Physics Photo of the Week

Warren Wilson College

Sept 10, 2004

Barometer readings for Tropical Storms Frances and Ivan

Recording Barometer

A hurricane, tropical storm, tropical depression is caused by the coalescence of thunderstorm convective cells over the warm tropical ocean.  The rising air in the coalesced thunderstorms develops into a large area of low barometric pressure.  The low barometric pressure is created by rising air rather than static or descending air.  As surrounding air flows into the partial vacuum created by the rising air, the rotation of the earth causes the converging air to spiral counter-clockwise (Coriolis effect) in the northern hemisphere, thus creating a cyclone (general term).  The strength of the cyclone determines whether it develops into a tropical depression, tropical storm, or hurricane.

The recording barometer, or "barograph" shown above is a barometric pressure indicator that records the barometric pressure on paper chart.  The drum holding the chart paper makes one complete turn in one week (assuming no electrical power failure).  The barometric pressure is sensed by the array of three diaphragms in the right center of the photo.  These diaphragms are hollow, evacuated, and "springy".  When the air pressure in the room in high, the hollow diaphragms are compressed, and the mechanical linkage causes the needle arm (with the pen) to rise.  The opposite occurs with a low barometric pressure. 

The recording below shows the trace of barometric pressure as the remnants of Hurricane Frances drifted over WWC causing excessive rainfall and the widespread flooding.  Unfortunately, the mechanical recording system suffered a power outage and a mechanical jam.

Barometer readings during TS Frances

Notice the low barograph reading for Wednesday, Sept. 8.  Typical barometric readings on the barograph hover around the white areas of the chart as indicated below in a 2-week run in the spring of 2004.

Typical barograph trace

Ivan barograph.

Barograph showing the strong drop in barometric pressure due to TS Ivan, Sept. 16, 2004

Eight days later, WWC experienced a second tropical storm - Ivan.  The Ivan barograph showed a deeper low pressure dip than Frances and brought stronger winds to campus creating much more extensive electrical power outages.  It is remarkable that power was restored in only 14 hours when much of the area had no power for several days.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

Click here to see all Physics Photo of the Week for 2004.