Clouds, Linings & Volcano - Montserrat July 2, '09 by Marc Bernstein. ©Marc Bernstein, shown with permission.

A low sun, moist marine air, scudding fractus, and the Soufriere Hills volcano make up this scene.

Long cloud shadows are cast through the moisture laden marine air layer. They are likely 3D shadows, volumes of darkness rather than 2D shadows cast on a thin haze layer. They are all parallel but perspective makes them appear to diverge radially from the sun's position.

The cloud has a 'silver lining'. Cumulus clouds have a high concentration of water droplets and light rays are scattered many times and attenuated - the cloud is 'optically thick' and appears dark when against the sun. All except the edges that is where there are fewer droplets along the line of sight. At the edges a ray is scattered once or only a few times and predominantly in the forward direction to give the bright 'silver lining'. But why are some clouds sharp edged and others fuzzy?

From the left (see the clear day view) there is a plume from the Soufriere volcano. Marc adds ".. the plume is mostly H2O and SO2 with variable amounts of fine ash mixed in depending upon trade wind speed. During the day it is mostly visible immediately downwind of the volcano mixed in with condensation from fumaroles and orographic clouds giving it a slight bluish cast. I don't know what the SO2 flux was yesterday evening (time of image) but typical outputs range from 500 to 1000 tons per day. Most of the time the plume is driven westwards by the prevailing easterly trades. Occasionally when the wind turns to southerly the SO2 smell can be quite strong at the observatory. In the late afternoon the lower sun angle helps make the plume visible westwards out over the Caribbean. The summit of the volcanic dome is around 1050 m but there is no crater. The plume originates mostly from numerous fumaroles on the dome and probably also from more diffuse degassing. The upper part of the plume probably rises to 1500-2000 m and the lower part drifts down maybe to 500 m."


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