There are situations where we can not be satisfied with weather forecasts. To plan a trip sometimes we have to know how to look to the past to understand the dynamics of the dominant weather elements changing across time and seasons.
The forecasts have a time limit. Usually no forecast should be trusted for more than 4/5 days . But when we are planning long journeys, or need to decide which is the best time to sail in an ocean, future predictions are not enough. We need to obtain a general understanding of the conditions prevailing during different periods of time. We therefore have to gather information by looking at past experience about weather conditions that we will find in a given period in an ocean, or along a route. In a sense, to safely navigate the future we have to make treasure of the past weather information.
If we want to cross the Atlantic on the route of the trade winds, which is the most favorable period of the year? When does the hurricane season ends? And when it starts?
If we plan a long trip, which will see us busy for a long time at sea, we must carefully choose the most favorable seasons to cross oceans and navigate safely. Short and medium range weather forecasts, we can recover in the course of our navigation, are not enough. First we need to understand what are the most favorable seasons and the months where we will meet the most benign weather for our navigation. To do this we must study the past weather, looking at the experiences accumulated over the years by those who have sailed before us and making use of climate data from the past, or by studying the Pilot Charts.
These are one of the reference points for all sailors interested in wind and currents conditions in all the months of the year and in the different oceans. They offer a simple and immediate way to see wind conditions (intensity and direction) and tides and ice on all the seas of the world, for all twelve months of the year. These charts were compiled by collecting data accumulated over time from boats that have sailed these seas, and report the monthly average values of the wind, currents and ice limits.
Pilot Chart, North Pacific, January
Pilot Chart can be downloaded in PDF format from the following link
The Pilot Chart offers a reasonable amount of information, but have limitations that we must take into account. The displayed data are monthly averages, built on non-homogeneous data. Unfortunately the majority of the data collected was concentrated along the commercial routes, where the transiting vessels have over time provided information. In other areas of the globe, little frequented by ships, the data used for the compilation of the Pilots are sparse or unreliable. However, the monthly average data provided by the Charts Pilots remain a useful tool for preliminary analysis of an ocean or sea area.
Today, however, we can do more. Thanks to the weather information systematically collected and made available in electronic format, we can view weather data of the past in a convenient and flexible format, such as GRIB files, and use this information to simulate our future cruises, day by day, in any region of the world .
To plan my future navigations I compiled wind speed and direction obtained from the GFS model in GRIB format for different periods of the past. This information can be of great use to those who wish to view the wind conditions of the past in preparation of crossings, or to plan cruises in certain areas of the planet.
With this contribution I want to make available to sailors an additional tool that can help them in route planning. The files that I prepared are a “modern” version of the classic and digital Pilot Chart, with the advantage of the electronic format which makes it possible to analyze with modern software the past weather conditions and to plan “virtual” navigations in the past. Furthermore, the data that I have collected are very detailed, being divided by month and day on a finer spatial grid. It is a remarkable improvement compared to the Pilots, which offer instead simple monthly averages on large scale charts.
From the GFS model, for some past years past, I compiled the wind speed and direction for each day of the year on a three-hour interval: 3:00; 6:00; 9:00; 12:00; 15:00; 18:00; 21:00; 24.00 (UTC).
The GFS model is initialized and updated with new data every six hours. Precisely at: 00; 06; 12 and 18 UTC. In order to provide the most realistic representation of the wind conditions on the globe I have only included the forecast data next to the model update times, according to the following schedule:
- Forecast at 03 and 06 processed from the model at 00.
- Forecast at 09 and 12 processed from the model at 06.
- Forecast at 12 and 15 processed from the model at 12.
- Forecast at 18 and 12 processed from the model at 18.
- Forecast at 21 and 24 processed from the model at 18.
For every day of the year, and for all 365 days of the years I have compiled for every three hours the wind forecast data in GRIB format. In this way we can have a very accurate wind data base useful for planning in detail any navigation in the globe.
The GRIB files that I made are broken down by month. They are quite heavy files, on average 120 Megabite for each month. They are therefore to be download only with a good internet connection. I think they are a very useful tool for those who want to plan and simulate routes and crossings worldwide.
The data is in GRIB format 2. Not all software that read GRIB files is able to read this newer format. Moreover, given the size of the files, not all programs are able to view them. I have not experimented a lot with the different GRIB viewer software available. I limited myself to use these data with a couple of Mac software that I use for weather analysis: LuckGrib and qtVlm.
GFS global model, wind, August 2014 – in LuckGrib
Display of wind data in August 2014, GFS model with LuckGrib
Route planning (Las Palmas – Bermuda) in August 2014, with GFS wind data with qtVlm
I hope that the data proposed here may assist sailors engaged in planning and simulating routes and crossings. For any further information please email me directly: Paolo.
Links to files follows, in brackets the size in MB: