Convocation & Insolation

William & Mary is back in business for another academic year.  I teach my first class at 9 a.m. on Wednesday—Geology 110: The Earth’s Environmental Systems; it’s an introductory class with 200 students enrolled.  Later, on Wednesday afternoon the College will gather for Convocation, and this is the ceremony that really kicks off the academic year.

In Earth’s Environmental Systems we’ll spend much time during the first week or two discussing the Sun and its relationship to the Earth.  Consider the Sun and its path across the sky here on campus during the first day of the new semester, August 29^th 2012.

The Sun will rise at about 6:36 a.m. (EDT) and will first appear above the horizon in the east-northeast.  As morning progresses the Sun will rise ever higher in the sky, tracking out a southeasterly course.  At local noon the Sun will be directly to the south and at its maximum solar elevation angle for the day (~62˚ on August 29^th).  Watch the animated graph to the see the path of the Sun across the Williamsburg sky.

Solar elevation and azimuth plot of the Sun across Williamsburg sky on August 29th.

Here’s a question:  why, on August 29^th, does local noon occur just after 1 p.m. rather than at 12 p.m.?

Insolation (incoming solar radiation) is the flow of solar energy intercepted by exposed surfaces.  Insolation is a rate per unit area (watts/meter²).  The amount of insolation received at any given point is a function of the solar elevation angle—the larger the solar elevation angle the greater the insolation.  Insolation varies greatly as the solar elevation angle changes during the course of the day.  Discerning how and why insolation varies across the Earth is fundamental to understanding the Earth’s environmental systems.

For many years Convocation has taken place in the courtyard of the Christopher Wren building, located on the west side of the Wren building.  Convocation begins at 5:15 p.m. at that time the Sun will be 28˚ above the horizon at an azimuth of 260˚ (to the west-southwest), and as the ceremony rolls on the Sun will progress ever westward.

Simplified map view of W&M Convocation layout and orientation of incoming solar radiation.

The Wren courtyard is well located to receive copious insolation during Convocation, and as such the temperature in the Wren courtyard is typically elevated to garish levels throughout the speeches and fanfare.  Just ask the choir and faculty sitting in the courtyard, all tricked out in their robes and academic finery, about the temperature during Convocation.  “It’s like being a disconsolate pig, all trussed up and stuffed into a three-sided brick oven…” groaned one of my colleagues.  A three-side oven is an apt comparison as the vertical brick walls of the Wren building receive up to twice as much insolation as the nearby Earth’s surface, this heats up the bricks which, in turn, heat the surrounding air much more effectively than any blustery speaker ever could.

A. A late afternoon Sun ray’s view of the western aspect of the Wren building, from Google Earth campus map with 3D buildings (http://gisfiles.wm.edu/campusmap/WMCampusMap.kmz). B. Cross section view of insolation and solar elevation angle during Convocation. Note that the sun rays that intersect the Earth’s surface are spread over a larger area than those intersecting the western brick wall of the Wren building.

For this year’s Convocation, the College is moving the ceremony from the western courtyard to the eastern facade and front yard of the Wren building- what a brilliant idea!  The venerable Wren building will provide shade for Convocation goers.  After Convocation the incoming class of W&M students will proceed westward through the Wren building and emerge onto campus as the newest members of our community.  This change in Convocation symmetry is a good idea.  Paying due diligence to the physical reality of late August insolation and seeking a shady refuge in the Wren building’s eastern yard is a brilliant idea.