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.
Hem - N. Hem 850 mb
This plot is comprised of 850 mb temperatures and 850 mb heights. The color contours are 850 temperatures in C with an interval of 2C. These are the temperatures at roughly the 5000 ft level and can be used to determine maximum temperatures. The line contours are 850 mb heights at an interval of 60m. This shows the location of high and low pressure systems at the 5000 ft level and the contours can be used to describe the flow at that level.