This video shows four things:
1) The weather - the mean-sea-level pressure as simulated by the 20th Century Reanalysis (black contours: solid=low pressure, dashed=high).
2) The observations - grey circles mark observations that we've had for a while, yellow ones observations recently recovered by the ACRE and oldWeather projects.
3) The fog of ignorance - grey fog masks regions where the reconstructions are still very uncertain (as there are too few observations to constrain the reanalysis).
4) The glow of discovery - yellow highlights mark regions where the new observations have produced a large improvement in the confidence of the reconstructions.