500 millibar key

The 500 millibar level is considered part of the midlevels of the atmosphere as well as being *near* the level on non-divergence. For more, read Dines' Compensation Principle in the Graphical Guidance on page 24.

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) 500mb 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. Contour the heights every 50 meters (try to stick with numbers ending with 5 (i.e. 545, 550, etc). Here is an example
5. 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
6. One can also contour the temeratures, or dewpoints if you like. When forecasting thunderstorms, it may be important to contour the temperatures as to find areas of cold mid-level temperatures for instability, warm temperautes to inhibit convection, and cold and warm advection areas to figure out areas in which a cap may be breaking or strengthenging and if the layer is destablizing. Example coming in spring.

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