude and speed of the moving air mass. 

 It is greatest at the poles and nonexistent at 

 the Equator. It increases as the speed of the 

 moving air mass increases. 



Centrifugal Effect 



According to Newton's first law of motion, 

 a body in motion continues in the same direction 

 in a straight line and with the same speed 

 unless acted upon by some external force. 

 Therefore, for a body to move in a curved 

 path, some force must be continually applied. 

 The force restraining bodies to move in a 

 curved path is called the centripetal force, and 

 it is always directed toward the center of 

 rotation. When a rock is whirled around on a 

 string, the centripetal force is afforded by the 

 tension of the string. 



Newton's third law states that for every 

 action there is an equal and opposite reaction. 

 Centrifugal force is the reacting force which 

 is equal to and opposite in direction to the 

 centripetal force. Centrifugal force, then, is a 

 force directed outward from the center of 

 rotation. 



As you know, a bucket of water can be swung 

 over your head at a rate such that the water 



Figure 12-10. — Coriolis effect. 



209.9 



does not come out. This is an example of both 

 centrifugal and centripetal force. The water is 

 being held in the bucket by centrifugal force 

 tending to pull it outward. The centripetal force, 

 the force holding the bucket and water to 

 the center, is your arm swinging the bucket. 

 As soon as you cease swinging the bucket, the 

 forces cease, and the water falls out of the 

 bucket. Figure 12-11 is a simplified illustration 

 of centripetal and centrifugal force. 



High- and low-pressure systems can be 

 compared to rotating discs. Centrifugal effect 

 tends to fling air out from the center of 

 rotation of these systems. Therefore, when winds 

 tend to blow in a circular path, centrifugal 

 effect (in addition to pressure gradient and 

 Coriolis effects) influences these winds. 



BERNOULLI'S THEOREM 



According to Bernoulli's theorem, pressures 

 are least where velocities are greatest, and 

 pressures are greatest where velocities are 

 least. This is true of liquids and gases. 

 (See fig. 12-12.) 



One of the practical uses of the theorem as 

 applied to meteorology is for forecasting winds 



278 



