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Weather information on board a sailboat

Weather information on board a sailboat

Every skipper, whether on a sailing or motor boat, must always have a clear idea of the current weather situation and the expected forecast.

Before leaving port, at least it is advisable to listen to the weather report broadcast continuously on VHF channel 68 or the relevant channel in your country. If you sail abroad, find out about the reference services in the country you are going to.

Even before arriving by boat you can keep an eye on the development of conditions via the internet. Even on the quay on your laptop, ipad, iphone or other smartphone, you shouldn’t ignore this source to stay up to date. Remember that a forecast is just a forecast and it is perhaps superfluous to say that the conditions you will encounter may be different!

Before the warm front

Acquire an overview

When looking at the forecasts, don’t focus exclusively on your navigation area and on too narrow a time window. Try to get an overview of the previous and next weather evolution for a large area around you. This is because while the forecasts are fairly accurate overall, the real weather could be slightly ahead or behind. A depressive centre could pass a little further south or north bringing very different conditions.

The first step would therefore be to view the synoptic maps with the current situation and subsequent forecasts. From here you can quickly check if there are any important weather systems approaching. In particular, warm or cold fronts with associated precipitation and probability of sustained winds.

Before the warm front

Synoptic and weather maps

You can view the synoptic maps of your country on your national forecasting website , I also recommend using the maps developed by the British meteorological service, MetOffice, which enjoy an excellent reputation for their readability and cover all of Europe.

The interpretation of synoptic charts

Synoptic charts require a bit of habit and knowledge to interpret them. A meteorology course or a book can help you understand better but with a little patience you will also find plenty of online resources. It is only your curiosity that will get you to understand them: The basic principles are not very difficult.

The synoptic maps are a graphical representation of atmospheric pressure but also contain a lot of additional information. The difference in pressure between two areas is what generates moving air flows, therefore the starting point for understanding the movements of the air masses. You will also hear about warm and cold, occluded and stationary fronts and stable and unstable air masses.

Before the warm front

High and low pressures

On a synoptic map you will find the main centres of high and low pressures. The corresponding value expressed in millibar, or more precisely ecto-pascal, will also be indicated. The average is 1013 ecto-pascals. Therefore the centre of an anticyclone will have a pressure higher than this value, for example 1030. While the centre of a low pressure we will find values below 1013.

For example, you will hear of a “very deep” anticyclone indicating very low pressure. For example, a depression whose centre is around 960 ecto-pascals will likely cause a lot of damage to the ground. The deeper an anticyclone the greater will be the intensity of the wind created by the disparity in atmospheric pressure.

The distance between the isobars on a synoptic map is called the isobaric gradient. The closer the lines are to each other, the stronger the air will blow. It is therefore possible to deduce the average wind expected in an area based on the distance between the isobars.

Air masses do not move in a straight line from high to low pressure centres. They move according to a clockwise rotary motion out of the anticyclonic zones and clockwise in entry towards the depressions. There is an angle of about 15 degrees to the isobars due to the Coriolis effect. The above is valid for the northern hemisphere, the directions of rotation are reversed in the southern hemisphere.

Weather: Warm Sector

Changes in atmospheric pressure

This is really the essential piece to understand the situation as a whole. It is no coincidence that the barometer is one of the essential tools on board even if few pay attention to it. To reach useful conclusions it is in fact necessary not only to observe the instantaneous level of atmospheric pressure but also its change over time.

Rapid pressure changes always correspond to strong winds even with increasing pressure. Knowing how to interpret these changes will help us understand if the weather is getting worse or better and what to expect.

A synoptic map, in addition to showing the graphic representation of the atmospheric pressure, also shows the main fronts. Warm fronts, cold fronts, stationary, occluded, lines of instability. Fronts are the points where air masses of different temperatures meet. This temperature difference always causes precipitations.

So understanding when we will meet a front will help us to understand if we will find rain and what weather will follow after the passage of the front. After a warm front we will find overcast and gloomy skies. But, after a cold front, the weather should start to improve and the sky should open up.

Weather: Warm Sector

From the current situation to the forecast

Among the synoptic maps, the first describes the current situation, the subsequent ones are usually forecasts every 12 hours. By observing these “snapshot” of the situation, you will therefore have to learn to understand the evolution of weather systems that always move eastwards from the west. Weather systems are very dynamic and constantly evolving and shifting.

At sea with our boat we are relatively static with respect to their displacement. So understanding the movements of the weather systems over the days and the time we can expect is very important. You need to understand what will happen before, during and after warm, cold or occluded fronts. This alone will lead us to understand the conditions we will encounter.

In fact, weather systems move very quickly compared to a sailboat. Only monsters like IMOCAs or powerful record-breaking trimarans can play cat and mouse with lows. We mere mortals will therefore “suffer” the time we find and we must learn to evaluate whether to postpone a crossing or seek shelter.

In the open ocean where there are no ports like the Global Ocean Race on a couple of occasions we have deliberately tried to avoid the worst. To avoid deep depressions we had to act a few days in advance despite being on a racing boat. This gives you an idea of the smallness of a boat in relation to a weather system. Above all, the insignificance of our movements compared to the movements of a depression.

Weather - the approach of the cold front

Synoptic maps and satellite images

The synoptic maps clearly show the depressive centres and active fronts associated with each low. If you have a good knowledge of meteorology, it will be easy to deduce the wind you will encounter.

In addition to this, the cloud cover, the type of clouds and the probability of precipitation. The clearest way to make this logical step is to display a synoptic map superimposed on a satellite image. The latter will also clearly show cloud cover not indicated on the synoptic chart.

Passage of the cold front

The weather bulletin and storm warnings

To view the bulletin associated with the synoptic maps you can use the your national weather forecasting website.These sites allows you to easily switch from viewing the observations and forecasts and reading the weather bulletin. The latter is provided by national authority of your country for weather forecasting. Here you will also find the State of the Sea and Wind at 10 meters as well as the storm warnings.

Personally, I find that the bulletin is perhaps the most cryptic way of presenting a prediction. Without having an eye on the synoptic map, we will therefore listen to the force of the wind and wave motion for our area. We will struggle to contextualise these indications with respect to the general situation.

There are therefore tons of other Internet services that deal with weather. I advise you to visit some of them both from your computer and with your smartphone and then save your favourites.

For example Windy.com is one of the most visited sites. All the data disclosed are the product of automatic processing. However you can switch from model to model and it is a very useful resource.

Passage of the cold front

The wind maps

There are many other sites that offer detailed forecasts for particular regions of the world. Many also report a graphical representation of the intensity and direction of the air, the so-called wind maps. Among the most known sites Windfinder, Windy, etc.

If you are using the wind maps for the first time, check the units of measurement. The colour scale can correspond to meters per second, knots or Beaufort. I would like to point out two other sites which I found Passageweather and Weatheronline easy to consult.

Cold front

Weather observations

If you want to know what conditions are in at the current time, you can use a site that also offers real-time observations, for example Windfinder and Xcweather.

The reality and the weather forecast

It is very important to get used to cross referencing forecasts with observations at sea. Check the correct calibration of your barometer and if you have instruments, check direction and intensity of the wind. This is even more important when you wait for a front to pass which will lead to a rapid change in conditions and a rotation of the wind. This could happen earlier or later than forecast. After all, it is useless to be at sea with heavy rain and strong wind and say it should be sunny.

Usually the problem is not in fact the availability of information and sources of forecasts. The problem lies in the ability to interpret this information and reconcile it with reality and notice the inevitable deviations and consequences.

Cold front

Grib files

The wind maps show a series of images at intervals every 3 or 6 hours updated periodically. The ultimate source of each of these images is a forecast model. Each model calculates air intensity, pressure and precipitation for a grid of points whose distance determines the resolution of the model. The most popular model is the GFS of the American Meteorological Service NOAA, but there are others.

The wind maps are often considered useful information only to the racing sailor but it is only a matter of habit. After all, it is only a different representation of the information that the bulletin already offers us. Once you get used to it, you can appreciate the immediacy of interpretation.

The gribs for advanced users

More advanced users can download information from a model such as the GFS via files called GRIBs. These are downloaded to a computer for further analysis and processing with dedicated programs.

Many dedicated websites allow you to request customised wind maps for your needs. You need to download and install a grib viewer or navigation software on your computer to use them.

I should also mention the Predictwind site, which for the most serious racers allows, among the various paid services, to obtain high resolution wind maps. With these you can calculate the optimal route for a transfer or a regatta online.

Local phenomenaLocal phenomena [/ caption]

Downloading weather data at sea

If you do not have an internet connection and for example you are sailing you can get personalised wind fields via satellite email. Once downloaded, you can view them with a grib viewer or your navigation software. The service offered by the Saildocs site is free and well suited for use via satellite phone.

To conclude this topic, I will briefly mention the site Wunderground. The site is interactive, one of the most advanced sites in circulation and allows you to superimpose wind fields with satellite images although overtime Windy.com has certainly gained more in popularity and offers similar features. Both let you compare wind maps obtained with different models. It is a bit complicated to use and is only for the most demanding users!

Local phenomena

How to interpret weather data

You can use wind maps published on one of the many sites available. Or even do your own analysis and evaluations on a navigation software with custom wind maps. We must never be under the illusion that this is the panacea for all meteorological problems.

The wind maps provide a clear and immediate view of the direction and intensity of the air masses. However, you absolutely must have an overview of the meteorological systems that generate these winds. This is to understand what you will find in the sky above your head.

The synoptic maps allow you to know the location of the main lows and highs. That is the areas of high and low pressure that are the basis of understanding the movement of air masses and therefore of the air. The synoptic charts also show the main fronts, hot, cold and occluded. From these you should also deduce the cloud cover, although for this the satellite images and subsequent simulations are even more effective.

Rain in the pot-au-noir (doldrums)


There are many tools we can use to understand what we can expect from the weather. From the humble on-board barometer to the powerful weather data processing models that can be easily used on a smartphone. As often happens, while this information is easily accessible, you still need to be able to interpret it. Especially in order to notice if reality and forecast do not go hand in hand.

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