Wednesday, January 17, 2018

Sadler Tropical Atlases

Before the internet, the US Navy and several universities were sources of climatic marine weather data that could be traced directly back to Matthew Fontaine Maury in the mid 1800s. It was his original idea to study old logbooks to extract and record weather and sea state observations and compile them into what evolved into modern Pilot Charts and now COGOW, which replaces even the Pilot Charts for climatic wind data. 

This work was extended in tropical waters worldwide in the 1980s by James C. Sadler at the University of Hawaii where he did the same thing with hundreds of thousands of ship observations from the mid 1800s to mid 1980s. 

Samples of Sadler’s Tropical Atlas for July are shown below. They remain an interesting depiction of average wind flow each month that could help in the planning of ocean voyages across the tropics—or at least help us understand why the traditional routes evolved as they have. 

Samples of wind and pressure data for July from James C. Sadler’s Tropical Altas. The wind lines are are called stream lines. They show direction without speed. Actual average speeds (in m/s) and the number of observations that led to the average are in the small numbers across the chart. The data are based on compilations of shipboard observations over many years. This wonderful work for its time has now been superseded by the COGOW program

Volume 2 covers twelve months for the tropical and sub-tropical Pacific Ocean. An electronic copy of Vol. 2 is available at the University of Hawaii meteorology web site  at, although you will not find any link to this location from their web site.

Volume 1 covers Atlantic and Indian Oceans, available in some libraries.

These plots are easier to use than Pilot Charts for seeing obvious sailing routes. They can be useful to investigating winning routes in ocean races and historic sailing routes of discovery. They would also indicate likely drift routes across the ocean.

LuckGrib is one viewer that can show true streamlines (example below).  OpenCPN has an option called "particle map" that simulates them on some level. Streamlines are a nice way to note forecasted convergence and divergence zones. Not many viewers show true streamlines as it is a difficult computation.

A streamline display of 10m winds from GFS shown in LuckGrib.

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