Reply to ASK-AN-EARTH-SCIENTIST

If Coriolis force drives weather systems in one direction in the northern hemisphere and another in the southern does this mean that hurricanes and other weather systems cannot cross the Equator?

Worried about hurricanes crossing the equator ? The Coriolis force is what we call an apparent force - it handles the problem that the earth is spinning and thus a point on the globe is always changing direction or always accelerating. You may also view Coriolis as a consequence of the conservation of angular momentum. At the equator you have a great deal as you are rotating about 25,000 miles in a day while someone on the poles is not moving anywhere in the same period. From your and my perspective air, rockets, and aircraft, are all deflected to the right in the northern hemisphere and to the left in the southern. (Yes, they correct for Coriolis when they shoot artillery shells!) The Coriolis "force" is a maximum at the poles and zero at the equator.

Observations show that no hurricanes form within 5 degrees latitude of the equator. People argue that the Coriolis force is too weak there to get air to rotate around a low pressure rather than flow from high to low pressure, which it does initially. If you can't get the air to rotate you can't get a storm. This is a reason why genesis does not occur at low latitudes but it does not explain why a developed hurricane does not cross the equator..

Could a hurricane cross ? Yes, because a well developed storm has plenty of spin that would dominate the weak Coriolis force near there. If it crossed the Coriolis force would be working against the initial direction of the spin, but it would be dominated by what we call the relative vorticity of the storm. Have we seen this happen ? Hurricanes can move south and get close to the equator but I cannot find an example of one crossing in the Atlantic or eastern Pacific. In the Indian Ocean some come closer to pulling off this trick. Why don't they cross ? The variation in Coriolis with latitude - called the Beta effect - actually will move a hurricane to the NW in the northern hemisphere even if there is no large scale wind pushing the storm along ! So, Coriolis not onlyseems to be a necessary ingredient to make a storm, but it may also pull them away from the equator making the crossing event a tough one to pull off.

Gary Barnes, Professor of Meteorology

Department of Meteorology

University of Hawaii, Honolulu HI 96822