Timekeeping in Navigation and Weather

There are a dozen or so timekeeping systems used in navigation and weather, and we cannot avoid using several of them from the nav station. In the end, the main time we care about is GMT, more properly called UTC, Universal Coordinated Time. All weather data is coordinated and reported according to UTC. In principle we should all be calling this UTC, not GMT, but GMT is still common. It is rather like the fact that we should all be using the metric system for these two subjects—it is in fact US law that we should—but no one does, and no one complains. The BBC still uses GMT.

Universal Coordinated Time (UTC)

UTC is the world standard time system used by all nations to coordinate weather and navigation information. It corresponds to the time used in Greenwich England for half of the year, formerly called Greenwich Mean Time. The town of Greenwich actually switches to daylight saving time in the summer, but all scientists and navigators continue with UTC. The official UTC system (like GMT) does not employ any form of daylight saving time.

Since all weather maps and GPS information uses UTC it would seem we would want to keep our watches and ship's clocks on UTC for convenience. It turns out this is not very convenient in practice. For daily activity—at home or underway—it is much better to have our clocks reading close to what we are used to, which is often referred to as local time.

Local time, however, is a nebulous term, which when used should always be followed up with the definition of what we mean. There are two basic times it could mean. Local time could be the local standard time (that is well-defined) or it could be the local zone time (also well-defined). To confuse matters a bit, local time is also sometimes used in phrases such as Pacific Time, which is intended to be Pacific Standard Time in the winter and Pacific Daylight Time in the summer—the one phrase covering both, depending on the season.

Zone Time (ZT)

Zone time is by far the more precise of the several local times. It is the one that commercial ships and navies use when crossing an ocean—or we all use when sitting at the USCG office taking a license exam! Zone time is determined entirely by the longitude of your vessel at the time you record it. It will differ from UTC by a whole number of hours called the zone description (ZD).

In this time system, the world is divided into 24 time zones , each 15º wide, centered at the standard meridians, which are the longitudes that are multiples of 15, ie 0, 15, 30, 45....165, 180. The borders between time zones thus take place at 7º 30' either side of the standard meridians. The only exceptions are the two zones (ZD = ±12) on either side of the International Date Line, which are only 30 minutes wide (7º 30' of longitude).

If you are keeping zone time (ZT), then you can find UTC from:

UTC = ZT + ZD,

where, again, the ZD is determined by your longitude. This formula is the one that determines (or helps you remember) the sign (±) of the ZD. If your location is slow on UTC, ie any west longitude, then the ZD of that location is +. Eastern longitudes have negative ZDs.

To find the zone description of any particular longitude, round the longitude off to the nearest whole degree, divide by 15, and then round the result off to the nearest whole hour.

Zone time never uses daylight saving time. It is used worldwide. Zone time is never used in civilian matters; it is only for ocean navigation. One could argue that official NOAA Tide and Current Tables are given in what is essentially ZT, but we are more likely to use a reproduction of these, which converts the times to standard times.

Gray lines and bottom labels define zone time zones; boundaries shown on map defines standard time zones. Unfortunately, this nice graphic from nist.gov has the time zones labeled in the landsman's convention. The navigator's ZD of the US West Coast is +8, not -8. Since Zone 0 (UTC) is labeled Z, UTC is often called zulu time, and noted ie 1200z. We leave it to those interested in history to discover why there is no J zone. There was a reason. And if you want to ask trick questions on your navigation tests, note that ZD +12 and ZD -12 are just 30m wide, not the normal 1h.

Standard Time (EST, PST, etc)

Standard time is the time system used for civilian matters and for near coastal and inland navigation. Some coastal weather forecasts specify the local standard time in addition to the UTC of the report. Standard time is essentially the local zone time modified by politics and geography, and then susceptible to changes for daylight saving time.

Standard time zones do not follow longitude lines rigorously as do the zone-time zones, but they will often be approximately along those lines, diverting to follow state and country boundaries, or maybe a river flow. We still speak of the zone descriptions of standard zones in the same way as zone times, namely Eastern Standard Time (EST) has ZD = +5. Eastern Daylight Time would be ZD = +4, and so on. In other words, we would have UTC = EDT + 4h.

A complexity arises because standard times are often described outside of marine navigation circles as, for example, EDT being 4h behind UTC or slow on UTC. This leads to writing EDT = UTC – 4h. This is the same equation (with sides swapped), but in this line of thinking the time zone is described or labeled as -4h. Thus we often see computer and smart phone apps using reversed signs for the time zones, so we have to keep an eye out on this detail.

North American time zones. From nist.gov. That site along with time.gov are good sources for various aspects and history of timekeeping.

Watch Time (WT)

Watch time is the practical solution to time keeping in navigation and weather. It is simply the time on your watch. Thus to navigate by WT, I just need to know the zone description of my watch. If I happen to have my watch set on Pacific Daylight Time, that would correspond to ZD = +7. Thus the ZD of my watch is +7 and that is all I need to know, no matter what longitude I am at as I cross the Pacific.

No matter where I am in the world, I find UTC by:

UTC = WT + ZD.

This is by far the best way to navigate, and we should always do so unless we are compelled to use ZT by labor laws or unions or some government regulation. It is easy to see that if you work day and night on ocean crossing vessels, you would want some semblance of order to your daylight and meal times, which would justify changing the ship's clocks each time you cross a time zone.

On a private vessel, however, this time changing just adds tremendous confusion to your weather and navigation. It is much better to just live with the fact that mid day might be 2 pm on your watch by the time you arrive.... or set it ahead before you leave. In other words, you go an hour or two off local time as you proceed, but that is not distracting. To minimize timekeeping errors do not change your watch time when underway. Wait till you arrive. You are free to set the ZD of your watch that works best for you.

Chronometer Time (CT)

For completeness, we include here also the very worst type of timekeeping, the one called Chronometer Time. It is UTC kept on a 12-hour watch face, without specifying AM or PM! Absolutely no one in the world would consider using such a time system—that is, almost no one. This is the time system used on USCG celestial navigation exams. It is the way they help support navigation schools, and we are grateful to them.

I should add that there are several forms of Universal Time and the conventions on terminology and abbreviations have not settled in yet, which is why so many folks hang on to GMT. The time that is equivalent to GMT is officially called Coordinated Universal Time, Type 1, and abbreviated UTC1, though this formal terminology is not often seen in navigation or weather resources. This is usually abbreviated as UT (Nautical Almanac) and UTC (NOAA weather maps). Some weather maps and forecasts also use the abbreviation Z (zulu) to mark a UTC, as we used to do for GMT. Thus when we see a map valid at 1200Z it means 1200 UTC.

There are other time systems that have implications to navigation and weather, but only indirectly. These include the Julian Time system used by astronomers to keep track of an absolute time stamp for events in the past and future and the Solar Time system used to keep track of time relative to the time the sun crosses your meridian. The latter was used commonly in the old days of navigation, but no longer, though it still comes to play when predicting the passage times of weather satellites in sun-synchronous orbits.

Chart Sounding Datums… who needs um?

I think the answer is no one, but the typical navigation text and reference publication would not be the place to learn that. 

Look at these sample definitions from the standard reference for chart reading called Chart No. 1:

There is no further clarifications of the terms in the booklet. There is definitely an implication here that we need to know what these chart datums are, but we do not. This type of presentation is just another way the government supports navigation schools.

In the top picture the phrase

"Drying heights and contours above chart datum" 

can and should be replaced by

"Drying heights and contours above the water level when the tide height is 0."

Likewise in the bottom picture, the level identification called

"MLLW (Chart Datum)" 

should be replaced with





"Water depth when the tide = 0."

In fact, there is no place in practical navigation where the navigator needs to know what tide level is being used for the chart datum. The depths printed on the chart (called soundings, because the numbers shown on the chart were actually measured at some point) are the expected depths of the water when the tide at that time and place is 0. If the chart reads 13 ft, and the tide is 4 ft at the moment, then we should expect the water to be 17 ft deep at that time. 

Put another way, we should always expect the water depth to be deeper than shown on the chart except in those short periods of time when there happens to be a negative tide.

All nautical charts of all nations work this way.

The chart datum used is actually some arbitrary level of the tide averaged over a long period, usually 19 years. On the US west coast where we have two highs and two lows each day and the two lows are not the same, NOAA uses the 19 year average of the water depth at the lower low of each day and they call that the depth of zero tide, labeled MLLW.  The idea is get some reference plane that will make the tide positive most of the time. 

They could have done something more like the Canadians and decided not to use the lower lows every day, but just the lower lows on the days near the new moon and full moon. This would lead to even fewer days with negative tides.

It really does not matter what they do, so long as the folks making the charts and the folks making the tide tables are still friends and working together. Beyond that the navigator really does not care.
_____________

By the way, with all that said, I must add that this lack of concern does not apply to the vertical height datum used on charts. That is always Mean High Water (MHW) and it is crucial that we know and understand this concept. In fact, the nautical chart is the only place to learn the value of MHW. This crucial data (ie for predicting bridge clearances) is not available in tide tables. It can also vary from one part of the chart to another, in which case there would be multiple listings on the chart..

The Ocean-going Nav Station

Several things will come to mind after a trip across the ocean in a small boat. First the seat. A curved one like this (A) is easy to build and then you are sitting up when heeled, but you still need the foot rest (B) so you can pin your thighs against the bottom of the chart table to stay in place.

The idea of a bungee cord across the top of the table (F) was discovered as a last minute rule beater in an ocean race safety inspection, but turned out to be a great solution i use all the time now.  It not only holds the chart table lid down but you can put books and charts or a laptop under it to hold them in place. To get into the table, just pull it aside. It would seem to be in the way, but it is not.

Pencils and tools holders (D) and (E) are essential. We need our tools outside of the table where they are easily accessible. Put another way, make a rule that absolutely nothing goes in the nav station except your stuff.  Then when it does fill to overflowing with random stuff from everyone, just forget about it. But most of all, do not depend on that space. Let folks use it for whatever they want. You won't be able to find what you need in there, not to mention the lid blocks the light when you lift it, which is a good reminder that you need to always have a pencil flashlight in the pencil holder for reading instruments at night and so on.

A roll-up screen (C) is valuable for blocking the light at night and also for keeping the water off the chart table in rough conditions. I have seen one rig that had two, one transparent plastic, the other solid for night work.

One could image using low lights or custom lights for this night work at night, but often a lot of work has to be done at night and good lighting helps…. but still we do not want to risk getting any light into the eyes of the helmsman.  Sometimes  light will get out of the port light and reflect off of the hull, so the port light has to be covered. (Red lights are useless for navigation. You can't see anything and colors are distorted. It is a myth that red protects night vision. Intensity is the main factor. Red is just usually dim. An equally dim white light would be just as protective.)

And a small fan (G) is a nice luxury in the tropics, since everyone else can be out on deck when the poor navigator has to sit at the table and work, sometimes in very hot conditions.

Not shown is a set of head phones for listening to the radio without disturbing the rest of the crew. Very valuable… as is a piece of rubber non-skid for under the laptop.

We have an extended article on the navigation station and its related tools and nuances in both our Celestial Navigation and our Inland and Coastal Navigation textbooks.

Concept and Use of Meteograms

Meteograms are graphic plots of hourly weather data. They can represent past data or forecasted data. A form of these are used by the NWS for their Hourly Weather Graph presentations that we discuss in the article War Horses and Secret Sources (which I will find and post here).

For now we stress one valuable application, namely a study of the behavior of frontal passage. A meteogram is a perfect record of how everything changes as a front goes by. We can use this to predict what might happen when a front approaches us.

This study can be done with plots of past data or forecasted data. For now we use the latter from this website:

http://www.emc.ncep.noaa.gov/mmb/nammeteograms

These are forecasts, based on the NAM model. To use these, go to that link, then click the region of interest such as SE US as shown below. We will use that data to investigate the passage of a cold front over Alice Town, in the Bahamas. The surface analysis map we use was valid at 18z on Mar 4, 2012.

The index map and an expanded map shows where the station is located. On the next page we see a section of the OPC marine map below, above the Unified Analysis (UA) map at the same time. The UA map is clearer for some analysis, but we get winds near Alice Town better from the OPC map. As a side note this shows how the UA maps can supplement the ocean maps in some cases. They are available every 3hr, whereas the marine maps are only every 6 hours.

The meteogram forecast is shown below the map. Now the job is to study the data before and after the frontal passage to see how the various factors were expected to change. Look at the map first to make your predictions of what will happen, then look to the meteogram to see how this pans out.

Before and after front passage (before /after).  

wind speed ________/ _________
wind direction ________/ _________
pressure ________ /_________
air temp ________/ _________
visibility _________ /_________

Check the map first to get your prediction, then compare with what the meteogram says, and then check too the station reports on the map.

Meteograms are a great way to learn about forecasting and what to expect as certain systems pass over us.  The hourly weather graphs mentioned above also show nicely when the air temp is expected to drop to the dew point, which is when fog will set in.

For mariners or weather watchers in the Pacific NW, you can access the very  nice meteograms at any location of your choice at the UW Atmospheric Sciences Make your own meteograms. These are output from their WRF-GFS  model runs which should be more accurate data in principle than corresponding  ones from the link above because they take into account the terrain and texture of the surfaces and other local factors.

To study front passage and expected wind shifts (ie always a veer) check out our online course on Marine Weather.  The books and software are also sold separately.