Seeing ships in rain radar

I’m starting to play around with “nowcasting” – doing weather forecasts based on recent rain radar images.  The Met Office provides a feed of radar images you can get from their Datapoint service.

The first plot I made of the data was number of rainy 15 minute periods during May (shown with and without a base map):

rawradar radarandbase

 

There are many interesting features here.  Firstly a few probably true observations about rain:

  • You can clearly distinguish land and sea – there is more rain over land.
  • You can see hilly regions with yet more rain.

However there are many other features that I guess aren’t true observations about rain:

  • You can see the two shipping lanes in the English Channel with the separation zone between them.
  • You can see the individual radar sites don’t give uniform coverage in all directions.
  • There are other linear features of which I’m not sure of the origin – any ideas?

All these features suggest that I can’t treat this data as perfect truth data for nowcasting.  😦

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Shipping 1750-1855 visualised

Inspired by Spatial Analysis’s blog post I thought I would look further at CLIWOC’s dataset of historic shipping.  In particular the previous plots don’t show the direction of travel so you can’t understand triangular trade or the nature of the trade winds. This struck me as a nice opportunity to experiment with semi-transparent plots in R as transparency on paths allow the eye to see aggregate behaviour.

We can use the cyclic nature of the colour wheel to view the month and direction of travel.  The key is in the middle of the plot.  By month, red means the month of January and cyan July.  By direction, red is north, west is dark purple, south is cyan and east is greeny-yellow.

A view of shipping 1750-1855

So what can we see?

In both the Spanish and French shipping you can see the effect of the trade winds which mean you want to go near the poles to get a westerly wind to drive ships east to home.  You can also see that ships seem to set off from the West Indies around June/July (does that correspond with harvests out there?).  You can also see the Spanish ships reaching into South America unlike the French ships.

The Dutch and British shipping reach out east too.  Similarly you can see the effect of the trade winds as ships go far south to go east but take the shortest path to come back west.  British shipping is doing a lot with India and east Africa whereas the Dutch shipping is concentrated out to Dutch East Indies.  The time of year story looks a little less clear but it looks like Dutch ships come home around January.

Also on the Dutch shipping you can clearly see triangular trade from Europe, down to Africa over to the West Indies and back to Europe.