Sea temperature, depth and wind, logged under sail

The route, measured

Colour the track by sea temperature, depth or wind. Underneath is the EMODnet seabed relief: pale over the continental shelf, blue where the ocean floor drops away. With depth, EMODnet’s own depth contours appear in white, so our soundings can be checked against the reference seabed model; zoom in close to the coast and every dot is a 10-second sounding. Hover or tap the track for the values at that point.

Sea temperature along the way

The hull sensor reported the sea temperature twice a second. Plotted against distance sailed, the passage south is a climb of 12 °C, from the cold, tide-mixed Iroise Sea off Brittany to subtropical water around the Selvagens, with a dip off Galicia on the way. Measurements taken in the water like these are what satellite sea-surface temperature products are checked against.

Depth soundings for the seabed map

Our echo sounder finds the bottom down to about 180 m, so we sound the coastal and shelf waters we sail through: 386 nautical miles of seabed so far, from the rocky Iroise Sea and the Breton coast to the Galician shelf and the anchorages of Porto Santo, Madeira and the Selvagens. Soundings like these, collected by ordinary boats, help fill the gaps in the global seabed map that Seabed 2030 is building.

From soundings to contour lines: Porto Santo

A depth contour joins points of equal depth, so the soundings first have to be brought to one water level, and the tide keeps moving it. While we lay at anchor in Porto Santo bay for 33 hours, our echo sounder recorded that tide (chart). We fitted a tide curve to the record and took it off every sounding of our passages in and out of the bay (map). Wherever the corrected track crosses 10, 20, 30, 50, 75 or 100 m, it gives a point on that contour.

To judge these points, we compared them with EMODnet’s seabed grid, whose cells are about 115 m across. Coming in from the east, where the grid rests on a detailed 2008 survey, it reads 1 to 2.5 m shallower than our 10, 20 and 30 m points, mostly because it counts depth from the lowest astronomical tide rather than from mean sea level, and 50.0 m at our 50 m point. South of the bay it rests on a sparser survey, and our 75 m and 100 m points fall inside a single grid cell: the seabed drops 25 m in 90 m there. One visit gives points, not lines. Crowd-sourced bathymetry pools the passages of many boats, for instance in the IHO Data Centre for Digital Bathymetry that Seabed 2030 draws on, until the points join up into contours.

Depths are measured from the water surface and, outside this example, are not corrected for the tide. We check every sounding before publishing it: readings that jump away from their neighbours, sit at either end of the sounder’s range (1.6 m and about 180 m), or show a shallow bottom where the sea is known to be thousands of metres deep are flagged in the download, not deleted. The map shows only the good ones.

True wind at sea

The masthead anemometer measures the apparent wind ten times a second. Combined with our speed and heading it gives the true wind, sampled wherever we sail, including stretches of ocean with no buoy or weather station. Forecasters and researchers can set it against model winds along our track.

It is a sailing instrument, not a calibrated weather station. It sits at the top of the mast, well above the 10 m height weather models use, and it moves with the boat. Averaged over ten minutes or more it makes a fair check on a forecast. We don’t measure air pressure or air temperature yet.

Download the data

Free to use under a Creative Commons Attribution licence (CC BY 4.0): use it, share it, publish with it, and credit Little Cloud Sailing. The files are refreshed each time we update this page; the date is in the file names.