The featured chart is a called a Hovmoller Diagram. It displays the time evolution of some averaged field. In this case, each row represents the meridonal (north-south) average daily high temperature departure for the midwestern US. Time increases as you go down the chart. The two vertical lines represent the areal average that includes Iowa. Values on the left are to the west of Iowa and likewise values on the right are to the east of Iowa. A neat aspect of this chart is that you can visually see the movement and modulation of air masses. For example in September, you can see movement from upper left to lower right. This represents air masses going from the western part of the domain to the eastern part, with the slope representing the speed of movement. The plot also shows a difference during the summer months to what has happened since early September. The difference is due to having storm tracks every week or so as opposed to persistent air masses during the lazy days of summer. The very dark blues (cold departures) showing up in October are where the snow fell in western Nebraska and South Dakota.

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