Buys Ballot’s Law

Buys Ballot’s Law 

In the Northern Hemisphere, if a person stands with his back to the wind, the low pressure area will be on his left. This is because wind travels counterclockwise around low pressure zones in the Northern Hemisphere. It is approximately true in the higher latitudes of the Northern Hemisphere, and is reversed in the Southern Hemisphere, but the angle between the pressure gradient force and wind is not a right angle in low latitudes. 

Typical questions related to this law are like: 

An aircraft is flying at FL 180 in the northern hemisphere with a crosswind from the left. Which of the following is correct concerning its true altitude? 

a) increases 
d) decreases 

the correct answer is “decreases”