Below is a graph from the time H. Lamb was working, showing changes in the time of year that Cherry trees bloomed in the royal palace in Kyoto, Japan. (Cherry blossom is sacred in Japan; monks have kept continuous, meticulous yearly records of these bloom dates at Kyoto for over a millennia.) This 1956 research has been overtaken by subsequent research and opinions revised accordingly. Yet this pioneering work, although crude by today's standards - and now decades old - is all Lamb (and his contemporaries) had to work with at the time he formulated his idea of the Medieval Warm Period. NB: I hope to return to the more recent research (Mikami 2008) into blossom bloom dates at some point.