11 November 2014

Red sky and weather forecasts

"Red sky in morning
Sailors warning"
goes the ditty. Despite the greyness and wetness to come,  to wake up to a world gone pink is astonishing ... and saves you from having to check the weather forecast. Today is definitely a raincoat day.
Looking south east

North east (there's a crows' nest at the top of that tree)

South west (Stroud Green Road towards Finsbury Park)

North west (SGR towards Crouch End)
That great British institution, the Met Office has looked at the science behind some weather sayings (here).

"The [red sky] saying is most reliable when weather systems predominantly come from the west as they do in the UK....

A red sky appears when dust and small particles are trapped in the atmosphere by high pressure. This scatters blue light and leaving only red light to give the sky its notable appearance.

... "Red sky in the morning, shepherds warning" means a red sky appears due to the high pressure weather system having already moved east, meaning the good weather has passed, most likely making way for a wet and windy low pressure system."

So it would seem that being surrounded by a red sky means the good weather (high pressure) is moving past.

But the forecast for London gives a 10% chance of rain [how do they figure that out? what does % actually mean?]. Further west - one cloud west - there are to be "outbreaks of rain, often heavy".
This analysis of the saying reports:

" If weather systems and their associated clouds are moving from south to north (as can occasionally occur), however, then the saying does not hold."

The weather in the UK is so complicated! It's amazing that forecasters get it right as often as they do. Back before the days of computer modelling of weather systems, the story goes that someone did an analysis of the accuracy of weather forecasts, and found that the most accurate forecast was "same weather tomorrow as today".

And sure enough, in the interests of transparency the Met Office has been analysing the accuracy of its forecasts, both temperature and weather types (here).

  • 73.3% of three hourly weather is correctly forecast as 'rain' on the current day (12-month period).
  • Target for 2013/14 is 70%.
  • It is difficult to forecast rainfall at a pinpoint location, therefore the target for this element is not as high as for others. For example, one location may have rain and a location a mile away may remain dry."

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