ASK TOM: How are wave heights measured?

Dear Tom,
During Sandy’s aftermath 21.7 foot waves were observed in Lake Michigan. How are wave heights measured?
—Bill Gunther,  Downers Grove

 

Dear Bill,
The National Oceanic and Atmospheric Administration deployed a buoy in the middle of Lake Michigan east of Kenosha, Wis., in 1981 that takes a variety of measurements, including barometric pressure; wind direction, speed and gust; air and sea temperature; dew point; and wave energy spectra. From these, significant wave height, wave period and wave direction are measured or derived.

 
The wave height is not obtained by direct measurement but derived in a complicated process based on accelerometers or inclinometers on board that measure the buoy’s vertical displacement during a specified time period.