In June 2022 we sailed from Australia to New Caledonia as the borders finally opened. Unfortunately, we had been long awaiting Starlink and therefore did not buy Predict Wind with Iridium Go. We are always on a budget because as you know we do all our sharing for free.
As the journey was only 4 days or so, we employed a weather router and did our homework on shore. When we saw a suitable weather window which the weather router agreed with, we left. The window did have a low which was tracking southeast at a good speed and it looked as though it was not going to affect us.
So, we left with only a Garmin Inreach and SSB. Brand new Gori props and good winds meant that we were doing over 200 miles per day whereas normally Impi is at about 180 nautical miles per day. https://gori-propeller.com/
Here is what happened and how we learnt from that
The low stalled and as our weather router did not warn us, we sailed right into it in the morning of day 3. We ended up in 6 m waves and 35 knots on the side of Impi, waves that looked like mountains.
I contacted New Caledonia MRCC on the emergency broadcasting channels on our SSB but had no reply on any channel. Brent meanwhile was hand steering Impi through the breaking waves that washed across the bridge. I wanted to know the location of the low which is why I contacted or tried to contact MRCC. We later found out that all SSB stations in New Caledonia have been decommissioned.
I contacted our weather router but no reply. We later found out that a dog had priority and a long walk had been in order!
Anyhow, luckily, we could contact friends in Australia via Inreach who gave us the location of the low and we steered north to Koumac to find wider isobars at 1013hPa. When we found those isobars, the winds dropped and the sea state improved. Big sigh of relief!
What are isobars?
Isobars are the plain lines curving across a weather chart. They connect points with the same mean sea level air pressure. Wider isobars portray a weak pressure gradient typical of light wind situations. Tightly packed isobars represent stronger winds.
How are isobars measured?
Isobars are measured in hectoPascals (hPa) which is the international unit for measuring barometric pressure. They are also sometimes expressed in millibars (mb) which is the same thing. For maritime use pressure is measured at sea level (SLP).
The central position of a shallow Low is above 1000hPa and of a moderate Low 980-1000hPa whilst a deep Low is below 980hPa.
The central position of a weak High is about 1015hPa, whereas an intense High is above 1030hPa.
Isobars also indicate the flow of air around a weather system. The strongest winds are usually near cold fronts, low pressure systems, tropical cyclones and in westerlies in south Australian waters.
Near a High’s center are light winds and sometimes areas of low cloud called anticyclonic gloom. Round the edge of a High, winds are sometimes strong. Intense Highs squeeze isobars together creating strong winds. (ref. RYA weather handbook)
The bigger Highs are, the slower they move sometimes, blocking fronts that are trying to follow them. Winter Highs often bring frost whereas summer Highs may bring thunderstorms and hail.
In our case the barometer read 1016hPa on departure in Australia, so a weak High. After two days the barometer dropped from 1014 to 1011hPa over 3 hours. Of course, this always happens at night. The wind increased from 20 knots to 35 knots and the waves were between 4 and 6 meters with breaking crests.
As we were watching the barometer we knew that by morning we would be in foul weather as it dropped to 1010hPa. There is a rule of thumb that if the barometer drops by 3 points over 3 hours you should not continue to head south in the Tradewinds belt.(ref. Astrolabe sailing) So we turned North! There are other guidelines which can be applied.
|Likely significance||Steady pressure drop over 6 hours|
|Alert||Less than 3hPa|
|Caution||3 to 4 hPa|
|Definite Warning||4 to 5 hPa|
|Too late for forecasting||More than 5 hPa|
What information goes into a weather chart?
In Australia the information of more than 700 automated weather systems goes into a weather chart. They add also the information of ships and meteorological weather buoys. In addition, international satellites supply information and monitor the upper atmosphere in terms of temperature and moisture profiles.
How can you get a weather chart when out at sea?
You can get Predict Wind, although it is an expensive subscription.
You can have a variety of charts sent to your phone including isobar charts. You will need a satellite connection to download the charts. For your connection you can use Iridium Go or Starlink, although the latter is as yet not fully operational.
For the different packages on predict wind consult their website https://predictwind.com/
You can use your SSB to get a weather fax, but communications can be unreliable as in our case because many stations are inactive.
Further research into the situation seems to indicate that after Australia it is hard to receive communications as the station in New Delhi India has weak transmission.
According to the Worldwide Marine Radiofacsimile Broadcast Schedules published in October 2022, the Naval Base at Simon’s Town South Africa is also no longer active.
You can get a Grib file via your Iridium phone. In Australia, you can’t get a SIM card or subscription on your Iridium phone which works out of Australia. Go figure! Not much use to international sailors. It is worth to note that the north of Australia has very poor cell phone connection.
The Grib file sent to you on Iridium is readable via additional software which you have to install on your laptop. You sent a request via your email and then download the chart. This worked quite well for us over the last 10 years of sailing but unfortunately it only gives one model, GFS which is not the preferred model in the Pacific.
If you find yourself in a low and have no weather chart what must you do to avoid it?
You can use Buys Ballot’s law. What is this? Well, in the Northern Hemisphere you can stand with your back to the wind, the low will be to your left, the high to your right. In the Southern Hemisphere the reverse would be true.
This movie by US Captains Training explains the law and how it can be used to avoid the dangerous sector of a low in the Northern Hemisphere. Remember that in the Southern Hemisphere, it is the opposite: winds around a Low would be going clockwise.
What did we do in our situation?
Well, our friends in Australia confirmed to us via Inreach that we had to sail North away from our course to Noumea to find the 1013 Isobar which was widely spaced. So, we used our barometer to find that isobar.
We like Inreach in that you can stop and start the payments on a monthly basis and that it is totally waterproof. You can hook the device to an App called Earthmate so that you can type with an Ipad or other device. Downside is that you can only get spot weather forecasts.
How can you estimate the wind speed by looking at the isobars?
You can guess the strength of the wind of a familiar area by looking at a weather chart. If you are in an unfamiliar area you can calculate the wind speed by using a mathematical formula.
Theory states that under stable conditions, wind speed over the open ocean and in areas away from the equator is proportionate to the pressure gradient divided by the sine of the latitude. Because of surface friction, the wind direction is about 15 degrees away from the isobar lines towards lower pressure. ( ref. Thomas McCullough 2003 full article available on oceannavigator.com)
To understand better why we need trigonometry formulae to calculate wind speed, have a look at this movie.
In order to find the sine of the degree of the latitude you need a scientific calculator. This movie explains how to do it.
What is a pressure gradient? Well it is the difference in pressure between high and low pressure areas. So as the pressure gradient increases, the speed of the wind increases.
What changes have we made in our communication and weather monitoring systems?
We have now an integrated barometer in our B&G systems as well as a barometer app (Marine Barograph) on a permanently assigned Iphone.
We have purchased Starlink here in New Zealand even though it will not work in a lot of countries West from here or in New Caledonia. We will probably have to combine this with Predict wind for crossings.
We have become a lot wiser! Now we will always ignore our destination and steer to widely spaced isobars to avoid the worst sea state and winds!