Credit: E. Wiebe
Spring time means unstable, changeable weather patterns. This photo (looking south) shows rain clouds moving in from the west on an otherwise mostly sunny, windy day at the end of April. There is a small patch of mammatus clouds in the centre of the frame where cold, moist air is descending in billows.
There are many possible scenarios and competing explanations for mammatus cloud formations. It seems certain that, in this case, the environment around these formations is complex and contains many areas of instability. The formation is occurring at the margin of two systems or air-masses that are slowly moving eastward (from the right in this photo). Convection at the cloud bottom boundary is the cause of the cell-like appearance of the clouds. There was strong convection and wind-shear in the area. The video below was taken shortly before the main photo (above). The video was made looking east toward James Island. Convection over the sun-warmed land is obvious but note the strong wind shear that caused rolling motion at the level of the cloud base.
The day, until this point in time in the early afternoon, had been sunny and windy. There were patches of cumulus from local convection scattered around to the west (right in this picture) and over James Island and the San Juan Islands to the east, (left, not seen here). Intense convection can be seen in the far south, (horizon of this photo) on the north slopes of the Olympic mountains in Washington State. The dark rainy skies that dominate this scene were slowly approaching from the west. Soon after this photo was taken it began to rain steadily.
This animation, made from images taken 10 seconds apart, shows the view eastward from Island View Beach on the Saanich Peninsula. An area with moderate convection is visible over James Island which is experiencing differntial heating with respect to the surrounding ocean. The bluff at James Island is approximately 60 m high, which gives an elevation of about 400 m for the bottom of the cumulus clouds. High altitude winds are blowing approximately to the east or a bit south of east. Watching the clouds forming above the island suggest that lower altitude wind is blowing north or west of north. There is a significant region of shear at the level of the cloud base. This is probably the same level where the mammatus cloud patch is seen the main photo above.
Here is the observed insolation at Cordova Bay school. The school lies about 5 km south of the point where the photo at the top of this page was taken and is near the shore as well.
|UVic / SEOS / Climate Group / About Front Page Picture / Dramatic Clouds at Island View Beach||Last updated: Friday, 20-May-2011 10:46:18 PDT|