It is true jtr that the majority of California's population lives in a relatively temperate climate. However, that climate is not representative of our State. Thus, when people visit or talk about California they inadvertently describe our State as having generally nice weather. Granted, the coastal locals you mentioned, such as the San Francisco basin, and the area from San Diego up to Los Angeles are beautiful. Included in this garden like area, is a narrow coastal strip that meanders between San Francisco and Los Angeles.
Nevertheless, these areas only make up a very small portion of the entire state. East of Los Angeles and San Diego, there is a series of small mountain ranges (peaking only at ~8,000 feet). These mountainous areas protect this small coastal region from a huge desert that consumes ¼ of our state. In this area, the average rainfall is less than 5 inches and the average temperature is disgustingly high (100F+ in July).
Now, the long center section of this State consists of a large flat valley that is a boon for our agricultural industry. The southern portion of this valley has hot summers and cool winters, the northern section is more pleasant and continues up through Glenn county which is in the northern 1/3 of our State.
The agricultural area is squeezed up against the Sierra Nevada mountain range. This mountain range dissects through ¾ of our State and connects with the Cascade Mountains that continue on into Oregon. The weather can be extremely treacherous, and is best enjoyed during the summer months. Any other time of year could see you stuck in freezing conditions. Yes, Lake Tahoe could easily have a light dusting of snow in September; I’ve even seen it happen in late August. However, this is a good thing. Considering what little rain we receive in the southern portion of this State, we rely on the snow pack in the Sierras to feed us with water.
Now, in contrast, the Northern part of our State, such as Humboldt, Trinity, and Del Norte counties, get lots of rain (annual average of 60 – 120 inches according to the SCAS). Their weather is a bit cooler, averaging 30F to 40F in January. These counties are part of, and are surrounded by the Klamath Mountains to the North and North West, plus they’re hemmed in by the Cascades to the North East.
Is this to say that our State has colder weather than New York or some other part of New England? No, not as a whole, but it does illustrate that our State’s climate is very diverse, and these are not microclimates (although we have those too). Thus, generalizing about our weather is indeed an error. Remember, the land in our State covers roughly 156,000 square miles (according to the US Census Bureau), stretches some 770 miles long and 250 miles wide at its furthest distance. Indeed, ours is an area of contrast, not generalizations.
Just as a side note, we do have some extreme microclimates. California is on record as having the snowiest city, Blue Canyon with an average of 240.8 inches a year, and Greenland Ranch as being the hottest recorded place in the USA (134F on July 10, 1913). Ironically, when the Spanish first arrived, they thought that our area was an Island. The name California comes from a mythical Spanish island ruled by a queen called Califia that was featured in a Spanish romance ("Las Sergas de Esplandian") written by Garcia Ordonez de Montalvo in 1510.