Philip Hodges' blog - a random mixture of data visualisation, UX/UI, interactive and visual design.

The HTML5 interactive charts can be viewed here:

http://lab.zoho.co.uk/lab/sandettie-lightship-buoy-data/

With this year’s late start to Spring the question was recently asked – “would this have an effect on the summer sea temperature?” This question was especially relevant to some friends of mine who are booked in for solo and relay channel crossings. Especially for channel soloists, where a couple of degrees diference to the water temp can have a huge impact when swimming 12-16 hours in open water.

So we attempted to answer this question :-)

There are various offshore monitoring stations but the most relevant to Channel swimmers is the Sandette Lightship Buoy which is owned and operated by the UK Met Office. Its position is 51.103 N 1.800 E (51°6’9″ N 1°48’0″ E) – roughly due North from Calais and due East of Dover.

The Channel Swimming & Piloting Federation (CS&PF) website has detailed charts plotting the data recorded from this buoy going back to 2004. However it is very “spiky” data and as you can only view the charts for each year it is difficult to get a quick overview of all the data. So the first thing I did was scrape their data and put it into a single time series chart. This gave you a better overview but was still difficult to interpret.

So the next thing to do was to plot the moving average and smooth out the spikes. This is a great way to get a better look at the underlying curve and thus makes it easier to visually interpret the shae and trend of the changes.

So it appears that although there is a “seasonal lag” between the water temp and air temp, it seems to only be over a 1-2 week period i.e. a cold Winter or late start to Spring does not seem to have a impact on the water temperature in July/August. The channel season water temperature is more dependent on the air temperature during June/July then it is Feb/March.

One important thing to note is that because Sandettie station is a buoy, the air temperature recorded there is very near the sea surface. It means that the sea will greatly moderate the air temperature reading. To get a better understanding of the air temperature effect on the water you really need to source the air temperature data from elsewhere. Possibly some Met data for Dover. If anyone can recommend a historical data set for air temperature in Dover/Calais then please send me the link and I will swap out the air temp data from the buoy.

The charts where created using dygraphs JavaScript Visualization Library which you can download from here: http://dygraphs.com/