850 millibar key

The 850 millibar level is part of the low levels of the atmosphere, and in higher elevations is often very near the surface..much like 925 millibars or maybe even higher pressure. Often in the warmer months, the air will mix up to 850 millibars allowing a forecaster to use the 850 chart and the surface charts to get a good idea of what is going on in the boundary layer. One can often forecast the daytime high temperature in the summer months based on the 850 temerature. 850 moisture, 850 winds, areas of warm advection and many other features are important to note on the 850 chart.

The McIDAS station plots are not like those seen on most upper air maps. Page 17 of the Graphical Guidance details how to decode an upper air station plot on most maps. Our Mcidas plots things a little easier for students.

The temperature and dewpoint are found on the left side (yellow numbers), with the temperature on top. Most charts plot the temperature and dewpoint depression (temp-dewpoint=dewpoint depression). The wind barb is in white and points towards the direction of the wind. If the line extends from the center straight down, it is a southerly wind (the wind is coming from the south). The light blue (cyan) numbers represent the height of 500 millibars for that station. Simply add a zero at the end to know the actual height, in meters (i.e. 563=5630 meters). The darker blue numbers represent the International station ID # which is not important for forecasting but allows one to know exactly which station he/she is looking at.

Practice:
1. If you are on the black background maps, please click over to the white background maps. The link is at the top of the McIDAS section
2. Print out a Floater (not a US) 850mb map. Please make sure that a staff member is present before you do this.
3. Draw in streamlines to represent the wind flow. Here is an example
(This is a 250 mb map, but it works the same way)
4. Pick out the fastest wind speeds. Start with the fastest one possible and draw a circle around it. Then find wind speeds a little less and contour them, and so on. Lines of constant wind speed are called isotachs. Here is an example of an isotach analysis
5. Contour temperatures. Contour every two degrees, using orange or red for temeratures above zero (freezing), and blue for temeratures below or equal to zero degrees celcius. Lines of constant temperature are called isotherms
6. Contour dewpoints. Contour every two degrees using green for areas of higher moisture (you may want to use different shades for each level of mositure) and brown for areas of drier air..especially areas of drier air that is intruding into moister air. Lines of constant values of moisture are known as isodrosotherms.

The Upper Air Help Page The Upper Air Page