This chart is provided by The University of Wisconsin .
Positive values are upward motion and negative values downward.
This comes from the fact that our atmosphere is generally stable and that upward motion is generally an artifact of precipitation releasing heat energy. Negative areas show descending air which are generatlly associated to areas of fair weather and often clear skies. If precipitation is occurring in these areas it will end soon. Sometimes descending air will trap cloudiness below it and result in a low overcast that can linger for days. The height field works very similar to the sea level pressure field.