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.
Sample 500 mb Plot
The 500 mb chart shows the absolute vorticity (in color contours), the height field (in white lines) and wind vectors.
500 height field The 500 mb level is often refered to as the steering level as most weather systems and precipitation follow the winds at this level. The winds follow the height contours and generally run from 30 to 100 knots. The speeds can be roughly estimated from the 300 mb winds as roughly 2/3rds their magnitude. This level averages around 18,000 feet above sea level and is roughly half-way up through the weather producing part of the atmosphere called the troposphere.
500 mb vorticity field
The vorticity field shows small eddies in the atmosphere that generally are not detectable on the standard height field. Values of 14 (green) or higher highlight those eddies. The higher the vorticity, the stronger the eddy. These eddies can help strengthen surface low pressure systems and induce precipitation and are often used as a predictive tool.
500 mb wind vector field The vector field shows wind direction and speed. Often this can be used to qualitatively show areas of convergence and divergence. In the middle and upper levels of the troposphere, this can be an indicator of existing upward (from divergence) or downward (from convergence) air motion.