But not all of those particles make it, so you’d expect to see more muons pass through regions of lower density or voids than through solid regions.

Territorial birds may be less stressed at lower density.

The southern areas of the city have mainly industrial and higher density residential sites, while the north features medium to lower density residential plots.

The heated air inside the solar balloon expands and has lower density than the surrounding air.

The lower density of helium reduces breathing resistance at depth.