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While the beautiful symmetry of our planet may suggest even gradations of climate as you move from the equator to the poles, in reality, there are a multitude of phenomena that belie the tidiness of such an understanding of the Earth's climates.
The first major factor is the Coriolis Effect, or the force created by the earth's rotation, that causes the circulation of the world's oceans. Easterners recognize the result of this effect as the Gulf Stream. A warm current flows north out of the tropics along the coast to the Grand Banks off Labrador. On the West Coast, we have a cold, southward flowing current that runs along the coast of the Pacific Northwest and northern California. Weather flow pattern is from west to east across North America, from the warmer waters at the center of the Pacific Ocean towards the California coast where it hits the cold ocean current along the California coast. The air cools forming a coastal strip of cool climate. As the air travels across the land mass, the temperature of the land changes the temperature of the air - warming it in summer and cooling it in winter. As a result, the immediate coast of California does not experience the extreme temperature variations as do parts of California further inland.
From a satellite's perspective, the West Coast of the lower forty eight would represent, if it were temperature coded, a long narrow strip running along the ocean's edge, likely colored blue to denote cool. In actuality, it's an inverted triangle with the fat end up by Portland, Oregon and the skinny tip down by Lompoc in Santa Barbara County. If you are inclined to check out the weather page of your local newspaper, you will likely observe the bands of forecast high temperatures revealing that sliver of coolness, something that is apparently overlooked by the shivering short and t-shirt clad tourists in San Francisco.
Now, let's consider the weather from a cross-sectional perspective. Out in the middle of the Pacific, the temperature of the air in contact with and just above the ocean is controlled by the temperature of the water. The moisture content of the air is relatively saturated, being in contact with a body of water. Near the coast, the air temperature drops along with the temperature of the water (due to cool coastal current, see above). Since cold air has a lower water holding capacity than warm air, some of the dissolved water in the saturated air changes from gas to liquid in the form of tiny droplets that we observe as fog. If you were to inspect this cross section of air near the coast the bottom layer would be cold and foggy, but the upper layers would gradually increase in temperature. This gradation is surprisingly well-maintained, free from mixing or churn, because cold air is heavier than warm air.
In interior valleys, this phenomenon is reversed. Inland the summer earth radiates heat warming the bottom layers that remain hot since there is very little air flow or circulation from the coast. As you go into the upper layers of the air mass, temperatures are cooler as experience has shown from backpacking trips you might have taken in the mountains. This accounts for why, in summer, you find cooler temperatures in the mountains of Napa as opposed to the valley, yet cooler temperatures in the river valley of the Russian River, near Guerneville say, as opposed to the surrounding hills.
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