OK. I'll take a stab at shedding some light on this one. But please keep in mind that I'm *not* a trained meteorologist!
Although I've taken a couple meteorology classes at U of Hawaii back in the 80's, including a 300-level course on Tropical Meteorology, my main interest (hence the style of my posts) is with Mesoscale Convective Systems (areas of persistent thunderstorms) as they relate to Tropical Cyclone formation, intensification and, when it happens, regeneration of remnant systems.
And while the study of vorticity can become quite complicated, with advanced mathematics involved, and the fact that there are various 'types' of vorticity, at least the *concept* of vorticity is not difficult to understand.
So, with due apologies to the Mets that contribute and 'lurk' on these forums, here goes.
Think of a figure skater, as she is spinning. She starts off spinning slowly, and as she pulls her arms in closer to her body, she spins faster and faster. Finally, as she suddenly extends her arms out fully, she stops spinning. Although used more often as the classic 'textbook' example of the "conservation of angular momentum", you can also think of it as a simplified example of one type of vorticity. Vorticity = Spinning. That's essentially the take-away of this whole discussion.
Now getting back to the chart with the cool color shadings you were asking about. This is a global computer *forecast* model of *where* vorticity will occur in the atmosphere, and how *strong* that vorticity (spinning) is forecast to be.
The stronger the 'Positive' vorticity, the faster the spin (*counter-clockwise* in the northern hemisphere) and is illustrated with shades of yellow, orange and then red, with values of 2 to 40.
Conversely, 'negative' vorticity (the spin is *Clockwise* in the northern hemisphere) is depicted in shades of green, light blue and then dark blue, with values ranging from -2 to -40 (think below zero).
And, as Ed has so succinctly pointed out ...
Quote:
The colors to the right are associated with areas of positive vorticity (rising air - bad weather - lower pressure) and the colors to the left represent negative vorticity (sinking air - good weather - higher pressure). The larger the number, the stronger the vorticity (positive or negative).
In areas where the atmosphere is NOT spinning clockwise OR counter-clockwise, but where winds are blowing in more-or-less *straight* lines from some direction on the synoptic (large) scale, the vorticity is nearly ZERO, and shown as WHITE on the color scale at the bottom of the chart.
Important to note in the following graphic that the atmosphere has nearly ZERO vorticity ... 2 units or less ... over most areas of the earth at any given time, at least at the 950mb (1000 foot) level shown in the chart.
Interestingly, you can see from the graphic that the small positive vorticity area near 10N and 145W (small elongated tan area) is associated with a weak tropical low that has now developed into Tropical Storm 'Kika'. It's expected to pass harmlessly, far to the south of the Hawaiian Islands. See the 'Other Basins' forum for the latest info on TS Kika.
And you can also clearly see the remnant circulation of TS 'Edouard' over Texas and the newly christened Eastern Pacific Tropical Storm 'Hernan'. Also of note is the Bahamas wave and the next wave coming off the west coast of Africa. And a handful of extra-tropical storms at high northern latitudes.
It's worth mentioning that atmospheric vorticity is best (most often) analyzed and modeled at the 500mb height in the atmosphere, whereas the chart you linked to is the 950mb vorticity analysis and forecast, which is much closer to the earth's surface.
Here are a few links to learn more about vorticity, but be aware that this subject can quickly become rather complicated and, frankly, over my head too !! But the following links are not too hard to wrap your mind around, and the Flash Tutorial below is especially user-friendly and highly recommended !! What's the word? Edu-tainment?
http://www.theweatherprediction.com/habyhints/56/
Here's the nice, short (two and a half minutes) Flash Tutorial that discusses vorticity as it relates to tropical cyclone development and movement.
As synchronicity would have it, it actually uses, as its' example, the same GFS chart, with the exact same color shading, as the link you provided! The only difference is that it depicts vorticity at the 850mb height, instead of the 950mb chart you linked to.
This is a MUST SEE if you want to see how vorticity can affect the tracks of tropical cyclones.
http://stormjunkie.com/training/models/fsu_models/fsu_850vort/fsu_850vort.html
Finally, there are (or can be) a number of misconceptions about vorticity, even among meteorologists. Credit goes to Meteorologist Jeff Haby for delineating these common misconceptions regarding vorticity. 'Misconceptions' 3, 4 and 5 are most relevant to this discussion and are the most easily comprehensible for most of us.
http://www.theweatherprediction.com/charts/500/vorticity/
Keep up your interests and keep learning and you will go far in your understanding! 
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"Don't Get Stuck on Stupid" - General Honore, following Hurricane Katrina
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