A type of synoptic chart showing the thickness of a certain physically defined layer in the atmosphere. Currently it almost always refers to an isobaric thickness chart, that is, a chart of vertical distance between two constant-pressure surfaces and is often proportional to the temperature of that layer. This chart consists of a pattern of thickness lines either drawn directly to data plotted on the chart or, more commonly, drawn by the single graphical process of differential analysis. See isentropic thickness chart, vertical differential chart.