# In meteorology, what is Ferrel's Law?

Ferrel’s Law is the rule that states that water which moves horizontally in the Northern Hemisphere is pushed to the right of its line of motion when air or water traveling horizontally in the Southern Hemisphere is deflected left of its line of motion.  The law was named after the American meteorologist William Ferrel (1817-1891).  His law is used to predict the “large-scale circulations of the Earth’s atmosphere and oceans.  Ferrel’s Law is the Coriolis force on a global scale.

To help visualize and understand how Ferrel’s Law operates, imagine an ant crawling across a compact disc. Let  “P” represent the ant’s angular momentum and its mass be represented by “m.”  Then multiply the square of the ant’s distance as it travels from the center of the disc  be “r2” by how vast the disc spins P = mr2

Imagine now that the ant is traveling in a straight radial line to the middle of the disc and that its radial velocity remains constant as the disc spins and that its mass remains constant as well, but the distance away from the center, r, decreases.  If the formula P = mr2  is applied, the angular momentum  increases.  However, a force is necessary to alter the angular momentum of any object.

So, if the ant walks directly to the middle of the disc, the ant has to feel some force that decelerates the spin rate. From the ant’s point-of-view, it is walking in a straight line and a “sideways push” seems to require it to balance itself in the same direction of the rotating disc.  This push of increasing force only appears to apply when the ant moves closer or further from the disc’s rotating axis is the Coriolis force.

However, if the disc is slippery to allow the walking ant to resist this force, the ant will be deflected in the same direction in which the disc is spinning. Consider the ant analogy to that of the Earth’s spin in orbit.  The Earth rotates eastward.  Air which moves toward either the North or South Pole typically retains its angular momentum as it travels eastward, as Ferrel’s law describes.

Deflected westward are movements away from the axis of rotation in both the northern and southern hemispheres; to the right of the line of motion in the northern hemisphere and to the left in the southern hemisphere. When seen from space, high pressure systems typically spin clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere.  The reverse is usually true of low pressure systems.

Finally, it is a myth that Ferrel’s Law can be observed in small basins like toilets, as the Coriolis fore is far too weak in such environments to be applicable