Figure 5.

Polarization underwater at a wavelength of 500 nm throughout the day. Data were collected at the Aquarius underwater laboratory in Key Largo, FL, USA on 15 and 16 August 1999 (see also [16]). Times given are Eastern Standard Time. Each part of the figure shows the polarization pattern looking upwards (this is why east and west are reversed from their positions on a compass card). The margin of each panel represents horizontal, and the concentric circles show elevations at 30° intervals. The lighter inner circle indicates the region within Snell's window, where skylight is visible through the sea's surface. At each location of measurement, the e-vector angle is plotted as the angle between a tangent to the elevation circle, and the degree of polarization is coded by the thickness of the plotted line (see the key at the lower right). The estimated position of the Sun at the midpoint of each series of measurements is indicated by the yellow symbol within Snell's window.