Sentence:  Like most human activities, ballooning has sponsored heroes and hucksters and a good deal in between. For every dedicated scientist patiently recording atmospheric pressure and wind speed while shivering at high <WSD>altitudes</WSD>, there is a carnival barker with a bevy of pretty girls willing to dangle from a basket or parachute down to earth. 
Meaning of the word marked with <WSD> tags:  The absolute height of a location, usually measured from sea level.