with what is called a southerly surge, when the northerly gradient relaxes, allowing the marine layer to "slosh back" up the coastline. Yet another variation occurs when the upper air becomes turbulent. Turbulence above the marine layer inversion can, depending on its severity, break up the marine layer. The most common causes of such turbulence are strong upper level low pressure areas, or the monsoon which occasionally extends northwestward from the desert areas of the U.S. There are also occasional extended spells when fog and stratus ("overcast") do not clear all the way back to the coast for several days. These extended periods of cloudiness are usually a consequence of a weak area of low pressure above the marine layer which increases its depth, making it more difficult for surface heating to evaporate the clouds within it. These periods of persistent overcast have inspired colloquialisms such as "May Gray", "June Gloom", "No Sky July" and "Fogust". Climate change Research published in 2010 showed that summertime fog in California decreased by 33% during the 20th century. The decline in fog is generally attributed to climate change, and is concerning for the local ecology, for example the redwood trees. Lower fog levels are also problematic for the agricultural regions fog patterns support, such as the Napa and Salinas Valleys. The attribution of the reduction in fog and of global warming itself to the Pacific decadal oscillation is generally rejected. Art and culture Writers, poets, and photographers have long been inspired by the fog, including Herb Caen, Jack Kerouac, August Kleinzahler, and Arthur Ollman. Sam Green made a film about San Francisco's fog for the Exploratorium, which premiered in 2013. In Kyle Boelte's 2015 book The Beautiful Unseen: Variations on Fog and Forgetting, San Francisco's fog becomes a metaphor for grief and the limitations of memory. In popular culture In 2010 an anonymous person began a Twitter account for the San Francisco fog, inspired by the fake BP public relations account that appeared after the oil spill in the Gulf of Mexico that year, and named "Karl the Fog" after the misunderstood giant in the 2003 film Big Fish. The name has since come to be widely used. There is a companion Instagram account. The name is also used throughout episode 8 of Carmen Sandiego. The name was disparaged by 3 local celebrities during the 2020 PGA golf tournament. Accidents Many accidents happened