Thickness of the Atmosphere
							q Most of the atmospheric mass is confined in the lowest 100 km above the sea level. q The thickness of the atmosphere is only about 2% of Earth’s thickness (Earth’s radius = ~6500km).
			q  Because of the shallowness of the atmosphere, its motions over large areas are primarily horizontal. è Typically, horizontal wind speeds are a thousands time greater than vertical wind speeds.  (But the small vertical displacements of air have an important impact on the state of the atmosphere.)