Classified as a Subtropical Mediterranean climate (Köppen Csb), Santa Monica enjoys an average of 310 days of sunshine a year. It is located in USDA plant hardiness zone 11a. Because of its location, nestled on the vast and open Santa Monica Bay, morning fog is a common phenomenon in May, June and early July (caused by ocean temperature variations and currents). Like other inhabitants of the greater Los Angeles area, residents have a particular terminology for this phenomenon: the "May Gray" and the "June Gloom". Overcast skies are common during June mornings, but usually the strong sun burns the fog off by noon. In the late winter/early summer, daily fog is a phenomenon too. It happens suddenly and it may last some hours or past sunset time. Nonetheless, it will sometimes stay cloudy and cool all day during June, even as other parts of the Los Angeles area enjoy sunny skies and warmer temperatures. At times, the sun can be shining east of 20th Street, while the beach area is overcast. As a general rule, the beach temperature is from 5 to 10 degrees Fahrenheit (3 to 6 degrees Celsius) cooler than it is inland during summer days, and 5–10 degrees warmer during winter nights.