Coriolis effect

 The Coriolis effect is the apparent deflection of a moving object when viewed from a rotating frame of reference.  An example is when two children on a merry-go-round try to throw a ball back and forth.  In flight, the ball appears to the children to follow a curved path.  Viewed externally, particularly from above, it is clear that the ball follows a straight path and it is the children who are on a curved path.  On the Earth, an object (such as an airplane) moving north from the equator curves eastward slightly (as viewed from the surface of the Earth).  Since we tend to locate everything using the surface of the Earth as a reference, we conclude that the airplane has curved to the right in its flight.  The same curved track is true for ships, but is inconsequential due to other, larger forces effecting the ship’s movement.  The mariner does feel the impact of the Coriolis effect in two other areas, though.  The movement of both air and water are subject to the Coriolis effect.  In the northern hemisphere, a wind from the south will gradually curve to the right (east).  Thus, air moving from an area of high pressure to an area of low pressure will curve away and eventually follow a line of equal pressure (isobar).  An illustration of this movement is the cyclone or hurricane, where the wind whirls around a low-pressure center in an endless loop.  Water currents behave in the same manner.  Large ocean currents in the northern hemisphere tend to adopt a clockwise movement.  In the southern hemisphere, the ocean currents tend to be counter-clockwise.  At the equator, the major currents flow toward the west, until diverted around large land masses.  The Coriolis effect is of no significance at small scales or low speeds.  Thus, the urban legend that water in a basin always flows into the drain in a clockwise manner is false.  But, knowledge of the Coriolis effect allows a mariner to make some rough estimates regarding the weather.  If you are facing into a steady wind, the high pressure system is on your left hand and the low pressure system is on your right hand.  Plot your course accordingly.