 While fog generally is the collection of water droplets or ice crystals suspended in the air at or near the Earth's surface, southern California's fog varies from the light 'ground fogs' to a dense almost "Tule fog" (pronounced ˈtuːliː fog) in the Winter and Spring, depending on the interaction of cold air brought down from the local mountains and the warmer ocean air masses.However, over the last few decades, clouds have become less prevalent on the coast, due to urban heat and climate change.When compared with data from the 1970s, scientists discovered that the stratus cloud cover has decreased between 25% and 50%.