Saturday, April 27, 2013

Finding Longitude from Sunset

With accurate time there are several ways to find your Lat and Lon without a sextant if you ever lose GPS data. We cover each of these in the book Emergency Navigation. In this note we do one example, and later will add to this several more that we now have data for.

The first measurement was kindly provided by Angeline Pendergrass during a research voyage on the R/V Thomas G Thompson off the coast of WA state. She took the data from the aft deck at a height of eye estimated to be10 ft.

Short of having an accurate watch at hand, the procedure was to take a cell phone picture of the sunset, just as the upper limb dropped below the visible horizon, which marked that time in her phone. Then she proceeded to the wheelhouse and took a picture of the GPS screen, which showed the UTC and the location of the vessel which was drifting at the time. The cell phone time showed the delay was 1m 7s, so she could then figure an accurate time of sunset with the associated position.

The results were:

Sunset (upper limb crossing the visible horizon)
03:17:49 UTC (4/23)
Lat 48º  48º 16.33’ N
Lon 123º 59.00’ W
Height of eye = 10 ft

(Though it is far beyond specs related to this discussion, this vessel has special props and instrumentation to hold a steady position in rough conditions. The actual position was known in principle to sub-meter accuracy during this period. The above numbers are rounded down.)

At this stage there are several ways to analyze this information.


The easiest and likely most accurate is to not go for an actual Lon, but rather just compute a regular cel nav LOP. To do this, we just interpret this as a “sextant sight” with Upper Limb Hs = 0º 0’ from an HE = 10 ft at the given time.

A key point to keep in mind for emergency determination of Lon is we are free to assume we know our latitude precisely. The reason is we have many ways to find accurate Lat without time, and with time we can get it very easily from a noon sight.  Not to mention that we also get accurate Lat from any set of star sights, even if the watch used is wrong.  The Lat will be right, but the Lon will be wrong by an amount directly related to the watch error at the rate of 15’ Lon error for each 1m of time error.

In this example we will assume we know our Lat and are just going for our Lon.  So for now we just choose some random value of what we might have thought our Lon was before we did the sight, Let us say our DR Lon before the sight was 124º 05’ W.

So we do a normal sight reduction with this data:

HE=10 ft.  IC = 0 ZD = 0, WE = 0
Hs UL sun = 0º 0’,  UTC = 03h 17m 49s on April 23, 2013
DR position = 48º 16.33’ N, 124º 05.00’ W
You can solve this with a Nautical Almanac and Sight Reduction Tables, or with a computer or calculator program.  We used the StarPilot program developed at Starpath and from this you will find an altitude intercept of  a = 4.3’ A 290.1 T.

This result is plotted in the picture below.

With the plot expanded to 30’ parallels we find a Lon of 124º 01’ W, which is off by only 2’, well within the uncertainties of this process, due primarily to refraction uncertainties at the surface.

You can get better results by expanding the plot to 6’ parallels, and you can check the sight reduction with this data from USNO (see

Celestial Navigation Data for 2013 Apr 23 at  3:17:49 UT          
 SUN        GHA 229 52.2   Dec N12 33.5 

So the summary so far is a timed sun set can get you a Lon from a known Lat using a simple sight reduction. The easiest way is to use a calculator for the process, but it is all doable by hand. Must be careful in figuring the a-value as you will be subtracting two negative numbers. (We will fill in that detail later. Students in our Advanced Ocean Nav Course get to slug that out in practice exercises.)

We should note that this level of accuracy was fortuitous. We should expect an uncertainty in this type of measurement of about ± 5’ ...even with everything well known.  In this case, the HE could have been off some.  If it was really 15 ft and not 10 ft, then this would change the a-value to 5.2’ A 290.1, which makes the final Lon about spot on correct.  (In fact, if the deck height were 10 ft, then HE was closer to 15 ft, but again, the uncertainty to expect is as noted. We need to check that with the ship specs.)


We can also get Lon without any cel nav by using Sunrise Sunset Tables. Short of going online for a quick answer (, we instead look at what you might have underway.  First option might be a Nautical Almanac as shown below.

Now you get a neat interpolation problem: find xx:xx

50º 00’ N            19:06
48º 16.3 N            xx:xx
45º 00’ N            18:56

and the answer is 19:02:33 for UTC sunset (observed at Greenwich)

The other option you might have are tables from the back of the Tide Tables, as shown below.

And now you have a really neat double interpolation problem. Note we were lucky with the Almanac since the date happened to be in the middle of the 3 days covered on each page. If that were not the case, then even the Almanac calls for a double interpolation.  You can do this by hand, or use tools we present at :

50 N
Apr 21    19:03
Apr 23    xx:xx
Apr 26    19:11
and we find xx:xx = 19:06:12 for 50 N on apr 23


48 N
Apr 21    18:59
Apr 23    xx:xx
Apr 26    19:06
and we find xx:xx = 19:01:48 for 48 N on apr 23

and now interpolate for Lat

50º 00’ N            19:06:12
48º 16.3 N            xx:xx:xx
48º 00’ N            19:01:48
and we find xx:xx: = 19:02:24 for 48º 16.3’N on Apr 23.

The first thing we see is the two tables do not yield the same results. the almanac gave us  19:02:33, and reason is simple, all times are rounded to nearest whole minute, so we have 29s floating around on each end of each number.  There is no way around that.  We must just accept right now that if you are using tables, we have an extra uncertainty which we might estimate at ± 30s, but that would be too harsh. That is, we are getting our result from at least 2 numbers and more likely 4 numbers, so the uncertainty in each of those could sometimes cancel each other out.  Thus are extra uncertainty is more like one fourth of square root of the sum of the squares, which would be sqrt(4x900)/4 = ± 15s.
In this example we might then say the best value we have is the average of the two ± 15s, which is 19:02:29 ± 15s.  The 15s automatically adds a Lon uncertainty of about ± 4’ of Lon.

Note too that if we had nautical almanac, we could compute the actual time of sunrise very accurately, but with those tools, i.e. almanac and calculator, we are better off just doing a sight reduction, which has all those steps buried in it.

So now we have this information:  Observed from Greenwich (Lon) = 0º the sunset would occur at 19:02:29 UTC but we observed it at 03:17:49 UTC.  Therefore what must our longitude be?

Observed time of sunset (UTC) = Greenwich sunset time (UTC) + Lon (West)

so Lon West = Observed  time - Greenwich sunset time = 03 17 49 − 19 02 24, which is negative, so we must add 24h:

      03h      17m     49s
    −19h    −02m  −24s
=      8h      15m     25s
then convert angles to time at the rate of 1h= 15º, 1m = 15’, and 4s = 1’

8h = 120º,  15m = 225’ = 3º 45’ and 25s = 6’, so Lon = 123º 51’ W, compared to known Lon of 123º 59’, or off by 8’, well within our known uncertainties.

So it all works.  We have about 8 sights taken from the JRH rowboat which we are still evaluating. We have the correct times, but have to go back to the logged positions to make the comparisons.  We will add that data shortly.

Thursday, April 25, 2013

How to Plot with Triangles

We  have had several questions on the use of these lately, so here is a quick answer. It takes two of them to do the job. They look like this:

 Problem: Draw a line through Point A in direction 057 T.


Step 1. Choose any point B on the nearest meridian line to point A, and orient triangle 1 (either one of them, they are the same) so that it points in direction 057 T. This will have the center point of the triangle on the meridian and then you rotate the triangle so the meridian crosses the 057 line on the protractor printed on the triangle. Double check by just looking at its orientation that it is about right... ie that you did not just line up some numbers without thinking of what the orientation was supposed to be. There are several scales on the triangle and you may have grabbed the wrong one if you do not look at the edge crossing the meridian to be sure it makes sense. You can do this alignment very precisely, relatively easily within ± 0.5°.

Note you are always using the degree marks on the outside edge of the triangle shown above. The inside scales are just there to show you the different orientations and subsequent meanings of the outside scales.


Step 2. Holding triangle 1 firmly in place, align triangle 2 under it so you can slide 1 over to point A as shown below.


Step 3. Slide 1 over to Point A and then draw in your line. If you have any doubt that you slipped some, then slide 1 back to the meridian to check it out.



Reading the bearing of a line on the chart is just the opposite. Align one triangle with the bearing line, then put another under it to slide it to the nearest meridian to read the bearing.


Hope that helps for now. We have triangles for sale in our online Accessories Catalog. They cost $14.95 each. 

Sunday, April 21, 2013

True Wind from Apparent Wind –– Revisited.

There are several ways to define the wind. For weather work at sea we care only about the true wind. This true wind is the speed and the direction of the wind relative to the fixed earth under the ocean. Tied up at the dock, we feel the true wind. Once we get underway, our own motion changes the wind we feel, and then it is called the apparent wind.  (In sailboat performance work there is a further distinction true wind and ground wind.)

Many sailors argue that the apparent wind is all they care about, and that can be well argued when it comes to setting sails and judging performance.  But to know about the weather patterns causing the wind,  we need to know the true wind. Furthermore, we often need to know this as accurately as possible. Shifts in the true wind direction are usually the first sign of changing patterns.  If we do not figure this properly, we can miss an important shift. This is not a simple observation––which is more or less the point of this article. Slight changes in true wind speed, for example, affect boat speed, and in turn the apparent wind speed and direction, which can easily mask a small but important shift in the true wind direction.

Let's bring in some shorthand:

AWS = Apparent Wind Speed (relative to the boat)
AWA = Apparent Wind Angle (relative to the bow, 0 to 180, starboard plus, port minus)
AWD = Apparent Wind Direction (relative to true north)

S = Knotmeter speed (relative to the water)
H = Heading (relative to true north)

DFT = Current Drift (speed, relative to fixed earth)
SET = Current Set (direction it flows toward, relative to fixed earth)

SOG = Speed Over Ground (relative to the fixed earth)
COG = Course Over Ground (relative to the fixed earth)

TWS = True Wind Speed (relative to the fixed earth)
TWA = True Wind Angle (relative to the bow)
TWD = True Wind Direction (relative to true north)

Since wind directions are almost always discussed in terms of true directions, not magnetic, we forget the compass for now, and consider all directions discussed as being true directions. Our actual use of compass directions in navigation do indeed complicate things a bit, but that can be sorted out later. It is not really related to the subject at hand.

To review the issues involved, we start with the basics.  It can be dead calm at the dock, and I take off under power headed due north with my knotmeter reading 5.0 kts, and sure enough,  I will have 5.0 kts of apparent wind, right on the bow. S=5.0, H=000, AWS = 5.0; AWA = 0, implying AWD = 000.

If I then turn to H=090, I will still have AWS = 5.0 and AWA =0, but now AWD = 090. In short, there is no wind here at all; I am making it all myself.

A bit closer to the point at hand, I could do this same thing, still with S=5 and then I notice that I now have AWS =7.0 kts of wind, still with AWA=0.  Something changed. I check the GPS and see that my SOG=7.0 kts, and that accounts for the extra wind.  I am in a current that is moving the boat 2.0 kts. Now I need to look at the COG. If the COG is exactly equal to my Heading, then this current is right on my stern, pushing me forward, toward. DFT=2.0, SET=090.

Now if I shut off the engine and slow to S=0, still with H=090,  I should see AWS=2.0, still with AWA=0, providing the COG=090, and SOG=2.0 (still H= 090), and I can conclude that I have measured two things: The true wind is calm, and the current is setting toward 090 at 2.0 kts. If this were not the case, one of these numbers had to be different. 

When TWS is not zero, this analysis gets more complex and a vector triangle must be solved, but the key point is always the difference between COG and H.  If COG = H, meaning you are moving the direction you are headed, then all of the standard vector triangle solutions for finding true wind will work fine. You just substitute SOG for the S that is in the equations or plotting routines.

Generally these formulas and plotting routines solve for true wind angle (TWA) based on AWA, AWS, and S. Then you apply the TWA to H to get the TWD. That all works fine in those cases, and we have several free true wind computers available at (downloads section) or there is a nice one in what we call the NIMA Nav Calculators a free download at

Once you are being set off course and COG ≠ H, then the standard formulas for computing true wind from apparent will not work properly, because we measure the wind direction relative to H, but our actual motion is in direction of COG. Thus you can no longer simply work with apparent wind angle (AWA); you have to switch to using apparent wind direction (AWD) and solve the vectors relative to COG, as shown in the sketch below.

Also it seems to me that the typical equations we see in books (including our own) that use some form of Law of Cosines might not be able to handle all the various combinations of directions. This has to be checked. I am not 100% sure of that. Just my impression.  It seems safer to get the answer from x-y coordinates, and so we present here these formulas, written in a way that can go direct into a spread sheet or calculator.

      AWA = + for Starboard,  - for Port
      AWD = H + AWA ( 0 < AWD < 360 )

      u = SOG * Sin (COG) - AWS * Sin (AWD)
      v = SOG * Cos (COG) - AWS * Cos (AWD)

      TWS = SQRT ( u*u  + v*v )

      TWD = ATAN ( u / v )

Remember in a spread sheet all the angles have to go in as radians, ie COG = COG(º)*(Pi/180). In a spread sheet you can write AWD = MOD(H+AWA;360).

Below is a sample computation of an extreme case of strong current showing how different the true wind results are if the COG-H difference is not accounted for. In that table, TWS-x = u, TWS-y = v.

Note the above picture is more complicated than we need in practice.  Once you have figured AWD, you can use your standard plotting method to get the true wind.  In other words, usually you do not have to compute SOG and COG, you measure them from the GPS.

However, it is likely simpler now to plot it with actual bearings, rather than as a relative plot using COG as 000.  Below is a sample. It is the green triangle that you plot, ie plot SOG/COG and plot AWS/AWD, and connect the end points to get TWS/TWD.

The spreadsheet format that does compute everything, makes it easier to experiment with various interactions of sailings and currents to see how this affects the final answer. You can download a copy of this spreadsheet with the equations in it at the tech support page for Modern Marine Weather ( with the CAUTION that you need to check it.  No guarantees that it is right!

Also please keep in  mind, that these measurements assume the instruments are calibrated and the wind sensors are located away from disturbing wind from the sails or other rigging on a power driven vessel.  We have seen cases where the masthead instruments are affected by updrafts from the sails, which is why on some race boats there is often a rather large arm holding the instruments a ways off of the masthead.

One easy test is to measure true wind on one tack compared to the other tack, as you tack back and forth in smooth water. This exercise might expose other important issues, namely that your speed varies noticeably on each tack, implying the speed sensing is not purely symmetric, assuming the sail trim is.  In big waves, you often expect the speed to be different opposite tacks, but if the speed sensors are working properly, the true wind will be the same on each tack.  Ditto for opposite gybes.

Friday, April 19, 2013

Modern Barometry and its Important Role in Marine Navigation

Talk presented at the commissioning of the World's Tallest Barometer at Portland State University on Apr 18, 2013.  (Videos of the instrumentMore news.)

I am grateful to Tom Bennett, designer and builder of this fine instrument, and to Portland State University for the opportunity to talk to you about barometers. It has been a favorite subject for many years, which started with my participation in ocean yacht racing in the early 80s. Barometric pressure is the key factor in the selection on an optimum ocean sailing route.

This facility promises to be a wonderful teaching tool. When you see the atmosphere push this column of liguid up three flights of stairs, there is no better way to appreciate Torricelli's insight that "We live on bottom of an ocean of air."  In just an hour of discussions with Tom as he demonstrated the instrument, we discovered a half dozen new informative study projects that could be added to the long list he had started.


When I mentioned I was giving a talk in the shadow of the World's Tallest Barometer, one of my associates said that he and a couple navy buddies learned about tall barometers on the deck of a ship, 35 feet above the water line. They had lowered a hose over the side to pump water onto the deck, and then could not figure out why the pump would not work; they had just used in on the dock that morning. Then his basic physics came back to him, and he realized they can’t pump water 35 ft, no matter how good the vacuum is at the top of the hose. Water will only rise 31 ft. This same observation is what ultimately led to the discovery of a working barometer in the early 1600s.

Now if the ocean had been made of this special vacuum oil used here, then their pump would have worked just fine, and no one would have learned anythingso we see the science sneaking into even the first thoughts of such an instrument.

And indeed, one of the things I would like to show is that the public understanding of barometers and barometric pressure is lagging way behind its prominence in our lives... and especially in our language.

The first thing Sarah Palin said to 70 million people in a vice presidential debate was “The barometer of the economy is a soccer mom.” Jerry Seinfeld said “The barometer of a relationship is the answering machine.” I am not sure what either of those statements mean, but the term is used as the holy grail of measurements, without anyone knowing what it means.

The public comments to the online article in The Oregonian about this project are another example. One stated: “If they wanted a barometer, they should buy one at Home Depot”which is cute enough, but misses the point more than they realize.

Barometers are not like thermometers. If you buy a thermometer at Home Depot it will probably workat least on some useable level. But if you bought a barometer at Home Depot it almost certainly would not work. It might go up and down to some extent, but it probably will not go up and down the right amount, and indeed it might only go so far, and then just stop. In fact, where the pressure matters most, at the two ends of the dial, the common consumer grade barometers are most likely not to work properly. In short, most such units are decorative, not functional.

But most barometer owners don't know they are not functional, because we do not know what function they are suppose to provide.

There is a certain awareness that pressure is related to weather: low pressure usually means bad weather and high pressure means good weather, and therefore a changing barometer means changing weather, but that knowledge is not useful if your barometer does not work right.

Ask most folks with that knowledge how high it has to be for good weather or how low for bad weather, or what is a fast rate of change compared to a slow rate of change, and there won't be many valuable answers... and it is not right to say that the right answer is there is no right answer, because there has to be an answer. You could say there is no short answer.

The problem is if your barometer does not work, you do not get any useful data to compile this answer from. Compare that with temperature. We know 90º is hot and 30º is cold. Going from 50º in the morning to 70º in the late afternoon is a big change, but not unreasonable. But 30º in the morning and 70º at noon is not possible, etc.

On top of that, there are perceived functions of barometric pressure that are demonstrably wrong, such as the correlation between atmospheric pressure and good fishing. There are tons of Fishing Barometers on the market. Yet if a fish changes depth an inch or two the pressure change it experiences is far more than would ever change with the weather. There could well be some weather conditions that do affect fishing that correlate with the pressure, but it is certainly not the pressure itself.... not to mention that fishing barometers are typically in the category of not working in the first place.

There are also Harvard Medical School scientists who have correlated large pressure changes with headaches, but it takes a black belt in statistics to appreciate the results. On the other hand there are tons of anecdotal evidence of that correlation.

There is also much anecdotal evidence that low pressure can cause the water to break earlier than expected in near full term pregnant women. That, it would seem has a reasonable model behind it, and one that should be easy to study from existing medical and pressure records, but I do not know of any real data.

There are of course many bonafide applications of accurate barometric pressure beyond its fundamental role in meteorology and weather. I will review a couple we have learned about over the past few years.


(1) Weather is of course No. 1. Pressure is a key input to atmospheric computer models used to predict the weather. I will come back to this one.

(2) Altimetry is a close second. As you rise above the surface or climb a hill, the pressure drops at a rate of about 0.44 mb per 12 ft (chosen so we can say “point four four per floor”). Altimeters used in aviation are barometers. They usually measure elevation more accurate than a GPS can do, except possibly in special cases using the WAAS satellites. Thus aviators, hikers, surveyors, and sky divers often rely on accurate barometric pressure measurements.

And now it is difficult to rank significance, so this is just a list, with no meaning to the order.

(3) The inverse barometer effect. It was known to mariners in the 1800s that “the fog nips the tide,” meaning that in foggy conditions the tide is not as high as normal. We know now this is not the fog at work, but the pressure. Fog is often associated with High pressure. When the tide rises, it has to lift the atmosphere and in High pressure the air is heavier so the tide does not go up as much. This is a small effect (1 mb of extra pressure inhibits the tide by 1 cm), but still a crucial one to many applications and again to our understanding of the role of pressure in our lives.

It is, for example, crucial to understanding the rise in global sea level, since this factor is much larger than sea level changes over the years. It has to be carefully factored out of all measurements so we learn what the sea level really is.

The roll of this factor is also strangely mis-represented on occasion by experts who know better. The head of NOAA stated after the record storm surges following Storm Sandy that this would not have been so bad had it not been for the global warning induced rise of the sea level. This is frankly not the case. Storm surge is water coming ashore that is much higher than predicted by the tides. The vast majority of the surge they saw was due to very Low pressure and strong onshore wind. In fact, tide surge by definition cannot be related to sea level height because the predicted tide heights are referenced to the known sea levels. From our context here, this is example of not giving atmospheric pressure the attention it deserves.

(4) Mining safety. The safe ventilation of mines relies on measurements of relative pressure throughout the mines, which are ultimately referenced to the station pressure at the surface entrance. This can be a tricker measurement than many interested parties are fully aware of. The issue is that at high-elevation mines (some are well above 7,000 ft) even some quality barometers might not work properly, because an are calibrated to work at sea level. This application requires careful consultation between mining engineers and the sellers of the barometers they use.

(5) Calibration and adjustment of various instruments.

Oncology labs that provide radiation therapy use dosimeters to calibrate the radiation intensity. There are several ways to calibrate these, but one popular method relies on knowing an accurate value of the station pressure at the time of calibration.

Crime labs must periodically calibrate the breath analyzers used throughout their jurisdictions, and as of last year, the method of choice now requires an accurate station pressure.

High precision Pick and Place assembly line robots use a vacuum to do the lifting, and the setting of these pumps require an accurate station pressure during calibration.

Inflatable life rafts must be certified for leakage under strictly controlled conditions, which include by law an accurate value of the station pressure at the time.

High performance racing car engines are tuned to meet atmospheric conditions before each race. We learned from one organization that they had a definite advantage over their main competitor because he knew the competitor was using his barometer incorrectly. There is often a misunderstanding about the relative role of station pressure and sea level pressure. All of these applications mentioned need station pressure at the time and location of the event. They do not care at all about the (sea level) pressure the local radio station is reporting, which could be dramatically different.

In analogy with the race car engines are the very much bigger engines used to generate power in electric power stations. The output from these engines is remarkably sensitive to pressure. A 0.3% error in the pressure causes a 0.5% error in the output. Considering the stations cost $100M, this is a real number.

(6) And then we come to a favorite of mine, world records in long track speed skating. For years it was thought that high elevation skating venues held all of the world records and athlete’s best times because of the “fast ice” at high elevation. There were even multiple theories published about why the ice is so much faster at high elevation. But it is not fast ice; it is “fast air.” This is such a fine tuned sport with records differing by tenths or hundredths of a second, that it is easy to show that the lower atmospheric pressure at high elevation reduces the drag on the skaters by more than enough to account for these observations. It is for the same reason that more home runs are hit in Boulder, CO than in Oakland, CA. The same hit goes about 20 ft father on average in Bolder.

During our research of this topic we found a former olympic skater who had been preaching this for years, but no one listened to him. He was right all along. To his credit he was telling the sport years ago that they should be recording the atmospheric pressure with every record set.

Marine Weather

The history of meteorology as a science owes much to the work of early mariners. Their logbook records of wind and pressure during the age of discovery were crucial to the development of modern meteorology. Their records led to early theories of global weather and to an understanding of storm formation and behavior.

The very concept of a weather forecast came from mariners, specifically Robert FitzRoy, the captain of Beagle who took Darwin around the world. (Darwin was originally there to provide the captain with intelligent company at dinner, but it obviously evolved into rather more.) It was FitzRoy's concept to have observations on the west coast of England telegraphed to the East coast so fishermen could be prepared. It was known by then that midlatitude weather moved toward the east.

FitzRoy was also the pioneer of the use of barometer for weather forecasting and analysis.

Barometer measurements are important to weather in all walks of life, but they have a special significance to mariners at sea, which has persisted for 300 years. Even today, the barometer means more to sea captains than to any other group of users—except maybe managers of power stations who are accountable to their stock holders!

When you are at sea, it is not a matter of just going inside if the weather gets bad. Any tool that can warn of bad weather is crucial so the vessel can be prepared and navigated accordingly. They are not going to pull off the road and wait this one out. The pressure makes the wind, and the wind makes the waves, and the waves can be a threat to any vessel.

Sailors use the barometer for weather warnings, but they also use their barometer to find more wind or more favorable wind more often than they use it to avoid too much wind.

The center of the Atlantic or Pacific ocean in the summertime is usually dominated by a broad mountain of high pressure, which creates a windless desert on the surface. To sail across an ocean, you must sail around this High or get stuck like the Ancient Mariner with no wind for many days. World sailing routes are determined by the intensity, location, and motion of these mid-ocean Highs. With study and experience you learn how how close you can get to the High and still have wind, but you need an accurate barometer to map out the mountain as you go around.

But you can rightfully ask: Don't all mariners now have satellite phones and modern weather maps giving them the best possible forecasts? Why is the barometer still so important?

The key to this answer are the words “best possible.” There will always be a forecast, that is certain. But marine forecasts are not marked good or bad. On land we get 40% chance of rain; but at sea you do not get 40% chance of gail. They either say gail or they do not. (Mountain weather and fire weather have probabilistic forecasts, but marine weather so far does not.)

And in fact, some configurations of the atmosphere lead to much more dependable forecasts than others. Thus it is the job of the mariner to gather all possible data to help them evaluate and correlate the official forecasts with what they observe.

Ironically, the more sophisticated the forecasting has become, the more valuable the barometer on each vessel becomes. A common way to get weather data at sea these days is via digital weather maps sent by sat phone and viewed in a computer program. These are beautiful products with very detailed data, which is easily accessed. But this super-convenient source of data is a direct output from numerical models that is delivered unchecked by human meteorologists. Most of the weather maps you see on TV etc are this same type of datain the US, usually from the Global Forecasting System (GFS) model. This model does a very good job in some cases, but not all.

Thus the mariner needs to check the model data, by comparing what the model predicts for the pressure compared to what they actually observe. If they agree, they can be more confident that the forecasts are good, but if they do not agree they know to be more cautious in their decisions.

Even when the forecasts, no matter what their source, are even right, it is not uncommon that the timing is off somewhat. The storm could have started to move faster than predicted, or slower. By watching the barometer they know how things are actually changing.

The barometer work is especially important for sailors looking for more wind in light air. Regions of light air are not predicted well by the forecasts, and the difference between 4 kts of wind and 8 kts of wind is huge factor under sail.

The other factor important to small craft mariners at sea is independence. They know that all systems, including their electronic connections to civilization, are vulnerable at sea, and if they lose this connection, the barometer becomes paramount to their shipboard forecasting. In the tropics for example, the pressure is very stable and with a small standard deviation of change. A detected change from the normal average daily pressure being observed of just 3 or 4 mb can be a strong warning of a tropical storm approaching, even if no other signs are apparent, especially if accompanied by the onset of a new long, low swell.

I commend Portland State University for building this beautiful instrument.

It is not only a unique teaching tool, it is a monument to the history of science. After all, if you look into who all was involved on some level (Aristotle, Galileo, Descartes, Pascal, Boyle, Hooke, Halley, Locke, Leibniz, Bernoulli) and look at the ramifications it had on the evolution of philosophy, science, and maritime achievements, it is fair to say that the “Barometer of the history of science” is “the history of the barometer itself.

Reference links:


Location of instrument if you get a chance to visit: 45º 30.560' N, 122º 40.880'W 
Go in the door and turn right.

To check for an accurate reference pressure, see
To see the historical accuracy of the various pressure stations nearby see:

Tuesday, April 16, 2013

North Pacific Wind Statistics

During an online discussion of best time to sail from HI to WA, considering late Apr or late Jul, my first reaction was Jul, and the question came up that the average wind stats from COGOW showed these about the same.  So the question was when looking at a more detailed picture of the statistics would shed any light on this. The pictures below are a step toward that answer. 

There are three pictures each at very roughly the start, middle, and near the end of the passage, first for late Apr followed by late Jul.

Now the same three pictures for late July. We see that there is more NE component to the trades, that is not a bonus, but the main issue to consider is the higher latitude winds.  Looking at these diagrams,  the amount of winds greater than 25 kts is the length of the dark part of the arrow.  This is notable from the in the NW and SW quadrants. In Jul the wind is mostly NW but not strong... on average.  The influence of very strong winds do not show up in these stats at all.  We can get another idea of this from the pictures at the end that show the coastal stats.  It would take more digging to get the actual storm stats.... As I write this on April 19 (2014), the winds off the NW coast are 30 kts.  That does not say too much, you can get 90 kts of wind in July, but just the stats are lower in July than in April.

These are coastal data that we have in the Starpath Weather Trainer Live software.  It shows that there is clearly a window in moderated conditions along the coast that runs from June to Sept or so. 

Sunday, April 14, 2013

Revisiting the Secret Sources

There has been for several years a nice way to look at marine weather from a conventional NWS presentation, which we have written up in the past in an article called "War Horses and Secret Sources." It is still there, but it seems to be expanded now to the Caribbean and maybe farther. It is still a bit of a secret (ie you do not find it on any marine weather page from NWS), but this service has some nice presentations for mariners.

Start by going to this link, which you can bookmark as from here you can get to other coastal waters or oceans.  I am still now sure where the home page is for this interface.

At this page you will see several sections of data, but they are all tied to the point you can click in the small Google maps window on the bottom, which can be zoomed and panned. This sample is for waters north of Puerto Rico, which we have some interest in now.

This shows a 4 day (96 hr) forecast for ocean waters that is very similar to what we see on land forecasts, except this is wind and waves.  Below this you will see:

Which is the text forecast for these days, giving more specifc info on wind and weather.  If you are providing this to a friend, you can just copy the text and email it (see bottom sample from the Printable Forecast link).  They will likely have something similar available underway, but it won't be quite so convenient.  And on the bottom of the page you see the key window that is your graphic index to the point or series of points you care about.

 Then for the location you selected you can get another view which might be called a "meterogram" for the area shown below which you get from selecting Hourly Weather Graph above.

This is worth checking out for the Caribbean or US coastal waters. It works great in comparing marine conditions in various parts of Puget Sound, SF Bay, or the Great Lakes... or parts of the Chesapeake.  Just be sure you are zoomed in enough on the little map that you click on the water. If you click adjacent land you get the land forecast.

My guess is the foundation of the predictions is the GFS model, but it could be enhanced in some areas with more local model computations or input on some level.

A broader interface to this presentation is given at which is definitely a new and experimental web page.  Seems a bit clunky yet, but the idea is good. In fact it was experimenting with this that made be realize that they have expanded the above presentation to the adjacent oceans, and that maybe this old way might be as convenient if not more.

Sample of the Printable Forecast

Tuesday, April 9, 2013

Small Craft Wave Danger as a Function of Wave Steepness

I found this in my notes, and then tracked it down to this link at the NOAA marine weather page, which has several topics of interest to mariners.

Very roughly, periods of less than 9 seconds are waves, and those with periods greater than 11 seconds are swells. I believe the average wave period over the ocean is about 7 seconds.

It is of course not gospel, but certainly a good guideline and perspective, since the forecasts just give the heights and sometimes periods, without such an evaluation.

Below the picture is the text that they present to describe the table.

"The danger presented to a vessel is a function of wave steepness as well as wave height and is unique to each vessel. In general for small vessels, for a given wave height the danger increases as the wave period decreases. Below is a table under development within the National Weather Service to assist forecasters in identifying sea conditions which may be of danger to vessels with a closed cockpit configuration of ~100 feet or less. The table is intended to be instructional only and the danger presented by waves to your own vessel may be quite different"

We can compute wave length from the period (with certain assumptions), and from the height and length we can compute steepness. See Modern Marine Weather for more discussion.

If we approximate the Length from the average sea way of the periods shown, then we can get average lengths and average steepness, being the ratio of Length / Height.  Waves generally break at Steepness of 7, but they look terrible and are certainly dangerous at Steepness of 10.  It seems from this table we calculate below, that they have called dangerous a value of 25 or less, which makes sense.  It is not clear why there is not more red at the shorter periods.

For a truly small craft, this graph should, I believe, be red for the shorter waves, with Steepness up to 15 or so.  We will have to look into this some more to see their logic.

Monday, April 8, 2013

High Definition (HD) and Broadband Radar by Larry Brandt

The latest marine radar buzz words are “HD Radar” and “Broadband Radar,” now being advertised by reputable radar manufacturers as the latest and greatest technology. But how are they different from the radar we have come to know over the past decade? Here is a quick overview of these two technology advances.

The term HD rides along with the recent high definition television wave. HD TV has brought enhanced reality to the screen: favorite TV personalities are presented life-sized or larger, with amazing details plainly visible. This has been a remarkable step forward for television viewing. In marine radar, the step is not so large, but it is noticeable.

HD processing in radar display is a manipulation of the video output of the received radar signal that can make the echo image appear sharper. Neither the transmitter nor receiver is radically changed to accomplish this, instead video processing after the receiver stage and the use of a color palette to display echo strength are used to make the target edges more defined. The result is a pleasing display appearance that lends confidence to the interpretation and may enhance target separation in some cases.
In short, this is an improvement in the display and not a change in the target detection capabilities of the radar.

Broadband Radar is an arguable marketing term, intended to differentiate one manufacturer’s technology from others’. Marketing folks come up with these terms that sometimes convey the opposite of their intent. We suspect this is the case here. We could argue that this version of radar technology is not at all broadband, and that, in fact, it depends beneficially on an extremely narrow receiver selectivity (i.e., the opposite of broadband) to function. Nevertheless, this has been a good choice of terms as it has caught on and is widely used.

Radar Dome from SIMRAD, part of the Navico family of broadband radars. 

The technology is indeed radically different from that used previously in the marine industry, although it has been used in aviation applications for decades.

The first change to note about this new marine application of a not-so-new technology is that it dispenses with the radar’s traditional high-power output vacuum tube–the magnetron. Instead, it uses a very low-power solid-state device as the transmitter’s main power generating element. A solid-state, low-power transmitter is a radical step forward in our industry.

The high-power magnetron is notoriously wasteful of energy: it splatters its output pulse across a wide band of the frequency spectrum. Talk about broadband! Adding disadvantage on top of disadvantage, the magnetron’s output pulse is not stable. Its transmitting frequency varies from pulse to pulse, and even within a pulse. This method of generating two thousand or more watts of pulsed power requires its paired radar receiver to have a wide selectivity that allows a lot of electrical noise to enter the receiver along with the faint target echoes.

A solid-state transmitter as in the new broadband radars generates energy that is more stable in frequency, and does not waste its energy in sidebands as much. This allows the radar engineers to design a receiver with very narrow selectivity that prevents much noise from entering the receiver where it competes with the very faint target echoes. If a magnetron radar can be called a blunderbuss, a solid-state radar is a rifle–that is, frequency-stable energy applied efficiently versus magnetron energy splattered inefficiently.
What has kept magnetron radars alive so long is their low cost. Microwave ovens we cook with need high power to warm the soup, so a solid-state power device would not serve well in the galley. On deck, though, magnetron technology has about run its course.

Screen image from SIMRAD broadband Radar.

The second change in broadband radar is that it doesn’t transmit a single output pulse. It transmits a continuous energy wave (called CW) that gradually sweeps across a frequency spectrum. If you could listen to the sound, it might be similar to a police siren’s whoop, whoop, sweeping in tone from low to high. Its paired receiver follows that frequency sweep with very narrow selectivity. In this it is very much like the function of an aviation product that we know of as the CW radio altimeter. As the transmitter sends out its constantly sweeping energy, its receiver keeps track of that output and compares it to the energy reflected from a target. When the incoming signal matches the outgoing, that identifies a target. Thus we come to the more generic name of this type of radar technology: Frequency Modulated Continuous Wave (FMCW).

One of the touted advantages of broadband radar is that it can see echoes virtually up to the rubrail of the boat. Much is made of this as a collision avoidance benefit; however, my own 2 kW radar mounted at the spreaders can readily see a small buoy barely 70 feet in front of the bow. My feeling is that that any situation within that distance that depends on radar to ameliorate is a collision that is unlikely to be avoided. Collision avoidance depends on proper watch at distances far beyond boat-length range–a virtue that some competitors still argue are lacking in the low-powered FMCW technology.

Today’s second generation broadband radars, however, are advertising much improved long-distance operation, which indicates that the first gen product probably wasn’t a great performer. There is no reason why a low-power solid-state radar cannot have long range performance equivalent to traditional multi-kW magnetron radars. The “No Substitute for Power” mentality in radar is based on myth. What matters is not transmitter power; what matters is Loop Gain, which is a function of transmitter effectiveness teamed with receiver sensitivity and selectivity, and modern video processing.

Eliminating the magnetron will reduce system power consumption because the filaments of the magnetron are a power-hog. Sailors will appreciate this power reduction, but the antenna drive motor and display back lighting, two other significant power consumers, remain unaffected.

Radiation exposure due to radar is a topic that wins too little consideration, in my opinion. Solid-state radar would certainly reduce or eliminate any perceived hazard in this area, which has to be considered a virtue. Accepted values of radiation exposure seem to go down with each improved study.

The most serious disadvantage of broadband radar, in our opinion, is that its low transmitter output power is insufficient to trigger a RACON. This is a serious weakness, especially in challenging navigational environments where many vessels operate. Aside from this one major drawback, we believe that FMCW (“Broadband”) radar is worth consideration if one were contemplating a purchase.

Modern magnetron radars are darn reliable. On a delivery this writer has used an old monochrome CRT Decca to cross the Columbia River Bar at 0300 in pea-soup fog, a radar that has probably performed flawlessly for 30-plus years. And so long as the radiation hazard is recognized and respected, a magnetron radar will serve just fine.

But progress marches on. When the magnetron was removed from aviation weather avoidance radar, many “experts” stated that low power (e.g., 25 watts peak power) aviation radar would never work properly. Today, thousands of such radars perform exceptionally well, providing pilots and passengers with benefits they only dreamed of—such as Doppler turbulence detection—which were not available on high power magnetron radars.

Technological progress in the marine radar field will eventually herald the demise of the venerable “maggie.” And along with the frequency stability of solid-state transmitters may come a host of new advantages, such as Doppler analysis of squall winds, or even Doppler analysis of wind gusts based on radar reflection from the water surface.

Stay tuned. There is a new radar world coming over the horizon.

Sunday, April 7, 2013

Weather Information for Voyages in the Australian Region by Kenn Batt, BOM

Long before (say 6-12 months) you embark on any voyage, either coastal or oceanic, you need to obtain an idea of the average winds and weather (the climatology) that you may encounter.

A broad overview of what drives our Australian weather at various times of the year, can be obtained from here

To gain an idea of the average coastal wind winds at locations around the Australian coast, then check out this link

For ocean areas check this one

An idea of other weather elements can be gleaned from here

Check out what weather information is available to you via HF and VHF radio whilst at sea (extremely important if you don’t have internet access on board)? All this information can be obtained from this link

How might an El Nino or La Nina impact on my voyage? Check out

At least two weeks before you plan to embark on your voyage, lock into (get in phase with) the weather. This can be achieved by monitoring computer forecast models. There are a range of different models available, some of the better ones are available from (If cruising plan to leave on a favourable weather pattern. If racing then there is generally no such option):

The Bureau of Meteorology website (ACCESS suite of models)

ACCESS model (out to 7 days)

Passage Weather (GFS model from the USA)

An excellent site (free trial then payment required) for higher resolution models is

At least the day before you set off, on the day and whilst on your voyage, regularly work down the following checklist:

  1. Are warnings current for my area of interest? or via HF/VHF radio

  1. What is the latest weather situation? or via the Weather Situation which accompanies a coastal or high seas forecast are broadcast on HF/VHF radio.

  1. What is the latest coastal or high seas weather forecast for my area?

[With internet access, the range of computer forecast models (listed above and more) can be accessed as well]

  1. What are the latest weather observations along the coast?

5. What is going on around me? Log wind (direction and speed), atmospheric pressure and cloud types at least every 3 hours and look for trends. This valuable information can assist you to fine-tune forecasts.  Make sure that you have a working barometer on your vessel.

Other information of interest could beTides? or from the tide tables that you have onboard.

Ocean Currents?

At the end of the day the one stop shop is

Do your homework before you depart. Whilst at sea regularly check and update the weather.

Thursday, April 4, 2013

New way to look at ASCAT data

We just figured out a neat trick way to display the ASCAT data, which we have set up for the tropical Atlantic in support of the OAR NW rowing team. 

The trick takes advantage of the fact that there are now two ASCAT satellites, Metop-A and Metop-B which circle the earth over the same ground track, but just 47 minutes apart. Thus we can show them side by side, and if we assume not too much has happened to the winds in 47m, we get a nice broad swath of data across the ocean, from where the boat is now (24N, 63W) all the way to Miami.

The pictures below should up date and be live, so keep in mind there might not have been a satellite pass when you are looking, in which case, check back in a couple hours.

Tuesday, April 2, 2013

Two New Books from Starpath

We are pleased to announce two new books. One is the second edition of our Modern Marine Weather ; the second is a compilation of about 3 year's of magazine articles on navigation and weather from Blue Water Sailing named after that column, Burch at the Helm.

We also offer both of these books as Kindle ebooks, as well as pdf ebooks and our own elibra ebooks.
The Kindle presentation of these at Amazon (linked below) offer an extended look into each book.