14 September 2007
Dean as seen by the naked eye
Optical thickness
Polarized light shows clouds in their true light
At this stage, we still know nothing about the clouds’ composition. Are they mainly liquid water or ice?
To find out, Parasol trains its “third eye” on the hurricane to see how light is polarized by the different particles reflecting it. By comparing these polarization measurements with data from the MODIS satellite-borne instrument, we obtain an image that shows precisely the state of water at the top of Dean’s clouds. Although the hurricane is over the Tropics, most of the water is ice (in blue). Why? To answer that question, we need a map of “apparent pressure” at the cloud tops.
Apparent pressure and cloud height
Of course, Parasol doesn’t measure pressure using a barometer, which would be useless at an altitude of 700 km. But its sensing channels allow it to “see” the amount of oxygen present in the cloud tops.
