During World Mathematical Year 2000 a sequence of posters were displayed month by month in the trains of the London Underground aiming to stimulate, fascinate - even infuriate passengers! Keith Moffatt tells us about three of the posters from the series.
Perhaps the most sinister weather phenomenon in the world is the twister - that dark, dangerous funnel drooping from the clouds that weaves its way across the landscape, leaving a narrow trail of devastation in its wake.