{"id":72256,"date":"2026-09-19T18:58:44","date_gmt":"2026-09-19T18:58:44","guid":{"rendered":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/?p=72256"},"modified":"2026-09-19T18:58:44","modified_gmt":"2026-09-19T18:58:44","slug":"interesting-lack-of-cloud-formation-on-the-california-coast","status":"publish","type":"post","link":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/archives\/72256","title":{"rendered":"Interesting (Lack of) Cloud Formation on the California Coast"},"content":{"rendered":"\n

The GOES-18 (GOES West) Advanced Baseline Imager showed something interesting off of California’s Cape Mendocino on the morning of Saturday, 19 September 2026. This cape is the westernmost point in California and is characterized by significant terrain changes over a relatively short distance. The Pacific Ocean next to the coast of California is frequently covered in stratocumulus clouds due to the relatively cold sea surface temperatures caused by the California current and its associated upwelling. These cold temperatures are clearly visible in the NOAA Sea Surface Temperature<\/a> maps derived from satellite observations. Here’s the map for the previous day, 18 September 2026; note how the temperatures around Cape Mendocino (the westward-poking nub near the intersection of 40 N and 124 W) are around 13 C or 55 F. <\/p>\n\n\n\n

\"Map<\/figure>\n\n\n\n

These cold sea surface temperatures cause the air above the ocean to also become quite cold, cooling to the dew point and forcing the formation of clouds. Since air is a poor conductor of heat, the air further away from the surface does not cool which causes a notable inversion to form and which limits the vertical extent of the clouds that develop. This can result in quite widespread deck of these clouds over cool ocean regions, as can be seen in this geocolor RGB image from GOES-18. Note how, outside of some valley fog, California is basically cloud-free while the ocean immediately adjacent is overcast with marine stratocumulus. <\/p>\n\n\n\n

\"GOES-18<\/figure>\n\n\n\n

But there’s an exception to this idea that the entire west coast is socked in with clouds. Let’s zoom in on the area around Cape Mendocino with this animated true color loop. While it’s always fun to watch valley fog dissipate with the rising sun, here pay special attention to what’s happening south of the cape. <\/p>\n\n\n\n

\"Animation<\/figure>\n\n\n\n

The winds are predominately northerly and it’s easy to see how the clouds are streaming southward because of that. But in the wake of the Cape, the skies are clear over the ocean. What’s going on here? To get a better idea, let’s take a look at the terrain of Cape Mendocino, courtesy of the US Geological Survey National Map<\/a>. Here, there’s a topographic map with a transect drawn across the cape to give an idea of the terrain. <\/p>\n\n\n\n

\"USGS<\/figure>\n\n\n\n

As you can see, over a short distance the terrain goes from the ocean at sea level to peaks near 3,000 feet. That’s quite a jump over a relatively short distance and the southerly flow is forced to deflect around the cape. In the wake of the northerly flow that is blocked by the terrain of the cape, drier continental air (enhanced by downsloping) is able to enter that region and prevent clouds from forming despite the quite cold sea surface temperatures. There even seems to be a hint of a terrain-forced mesolow with some southerly flow appearing right next to the coast in this region. <\/p>\n\n\n\n

Note how difficult it is to see this clear spot in the infrared imagery, however. This is because the clouds are so shallow that there is very little difference between their temperature and the temperature of the ocean. Because we know what we’re looking for, we can probably identify the clearing region, but we’d likely have to resort to some other product to help discern between clear sky and cloud cover for a region that we hadn’t already been looking at.<\/p>\n\n\n\n

\"GOES-18<\/figure>\n","protected":false},"excerpt":{"rendered":"

The GOES-18 (GOES West) Advanced Baseline Imager showed something interesting off of California’s Cape Mendocino on the morning of Saturday, 19 September 2026. This cape is the westernmost point in California and is characterized by significant terrain changes over a relatively short distance. The Pacific Ocean next to the coast of California is frequently covered […]<\/p>\n","protected":false},"author":22,"featured_media":72262,"comment_status":"closed","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[114,8,38],"tags":[],"class_list":["post-72256","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-goes-18","category-marine-weather","category-what-the-heck-is-this"],"acf":[],"_links":{"self":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts\/72256","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/users\/22"}],"replies":[{"embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/comments?post=72256"}],"version-history":[{"count":1,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts\/72256\/revisions"}],"predecessor-version":[{"id":72264,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts\/72256\/revisions\/72264"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/media\/72262"}],"wp:attachment":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/media?parent=72256"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/categories?post=72256"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/tags?post=72256"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}