Archive for the ‘Divergence aloft’ Tag

Hurricane Kyle Is Heading Northward Toward Maine and Nova Scotia

Source = National Hurricane Center

Source = National Hurricane Center

Left click to enlarge this image.

At 8 PM EDT this evening Kyle still was maintaining hurricane strength in spite of high wind shear aloft.  Generally, a 15 mph wind shear is about the break-off point for being slow enough to allow a hurricane to hold its strength or intensify.  It has been greater than that today and is expected to get up to 25 mph tomorrow.  However, the winds over the storm are diverging as two cars going down the highway together would diverge a bit if one of them were to move to a lane further from the other car.  So, the air over the storm is moving in the same general direction but spreading a bit.  When air aloft converges it tends to sink and the opposite happens when air aloft diverges; there tends to be an increase in the amount of air rising from below.  This could allow Kyle to maintain hurricane strength tomorrow in spite of the shear.  It’s a fine balance and there is some disagreement as to whether it will still be a hurricane tomorrow since at 8 PM the maximum sustained winds were 75 mph and 73 mph would demote it a tropical storm.

It will be interesting to see what happens.  Of course, for the sake of the landfall regions and the ships and boats at sea in that area, I hope it weakens quickly.



Sea surface temperatures can be important variables regarding the likelihood for strengthening or weakening of a tropical system.  Conditions aloft are generally more important.  A tendency toward divergence aloft is likely to increase the strength of a hurricane and a tendency toward convergence aloft is likely to decrease the strength of a hurricane.  Gustav appears to be one of those typical noteworthy storms being influenced significantly by the warm temperature (and higher evaporation rates) at the surface.  The chart below shows heat potential for yesterday (Saturday, August 30).  I added the labeling and I drew the current location (which is nothing more than a guess on my part).

Hurricanes are very unstable.  Vertical motion is imperative to the maintenance of the system.  Though tap water in a sauce pan is stable when you first put in on the stove top, heating it from below tends to make it unstable.  The extreme case of boiling (by heating from below) provides us an example of violent instability.  Clouds are the markers in our otherwise transparent air (I’m ignoring visible pollution here) and the cumuliform varieties show us some of the vertical action occurring – akin to a slow motion rendition of the water boiling in the saucepan.

Conversely, cooling the air (or cooling the water in the saucepan) from below tends toward stability.  No one, wanting to get water up to a boil would put the saucepan on top of a block of ice.

Let us all hope that when Gustav moves over the relatively cooler water that is becomes “less unstable” and weakens significantly.  THE PROBLEM FOR ME IS THIS:  The color coding in such charts is misleading for some because the blues leave one with the impression that the water is cold.  It is not “cold.”  The second chart in this posting gives you a clearer impression of the actual temperatures yesterday.