Upper air data is taken from balloons which measure upper air conditions over a particular location. This includes plots of data on the standard mandatory and height levels, contours of various upper air parameters and a set of sounding plots for most US sites. These data update once every 12 hours at around 9:45 EST.
This is contour plot of thickness in meters with an interval of 60 meters. Thickness is a measure of mean temperature in a layer of the atmosphere and ranges from 4900m in cold wintertime atmospheres to 5900m in warm summertime atmospheres. The 5400 meter line is often used as an indicator of the rain-snow line.