The Coriolis Force (Meteorologist’s term for Angular Momentum)
certainly exists, but because the Earth's angular velocity is so small (360 degrees per day, or about 7 x 10-5 radians per second), the Coriolis effect isn't really significant over small distances.
The Coriolis effect is not the determining factor in which way your pan, sink, or toilet
drains. In a system that is as small, and rotating as slowly, as the water in these, the effect is inappreciable when compared with other factors such as the initial motion of the water, and the shape and orientation of the container. This is especially true in a sink or toilet, where there are jets of water shooting in.
Theoretically, you might detect the Coriolis effect by building a perfectly round, flat basin and making sure the water is initially perfectly still, and the drain is opened very carefully.
The Coriolis Force is calculated thus:
F = -2m
is the mass of the deflected object (divide the water in the basin into small volumes, and consider each an object)
is the angular velocity of the rotating object (for Earth, 360 deg./day or about 1E-5 radians/sec)
is the velocity of the deflected object
indicates a vector cross-product.
Hence, it takes a:
- large mass
- large angular velocity
- large object velocity
- an object velocity perpendicular to the angular velocity
for the deflection to take place, contributing to a large and noticeable deflection.
The water in a sink might make a rotation in a few seconds and so have a rotation rate ten thousand times higher than that of the Earth. It should not be surprising, therefore, to learn that the Coriolis force is orders of magnitude smaller than any of the forces involved (ie: gravity). The Coriolis force is so small, that it plays no role in determining the direction of rotation of a draining sink.
Coriolis, Wind, and Weather:
Atmospheric pressure differences (Hi’s & Low’s) tend to push winds in straight paths. Yet, sailors know that winds follow curved paths across the Earth. As air begins flowing from high to low pressure, the Earth rotates under it, making the wind follow a curved path.
In the Northern Hemisphere, the wind turns to the right of its direction of motion (counterclockwise, as seen from above) . In the Southern Hemisphere, it turns to the left (clockwise) around low pressure areas. The Coriolis force is zero at the equator.
On the scale of hurricanes and large mid-latitude storms, the Coriolis force causes the air to rotate around a low pressure center in a cyclonic direction. Indeed, the term cyclonic not only means that the fluid (air or water) rotates in the same direction as the underlying Earth, but also that the rotation of the fluid is due to the rotation of the Earth. Thus, the air flowing around a hurricane
spins counter-clockwise in the northern hemisphere, and clockwise in the southern hemisphere (as does the Earth, itself). In both hemispheres, this rotation is deemed cyclonic. If the Earth did not rotate, the air would flow directly in towards the low pressure center, but on a spinning Earth, the Coriolis force causes that air to be deviated with the result that it travels cyclonically around the low pressure center.
The “Global Circulation Wind” rises from the equator and moves north and south in the higher layers of the atmosphere. At around 30̊ latitude, in both hemispheres, the Coriolis force prevents the air from moving much farther. At these latitudes, there is a high pressure area, as the air begins sinking down again. As the wind rises from the equator, there will be a low pressure area close to ground level attracting winds from the North and South. At the Poles, there will be high pressure due to the cooling
of the air.
Keeping in mind the bending force of the Coriolis force, we thus have the following general results for the prevailing wind directions:
Worldwide Prevailing (Trade) Wind Directions
Latitude ~ Direction
90-60̊N ~ NE
60-30̊N ~ SW
30-0̊N ~ NE
0-30̊S ~ SE
30-60̊S ~ NW
60-90̊S ~ SE