Dave, got your PM but hadn't taken the time to answer it until today, so here you go ......
Here's a graphic I posted a while ago
The inversion can be seen starting @ 900 mb (or
about 3,000') where the red line starts moving from left to right, getting warmer as the balloon rises (it should be moving from right to left indicating the temperature is droping as the balloon rises).
In this graphic there are basically 3 inversions, one at the ground layer (not very significant), another at 900mb, and another around 700 mb.
Information on the Feet to Mb conversion
Commonly viewed levels in the atmosphere:
200 MB about 35,000 Feet Good view of the jet stream
300 MB about 30,000 Feet Good view of the jet stream
500 MB about 18,000 Feet Good for detecting significant waves (ridges and troughs) in the atmosphere
700 MB about 10,000 Feet Good for viewing moisture and vertical motion in the atmosphere that may impact surface weather
850 MB about 5,000 Feet Good for viewing moisture, temperature, temperature advection, and lower level winds.
1000 MB near the surface The closest common pressure level that is near the surface
MSL (mean Sea Level) This level is commonly used for identifying fronts, surface lows and highs, forecasting surface winds
Also called MSLP (mean sea level pressure) in graphics.
If your inclined to read more then look
here.