Stanley to Archangel, and all points in between.

It probably won’t surprise many of you to hear that hear that the Earth is generally warmer at the equator, and colder towards the poles. I base my holiday plans heavily on latitude: going north (from England) for snow, and south for sunbathing. We all know this, but OldWeather has now completed enough log pages that we can prove it just from the logbook observations – the image below shows how air temperature changes with latitude, using the 120,000 temperature observations from pages that have already been examined by the three people we need to provide reliable results.

Air temperature as a function of latitude, with some of the more common port locations marked.

Air temperature as a function of latitude, with some of the more common port locations marked.

So it’s warmer (on average) in Singapore, and colder in Scandinavia; we didn’t need the logbook records to tell us that, but that doesn’t mean that this way of looking at the data is not interesting – partly because comparing the temperature records with others made at the same latitude is a good way of finding outliers: values that are likely to be errors in either recording or transcription.

One thing we can immediately see from the figure is the spikes at locations associated with ports. The spikes go both up and down, meaning there more of both high and low temperatures at these locations. Partly this will be a a physical effect – temperatures over land do vary more than those over the ocean – but it’s also partly an artefact of the way I’ve made the plot: The Navy ships spend a lot of time in port, so we have many more observations from those locations, and so more unusually high or low values. Even in the ports, however, there are very few really way out values, but some are suspicious: are there really marine temperatures below 0F at about 45N? (Seawater freezes at 29F) Those values come from HMS Bayano, off the Canadian coast in December, (thanks captain spudman and lieutenant Dinsdale, among others) so very low temperatures can’t instantly be ruled out, but they will need further investigation.

The variation of barometer height (air pressure) with latitude is less well known, but just as interesting: this picture is dominated by the low pressure variability in the tropics (steady weather) and the much more variable pressure in the higher latitudes (anticyclones, depressions and storms). We can see very nicely the transition, in the southern hemisphere, from the steady trade-wind regions to the famous ‘roaring forties’ and ‘furious fifties’.

Barometer height (pressure) as a function of latitude

Barometer height (pressure) as a function of latitude

Captains care about the air pressure because it warns them of changes in the wind. This sort of plot isn’t ideal for showing winds, because the wind measurements are restricted to the Beaufort scale categories, but we can still see where the strong winds are to be found. Cruising in the North Atlantic, the Royal Navy’s main stamping ground, was clearly no picnic: with temperatures down to freezing, variable weather and strong winds.

Wind force as a function of latitude.

Wind force as a function of latitude.

The Beaufort scale only goes up to 12; extensions are sometimes used for severe tropical storms, but the value of 15 recorded by HMS Cambrian in Rosyth dockyard in March 1919 is not credible. (Though I congratulate captain MamaLizard and the crew on correctly entering the value in the log – we always want the value written, even when it’s obviously an error). If we disregard the Cambrian’s exaggerations, there are four reports of wind force 12 so far, but they are all typographical errors – it’s not much of a slip of the pen to turn ‘1-2′ into ’12’. We’re still waiting for our first real hurricane.

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