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 500 mb
This map is a composite 500 mb height and sea level pressure maps for the entire northern hemisphere. The color contours are the 500 mb height field which shows middle level air flow around the mid-latitudes. Air follows the contours and the tighter the packing, the faster the flow. Where air is moving from north to south, temperatures are generally below normal. Where air is moving south to north, temperatures are generally above normal. The sea level pressure field is plotted in black lines. High and low pressure systems can be determined from the locations of Hs and Ls on the plot. Low pressure systems usually have some kind of precipitation associated with them whether it be rain or snow.