3.1: Overview of Weather and Climate

In seeking to learn the climate of a country we first think of its latitude, since that is usually the most important factor. In the case of California, however, this inquiry does not help us very much. California has a coastline nearly 1000 miles in length, reaching through ten degrees of latitude, and we would expect that its northern part, which is nearly the latitude of Chicago, would be cold in winter, and that the southern part, which reaches to about the latitude of Savannah Georgia, would be very warm, but there is, near the coast, only a few degrees difference in temperature between the north and the south. Spring fruits appear first, and oranges ripen earlier in Northern rather than in Southern California.

Lecture Time: Intro to Weather & Climate

Want to learn more? Either scan the QR code or visit this link to watch a lecture that introduces the basics of California’s weather & climate (Video length: 23 min).

Regarding the small range in temperature as we go north or south along the coast, the temperature of the ocean changes but little from winter to summer. The California current is a cold ocean current, which makes the west coast of the USA much drier than coasts with warm oceans. We also know that California, being in the temperate zone, is situated in the belt of prevailing westerly winds. The temperature of the air over the ocean is determined, to a large extent, by that of the water, and since the ocean varies but little throughout the year, and the winds blow mostly from the ocean toward the land, the latter is going to be cooler in summer and warmer in winter than it otherwise would be.

If not for mountain barriers, the cool ocean winds would sweep far inland and temper the climate of the whole State. The fact that there are mountain ranges lying close to and parallel to the coast has resulted has aided in making the climate of the interior subject to much greater extremes of temperature, as well as decreasing its rainfall. By the time the winds have passed the Coast Ranges their moisture and coolness have been greatly reduced, while still farther eastward, on the opposite side of the Sierra Nevada, the summers are not only extremely hot, but the lack of moisture makes the region a desert.

During the summer, except for the thunderstorms in the mountains, it is very rare that rain falls over most of the State, although the air sweeping in from the sea is saturated with moisture, as shown by the fog it bears, but if the Pacific had warmer ocean waters and currents—like the Gulf of Mexico or Atlantic, California would experience more rain in the summer, as warmer water provides more humidity than colder water. To have rain, air must not only be cooled to the dew point, but below that point, and the only thing which will do that is exemplified in those irregular and violent disturbances of the atmosphere which set up currents carrying the moisture-laden air to a great elevation where the temperature is sufficiently low to bring this about.

Many storms that pass across Northern California fail to reach the southern portion, and they also usually decrease in intensity toward the south, so that outside of the mountain districts the southern portion of the State has a very light rainfall. The influence of mountains on precipitation is extremely important because of the low temperature of their lofty tops. In the case of many storms in Southern California which pass over the lowland regions with but a slight rainfall, when they encounter the mountains there is a heavy fall of snow or rain. It is because of this fact that we find such great differences in the rainfall during a few miles.

During the summer there generally exists an area of low air pressure over the Gulf of California, and here originates the "Sonora" (so-called from the province of Sonora, in Mexico) storms which bring frequent rains to Southern Arizona and New Mexico. These storms sometimes reach into Southern California and give heavy summer rains in the mountains and in the deserts. In fact, the heaviest rainfall of the whole year may occur in the Colorado desert during summer.

Summer thunderstorms are frequent in all the higher mountain regions of the State. They are partly due to the low-pressure areas just mentioned and in part to the influence of the cool mountain tops on the upward-moving air currents. They add quite substantially to the summer water supply, and some monsoonal moisture from the south.

During the clear cool periods of winter, a heavy blanket of fog, known as "tule fog," occurs in nearly all the lowland valleys to a greater or less extent. This generally breaks away during the day, but in the lower San Joaquin-Sacramento Valley, it may last continuously for weeks at a time, completely shutting out the sun. The name is derived from its prevalence in the tule region of the district just mentioned. This fog is the result of the settling of the heavier and cooler air into the hollows of the land, where the evaporation from the moist earth following the early winter rains finally produces a saturated condition.

Destructive winds of cyclonic character are rarely known in California. The most disagreeable, as well as harmful, winds occur in the spring and fall. They are dry and hot, and from their direction are known as ‘northers"’ or diablo winds. In Southern California, this wind is locally known as the ‘Santa Ana’. During its occurrence, the air is filled with dust, and for this reason, is often called a dust storm in the drier parts of the State.