Marine Stratus and Orographic Stratus
Orographic stratus is a much more localized cloud formation. It occurs simply by air of sufficient moisture content being forced to rise by Marys Peak. If the Lifting Condensation Level (or LCL for short) is low enough, the rising air will then condense as it rises over Marys Peak. The orographic stratus is a stationary cloud remaining just over portions of Marys Peak. When the air sinks back down the leeward side of Marys Peak, it drops back below the LCL causing the cloud to dissipate. This cloud type is most commonly seen in the morning, around dusk, or after or between precipitation. At these times the air is the most saturated and the LCL tends to be lower. As you can see, the LCL is lower in the first figure on the left. Grass mountain in the background it able to form orographic stratus in addition to Marys Peak. In the second picture, where the LCL is higher, Grass Mountain struggles to make much orographic stratus, and the orographic stratus over Marys Peak is much thinner and higher up that in the previous image.
Stratus clouds are low-based clouds that tend to stretch over the land like a blanket. One of the more interesting types of Stratus, particularly in an area of coastal mountains is Marine Stratus. The marine layer is often not very thick in height. Thus, the Oregon coastal range, including Marys Peak, tends to block the flow of the maritime air all the way inland. This is why during the summer temperatures can sometimes hit 95 degrees while towns like Newport along the coast stay in the 70s. However, under the right synoptics (such as a weak cold frontal passage during the summertime) the marine air gets pushed up and then over and around Marys Peak. The below video illustrates the fluid of the atmosphere, where the marine stratus can be seen pouring over the top and the sides of Marys Peak. As the marine air sinks down Marys Peak into the Willamette Valley, it warms slightly thus lowering its relative humidity. This causes the clouds to dissipate and skies to remain clear while temperatures moderate in the Willamette Valley For more information on general stratus clouds, click here.
Our MissionThis webcam was funded by the National Science Foundation, with the original camera being installed by the Ocean Mixing Group on the roof of Burt Hall at Oregon State University on April 28th, 2010. The webcam is a part of experimental investigations into the physics of form drag in geophysical flows. The Biomicrometrology Group maintains the site as part of ongoing studies to understand and quantify interactions between the air, vegetation, and the land surface. The images obtained here are intended to complement studies of controlled flows over topographic obstacles in ocean and atmosphere. On August 4th, 2014, the Biomicrometeorology group at Oregon State University installed a new camera with near-infrared sensitivity to increase the window of viewability.
A collaborative, NSF-funded project by
C. Thomas, S. deSzoeke, L. Mahrt & E. Skyllingstad / OSU Atmospheric Sciences
J. Moum & J. Nash / OSU Ocean Mixing Group
Cloud Atlas compiled by REU student M. Spagnolo