Monday, January 16, 2017

Buys Ballot Law to find wind direction from isobars

Reading wind speed and direction from the lay of the isobars on a weather map is a basic skill in marine weather. We need it because surface analysis maps only contain spot winds from observations and forecast maps only include winds greater than 34 kts.  On other parts of both maps we are left to deduce the corresponding wind from the isobars alone.

The ubiquitous use of GRIB formatted weather forecasts has dampened the motivation to learn this skill because looking at one of these forecasts you can turn on and off wind and isobars at will to see the correlation, and if that is all we used we would not need to know more. But that is poor policy to rely on these GRIBs alone; for most effective analysis and forecasting we need to look at the actual maps made by the NWS, and to read these we need this skill.  Even with GRIBs at hand, it is valuable to see if the correlation makes sense or not.

The procedures are discussed in Modern Marine Weather and we have several videos on the subject as well.

The most challenging part is usually figuring the speed of the wind, which takes either tables or a formula (Section 2.4 in our text), on top of reading latitudes and distances carefully from the map.  The wind direction should in principle be easier to determine, but we have found there are still some cases where the in-principle easy solution can be evasive.

Thus we take here an all new approach to resolving this that relies only on the Buys Ballot Law. This should work in all cases the same way.  Normally we started with the rule that wind flows (in the NH) clockwise around Highs and counterclockwise around Lows, and assumed that is all we need to figure the wind direction at any point on a map.  But when we do not know where the local Highs or Lows are located it could be distracting.

Here is the short depiction, followed by a (probably longer than needed) video showing it in action.


In the Southern Hemisphere, the wind circles the other direction so the hand are reversed... but don't even think about that now.





Here is then how you can follow up on choosing the wind direction more precisely.

(1) Plot the point you care about on the map.  

(2) Through that point sketch in a new isobar that is parallel to the isobars on either side.

(3) Draw a line through the same point that crosses your new isobar at an angle of about 20° pointing toward the lower pressure.

(4) That line is marking with the wind direction. (Put an arrowhead on the end of the line on the low pressure side.)








Wednesday, December 14, 2016

NPR lets slip poor reporting of the El Faro case started by the AP

In yesterday's news report, NPR carried on a policy that we see in many news broadcasts these days. Namely, one reporter from the agency quoting another reporter from the same agency as their source, which, in itself, is sad enough.

In this case, NPR reporter Rebecca Hersher presented this: 

"As NPR's Greg Allen has reported, Davidson was an experienced captain: When he left port on Tuesday in the El Faro, Joaquin was a tropical storm, not a significant concern. On early Thursday when he made the radio call to the company, Davidson said the ship was listing to one side, had lost its propulsion, was taking on water."

It sounds like she is relaying an evaluation of an NPR reporter, Greg Allen, about the situation, i.e., when they left port, Joaquin was not a concern. Or maybe he is putting words into the mouth of the deceased captain? In any event, this statement is tragically misleading and inappropriate. 

It is true that when they left port there was, in another part of the world, a storm that may not have been a concern—had they been in that part of the world, at that time. However, he was headed in that direction, and the forecast at that time, was that they would meet a hurricane when they got there.  And they did indeed meet the storm more or less right at the spot and right at the time it was forecasted. These forecasts are all archived at the NHC and this can all be checked.  

In short, when this vessel pulled into the ocean, it could be anticipated that they would meet a hurricane, and this is indeed a concern. I posted a short article on this the day the vessel was lost,  which focused on the same misrepresentation of the facts, in this case, by the AP. That analysis remains tragically valid.

NPR (and anyone else parroting this same analysis) is doing a disservice to the NWS and to all mariners by this type of reporting. We teach daily the obvious fact that a storm's condition when you are not there does not matter. What matters is the condition of a storm when you are where it is. We must rely on forecasts.

As I understand, it has not been established why the vessel proceeded on the path it did, but the NPR report flies in the face of all prudent navigation practice. They are presenting, in essence, a navigation and weather routing lesson to their readers that is totally wrong and can be disastrous.


This morning I hear that NPR is proud they do not present fake news—a low bar at best—but the Greg Allen assessment presented is borderline, in that it is a thoughtless statement that distracts from a productive understanding of the tragic event.

I must add, NPR is indeed one of the best news sources in the world. This is a rare transgression, which I mainly point out because I care about them and respect their work.