OPC maps for Google Earth

The Ocean Prediction Center now offers links that will put some of their maps (including Gulf Stream and Unified Analysis) directly onto Google Earth. Once you have done this, they will update automatically next time you open Google Earth.  Hard to think of a more convenient way to get a quick picture of what is going on weather wise on a large scale.

Get the products and instructions at this OPC webpage. Then just save the files anywhere and drag them onto Google Earth. Then save your My Places when you close GE (it should ask you to save), and they will be there and update automatically for you. Very slick.

Here are a couple samples:

We have also made ourselves similar links for getting the ASCAT winds onto GE. This is a project I would like to expand, but will need some help. I call it the ASCAT Genome Project. We want to get all ASCAT and OSCAT winds worldwide linked to GE. It will be a great service, and once I figure a way to coordinate the help of others we will post a note here to join us if you like.

Track of HMS Bounty and the NHC Forecasts

In Modern Marine Weather we point out that if you want to sail in a hurricane you can. We know where they take place and when they take place—and when they do occur, their location and projected tracks are remarkably well forecasted. To sail in one, just look up this information and go there.

On the other hand, if you do not want to sail in a hurricane, look up the same information and then do not go there.

Seems simple enough, but this nutshell summary of avoiding hurricanes is obviously oversimplified. Huge areas of the ocean and long seasons would be blocked out by such a simple guideline. Fortunately, the National Hurricane Center (NHC), in collaboration with experienced mariners over the years, has come up with more realistic guidelines for safe navigation in the presence of tropical systems. It comes in the form of two simple rules: The 34-kt Rule and The Mariner’s 1-2-3 Rule.

The 1-2-3 Rule is simple and easy to apply based on text or voice reports of the storm locations, which are given several times a day in the high-seas broadcasts. Namely, the danger area to be avoided expands the forecasted danger zone by 100 nmi per day, as shown in Figure 1.

The danger zone for this application is characterized by the NHC as the radius about the storm center that includes winds greater than 34 kt.  Thus we start with what the NHC calls The 34-kt Rule: “For vessels at sea, avoiding the 34-kt wind field of a hurricane is paramount. Thirty-four knots is chosen as the critical value because as wind speed increases to this speed, sea state development approaches critical levels resulting in rapidly decreasing limits to ship maneuverability.” They add the natural precaution that sea state outside of the 34-kt radius can also be significant enough to limit course and speed options, so we should monitor this carefully.

Figure 1. The 34-kt Rule and The Mariner’s 1-2-3 Rule

We can use the forecast of Sandy on Thursday, Oct 25 (below) as an example of a long-term forecast for a storm headed north:

NWS NATIONAL HURRICANE CENTER MIAMI FL      
0300 UTC THU OCT 25 2012

[Day 0 - Thursday = 00h]
HURRICANE CENTER LOCATED NEAR 19.4N  76.3W AT 25/0300Z.  POSITION ACCURATE WITHIN  20 NM
PRESENT MOVEMENT TOWARD THE NORTH OR  10 DEGREES AT  11 KT. ESTIMATED MINIMUM CENTRAL PRESSURE  954 MB. EYE DIAMETER  20 NM. MAX SUSTAINED WINDS  80 KT WITH GUSTS TO 100 KT.

64 KT....... 25NE  20SE  20SW  20NW.
50 KT....... 50NE  60SE  40SW  40NW.
34 KT.......110NE 120SE  70SW  60NW.
12 FT SEAS..120NE 300SE 120SW 120NW.

WINDS AND SEAS VARY GREATLY IN EACH QUADRANT.  RADII IN NAUTICAL MILES ARE THE LARGEST RADII EXPECTED ANYWHERE IN THAT QUADRANT.

[Day 1 Friday = 24h]
FORECAST VALID 26/0000Z 24.4N  76.2W
MAX WIND  70 KT...GUSTS  85 KT.
64 KT... 20NE  20SE   0SW   0NW.
50 KT... 70NE  70SE  40SW  50NW.
34 KT...150NE 120SE  70SW  90NW.

[Day 2 Saturday = 48h]
FORECAST VALID 27/0000Z 27.6N  77.2W
MAX WIND  65 KT...GUSTS  80 KT.
50 KT...120NE 100SE  90SW 120NW.
34 KT...250NE 160SE 100SW 230NW.

[Day 3 Sunday = 72h]
FORECAST VALID 28/0000Z 30.5N  74.5W
MAX WIND  60 KT...GUSTS  75 KT.
50 KT...120NE 120SE 120SW 100NW.
34 KT...300NE 270SE 180SW 300NW.

EXTENDED OUTLOOK. NOTE...ERRORS FOR TRACK HAVE AVERAGED NEAR 175 NM ON DAY 4 AND 225 NM ON DAY 5...AND FOR INTENSITY NEAR 20 KT EACH DAY.

[Day 4 Monday = 4-day]
OUTLOOK VALID 29/0000Z 33.5N  71.5W
MAX WIND  60 KT...GUSTS  75 KT.

[Day 5 Tuesday = 5-day]
OUTLOOK VALID 30/0000Z 37.0N  70.0W...POST-TROPICAL
MAX WIND  60 KT...GUSTS  75 KT.


Plot the location of the storm at forecast time, and then plot on your chart the forecasted locations on Day 1, 2, and 3. Then check the forecast for the maximum radius of 34-kt winds. They are given for each quadrant on each day’s forecast.  On Day 1 this was 150 nmi, which occurred in the NE quadrant. With a drawing compass draw a circle around each of the 3 locations with 34-kt radii (in this example) of 150, 250, and 300 nmi. You then have a plot of forecasted storm sizes on these 3 days, which is shown in Figure 2.

These are not the safety zones of the 1-2-3 Rule, these are the actual forecasted locations and sizes of the areas that should be avoided. Next we apply the Mariner’s 1-2-3 Rule to account for uncertainties in forecast accuracy.

To each of these 3 radii, we then add 100, 200, and 300 nmi to account for historic uncertainty in the forecasted track. This guideline is based on the NHC’s records over the past 10 years of past errors in forecast track location. The track locations are actually more precise than this in recent years, and indeed getting better continually, but the uncertainty in intensity and size of the forecasted systems remains more of a challenge and hence the larger safety zones.

Furthermore, this guideline as presented is for tropical systems that are fueled from the warm water below them. Once a storm moves out of the tropics and becomes extratropical, it begins to gain energy from a much broader source—the temperature difference between cool northern air and warmer southern air masses. When this happens the system can quickly become much larger and more intense. This is precisely what happened with the transition between Hurricane Sandy and “Superstorm Sandy.” It got much larger, with a much broader band of strong winds and high seas.

This one became known in the media as a “superstorm” (not a defined meteorological term) because that transition took place at a time that was uniquely favorable to enhanced extratropical development. Extratropical storms on the surface are strongly influenced by the wind patterns in the upper atmosphere, and this one just happened to move north at the worst possible time—not only for enhancement, but for a forced turn to the west, rather than the more normal route to the east. This, however, was not a surprise. This storm was indeed a testimony to the skill of modern numerical weather prediction, which had accounted for these effects.  They were included in the forecasts as shown by the examples here.

In short, we should not look at The 34-kt Rule and The Mariner’s 1-2-3 Rule as an over-conservative guideline. It was spot on in the case of Sandy, with tragic consequences for the Bounty.

Figure 2. Plot of the 3-day and extended forecasts for Hurricane Sandy issued on Thursday, Oct 25, 2012, valid at 00z. Also shown is the track of the HMS Bounty which departed for Florida on that date. The shaded areas encompass the 34-kt wind regions for the next three days, with the radial lines marked that showed the largest radius in each case. The associated colored dashed circles mark the limits of the Mariners 1-2-3 Rule. The two top storm locations mark the extended forecasts, with the dark circles marking the quoted uncertainties in track location. These two dark circles do not imply any wind or storm size predictions. The 5-day forecast of the ECMRF model called correctly for an earlier turn to the west than the GFS model did, but the NHC always balances the input from multiple models when making their forecasts.

In this storm, it was well forecasted that the hurricane would turn extratropical, expand, and intensity, so the 34-kt Rule alone blocked off essentially all of the coastal waters before even applying the 1-2-3 Rule. The 1-2-3 Rule showed how far out to sea the risk extended and remained valuable in the waters off Florida for several days. Figure 3 shows this extra caution was justified.

Figure 2 shows the forecast at the time of departure. The inevitability of the encounter with the 34-kt wind field did not diminish with time.

Figure 3. Satellite wind measurements at 1710z  on Sunday, Oct 28, 2012. For comparison, the 72-hr forecasted location of the storm for that time made four days earlier on Thursday is shown, along with the 34-kt wind and 50-kt wind radii (not part of either of these rules).

The 50-kt wind field was very well forecasted, and for the most part the 34-kt winds as well. But there were large regions of the ocean beyond that 34-kt radius that had winds well above 34-kt, which shows the value of the 1-2-3 Rule. That rule at 72 hr off calls for a large clearance in such a large storm. For storms within the tropics the 34-kt wind range might  not be as large, so maneuvering options could be larger, but tropical storm sizes vary significantly within the tropics.

The NHC website publishes the 1-2-3 Rule boundaries for all storms they track, so watching these online is a good way to get a feeling for how they evolve.

The satellite data here are from the Indian scatterometer OceanSat2 (OSCAT). The European instrument ASCAT is a primary source for this type of data, but it did not have a pass at this time. The sections it did show, confirm that the winds were at least as strong as shown here. We have made a link to all of the scatterometer data along with a convenient way to get ASCAT winds by email at www.starpath.com/ascat.

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To see how the 34-kt wind field predictions varied over this period, along with the related probabilities that give some insight into the 1-2-3 Rule, look at this video of the 39-mph winds.

For further information see:

www.nhc.noaa.gov/marine
www.nhc.noaa.gov/prepare/marine.php
www.ecmwf.int

The direction of sunrise and sunset—the old fashioned way

…or another value of a good compass to sailors, photographers, and lawyers.

Sailors care about accurate compass bearings for position fixes and for evaluating the risk of collision as required of all mariners in the Navigation Rules:

Rule 7, Risk of Collision

(d) In determining if risk of collision exists the following considerations shall be among those taken into account:

(i) such risk shall be deemed to exist if the compass bearing of an approaching vessel does not appreciably change;

Note the word “shall.” This is not optional. The only way to measure such bearing changes in the allotted time is with a high quality compass.

Photographers sometimes need to know the precise direction of sunset so they can frame a picture precisely as they want it. A cheap compass will not do this as they cannot take bearings accurately enough to pin point a spot on the horizon. A good candidate for this task is the famous French model, known for some thirty years or more as the “hockey puck compass,” though no importer uses that name directly.  It sells for about $120. It can read a bearing to within ±1° or so, but we cannot count on that being the exactly correct bearing because local disturbances can throw this off somewhat, no matter how remote.  Leaning on a car would make much bigger errors than that. Your eyeglass frames could also, as well as your watch when holding this hand held instrument up to your eye.

So standard compass precautions must be taken, but that done, this compass will do the job nicely. It is is also small and rugged, which are bonuses in the field.  We would like to think that the compass in our iPhone might do the job, but they are not dependable for this precision. All electronic compasses are very sensitive to tilt angle.

But having the right compass is just step one. We then need to know what the direction of the sun is when we want to photograph it. If we are sticking with sunrise and sunset, we do not have to worry about time keeping. It happens when it happens.  Lawyers, on the other hand, might want to prove the sun was shining in their client’s eyes at any random time of day, so they do have to worry about time keeping.  We will come back to that.

I do not know of any one magic table that tells us exactly what we want, namely the magnetic bearing of the sun on the horizon for any latitude and longitude on any date. So we have to do a couple simple computations, after which we could create special tables for special locations.

First, the direction of the sun is an astronomical properly, totally independent of the magnetic field of the earth. Thus we must start with true directions. From the Nautical Almanac we can compute the direction of the sun at any time from any place, but this will be true directions (labeled T). That is, north is 000 T, east is 090 T, south is 180 T, and west is 270 T. Southwest would be 225 T. Or we can be more specific, as we will soon want to be, and the direction that is 20° south of west would be 250 T.

Now that the math is done, what do we need to know besides the date?  We need to know our latitude and we need to know the local magnetic variation. You can get your latitude from Google Earth. Just find the location you care about and look to the bottom of the screen. Magnetic variation (often called declination on land) is the difference between True North and Magnetic North. You can get it from the National Geophysical Data Center (www.ngdc.noaa.gov). The variation will have a label, E or W. It is defined in such a way that true bearings = magnetic bearings + Var E (or – Var W).  For our application, we will be going backwards, so:

Magnetic bearing = True bearing + Var W

or

Magnetic bearing = True bearing – Var E.

Now we are left to finding the true bearing of the sun at sunrise and sunset. A celestial navigator can compute this readily from the Nautical Almanac, but this takes tools we do not need.  There is no one table that does this job specifically, but there is one we can use. It is called Table 22. Amplitudes ofthe Sun. It is from Bowditch’s American Practical Navigator (1977 or earlier). 

In Table 22, the word amplitude means the angular difference between the direction of sunrise and due east or the direction of sunset and due west. The motion of the sun is symmetric across the horizon, so these values are the same on a given day. If the sun rises 20° south of east, it will set 20° south of west. In this case the amplitude would be S 20°.

We do have to keep the mind engaged, however, because the arithmetic switches. This is apparent if you look at a compass rose—always a good idea at this stage. That is, 20° south of east means 090T + 20 = 110T, whereas 20° S of west means 270T – 20 = 250T.

And we are now almost done. We are asking for a rather sophisticated result, so it should not be a surprise that we have a couple steps to take. Bear in mind as well that we are doing this with paper tables. In the end, we have apps for this and you can punch a button and get it from your cell phone!

To simplify its presentation, Table 22 does not use date, but rather uses the astronomically more significant parameter called the sun’s declination. This is the same word as used on land for variation, but it is a totally different concept; it is the latitude directly below the sun at this day of the year. (You see now why mariners like to use magnetic variation rather than magnetic declination.)  We are using sun’s declination just as an index to access the tables. Table of the sun's declination.

Suppose we are at latitude 38N, on May 13, and the local magnetic variation is 10° W. What is the compass bearing of sunrise and sunset?

Refer to Declination Table to learn that on May 13 the sun’s declination is N 18° 14’ (18.23°).

Then turn to Table 22 for latitude 38 and see that the amplitude is about 23.5° (ie about half way between 23.1 and 23.7 — fractions of a degree do not matter here at all. You can round up or round down.)

Then apply the label N or S to the amplitude that is the same as that of the declination. North in this case, so amplitude is N 23.5°.

Then the true sunset direction would be located at 270 + 23.5 = 293.5T

And the magnetic (compass) direction would since we have west variation:

Compass of sunset = 293.5 + 10 = 203.5 M.

This method works with these tables included here alone and nothing more.  If you have a computer you can in principle get this data from various sources, but it is not quite as simple as one might guess. The main problem if you want accurate data (ie ±2°) is you have to get involved in time keeping, because on that approach you need to know the time of sunrise or sunset and the values you find in the newspapers will not be accurate and the accurate ones will require you to adjust the times for your longitude… something we have not even mentioned here, etc.

If you care to pursue this, the place to start is www.starpath.com/usno find the time of sunset for your location then come back to that page to get the precise true direction. You still have to apply the magnetic variation on your own... or post questions here and we will try to help.

Gill Pressure Ports

A barometer reading is sensitive to the wind. A sensor input directly exposed to the wind can lead to variations of 2 or 3 mb with strong wind gusts [1], and the effect depends on wind angle as well as wind speed. Even with a sensor indoors or below decks with a leaky seal to the outside, you will see variations of up to a mb with outdoor gusts [2].  Warships and first responder vessels all have to have pressurized pilothouses, so the barometer must be read from a lead to the outside, usually just a small Tygon tube.

Here is a version from RM Young [3]. This one sells for about $140.  Very simple; no moving parts; but the engineering that goes into them is not so simple and has evolved over the years, starting in the mid 70s. They are now often called a Gill Pressure Port, in honor of the inventor Gerald C. Gill who developed it for what is now called the National Data Buoy Center.

We have tracked down his original extensive technical report on the development of this device [4], which we will make into a pdf ebook that is inexpensive and easy to access. (For now it is too big, and takes some cleaning up.)

The history of this device is quite fascinating, and seems to be an example of some lack of communications between researchers–at least several of the key players do not reference each other during the days of its development.  See for example the early work of Miksad [5] and a related  patent app [6] filed in 1989, canceled in 1996... maybe there is a story there as well!

As should not be surprising, the best of the devices comes from Paroscientific, our neighbors just across the bridge in Redmond, WA. They are the world leader in all matters relating to high accuracy barometric pressure measurement. [7]  Paroscientific has somewhere online at their website a report of the testing of this device, but we have not found this yet.

And we should of course mention Vaisala, the world leader in production of the full range of weather instrumentation, which has a top of the line model as well [8]. These devices are often refereed to as static pressure ports, without reference to Gill, but it seems he was a pioneer in the development of the engineering.

For completeness,  note that the pressure port requirement has applications in other areas, such as pressure drop measurements in air conditioning conduits where there is a varying wind flow through the system [9].


THIS ARTICLE IS A WORK IN PROGRESS. IF YOU HAVE AN IMMEDIATE NEED FOR MORE DETAIL, LET US KNOW AND WE WILL PUSH IT UP THE LIST. FOR NOW I JUST WANTED TO COMPILE THE PIECES WE HAVE SO WE HAVE A REMINDER TO MAKE THE GILL EBOOK... which is in keeping with one of our ebook goals, to preserve obscure but important texts that somehow Google did not do for us. Ed. note: we published this one in 2019. see below.


References:

[1] Guide to Meteorological Instruments and Observing Practices, WMO-8 (related section cited in [7], below. Many copies of the full document online.)

[2] The Barometer Handbook
 
[3] RM Young model 61002

[4] Development and Testing of a No-moving-parts Static Pressure Inlet for Use on Ocean Buoys, Gerald C. Gill 1975-76. 120 pages.  Now available as a Kindle ebook.

[5] An Omini Directional Static Pressure Port, Richard Miksad, 1975-76

[6] Pressure Port Patent app (I do not know where we found this. It was part of the research for the Barometer Handbook. It at least shows the insides of one design, as does the Gill original article. We will, of course, never see the insides of the RM Young or Paroscientifc models online, unless we buy one and take it apart.)

[7] Paroscientific precision pressure port model 8007 (manual)

[8] Vaisala Static Pressure Head SPH10 / SPH20

[9] An Inexpensive Method for Measurements of Static Pressure Fluctuations, Liberzon and Shemer, 2009