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Low Level Air Flow
Up the Ojai Valley
Air will move from areas of relatively high pressure to areas of relatively low pressure. There are a number of things that will affect the local relative pressure distribution throughout the daily cycle (diurnal).
Generically, during the diurnal cycle of a day, the surface air will absorb heat and become lighter. The lighter air will lift. We use the term “adiabatic” flow, to mean the expansion and cooling of an air mass due to lifting (without loss of heat). The lifting air will leave a surface low pressure draw, and filling flow will occur. Since the temperature dynamics are stronger over the higher terrain, the higher terrain will convect (transport or wick) more air vertically into the upper atmosphere than lower terrain. So, air will move from the lower terrain toward the higher terrain to fill the void left by the rising air over the higher terrain.
At night, the process reverses. The ground cools the surface air and down slope drainage occurs, much like water shedding into a watershed drainage. We refer to this contraction and down slope flow as catabatic.
Locally, we have an ocean and desert as well as high terrain. During the summer, the desert gets quite hot relative to the coast, and draws filling flow towards it. During the summer costal inversion, the filling flow is relatively heavy near the coast, and will usually be stronger at lower altitudes. We say the air is “heavy”, because it’s heavier than the air above it. Since it is heavy and wants to take the lowest route inland, it will accelerate and “venturi” through the gaps in route to the desert. It will also accelerate around edges and over ridge tops (horizontal edges).
In the winter, the desert can get relatively cold compared to the coast, so drainage flow is more pronounced in the winter than in the summer. In the summer, it tends to flow up hill in the afternoon;
Whenever you put an obstruction in a “heavy” flow path, the flow will try to go around it rather than over it. What flow does go over it will accelerate and venturi over the edge, because it doesn’t want to rise (it’s heavy). If there is a path around both sides of the obstruction, it will tend to converge on the back side.
The Topa Ridge is an obstruction to the air trying to flow toward the hotter inland zones and higher mountains. The flow will accelerate through the narrows of the Upper Branch of Matilija Creek and over the ridge line. Sulfur Mountain is an obstruction to the flow trying to reach (relatively hot) Upper Ojai from the ocean. Upper Ojai sits behind Sulfur Mountain and is relatively high (~ 1,500') compared to downtown Ojai or Santa Paula. There is a path around both sides of the mountain, but the path from the west is more pronounced. The upper Ojai convergence usually sets near The Summit for a number of reasons including the principle discussed above.
In the Summer, Lower Ojai almost always flows from the SW at the lower altitudes during the heat of the day. As you move west at lower altitudes, the flow is more channeled, and thus stronger. Towards Upper Ojai, the flow from the west tends to be weaker, and is often light and variable in a convergence zone over The Summit.
When trying to get to Nordhoff Ridge from Chief’s in the Summer, I usually try to avoid the lower level west wind. If I make a run for the low point on the end of the SE spine coming off Nordhoff Ridge, and come up short against the stiff lower level SW, I’ll turn around and fall back for the base of The Stooges, hoping to get back up and maybe try again. What I usually try to do is work back, up the spine behind the Stooges. I’m trying to get away from the valley flow and protected from the SW behind Nordhoff Peak. It’s a bit risky because by getting deep, there’s potential to get stuck in the low level up-canyon wind.
Paragliding comps run the pilots upwind 70% of the time, but for me, it’s more fun to go downwind when I can. I tap into some of the navigation tools used by balloon pilot and try to recognize the micro flow. My tactics and strategy vary with the obstacles and opportunities. Watch the buzzards (Turkey Vultures), they follow the seams and rarely fly in a straight line. A day's meteorology is like a finger print, no two days are the same.
© copyright 10/7/01