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By Mel Zeldin With September comes the start of the Santa Ana Wind season, and with it, the increased potential for large wildfires. The "Apple Fire" in Cherry Valley in August is an early reminder of the devastation that wildfires can produce. In addition, Santa Ana winds occurring in September are often associated with some of the hottest temperatures of the year, especially closer to the coast. Santa Ana winds can occur most any month from September through May, but they are most notorious in the fall, coinciding with and exacerbating, the fire season. So why do we have them? It's really an interesting combination of weather pattern and topography. Air generally moves from high pressure to low pressure, and high pressures are usually associated with colder temperatures. In the summer months, the warming of the land masses, coupled with cooler ocean temperatures, generally keeps high pressure off the coast and low pressure in the interior deserts. The resulting air movement, called the sea breeze, moves from the coast inland, and here in Beaumont, we tend to have afternoon westerly breezes. Beginning in September, daylight gets shorter, and cooler air begins to develop to the north. Sometimes, under the right weather pattern, cool, dry air is carried into the Great Basin area, encompassing northern Nevada and parts of Utah and eastern Oregon. That area is generally 4,000 to 5,000+ feet above sea level. At the same time, lower pressure forms off the coast of Southern California, creating a situation for the flow of air from the Great Basin to Southern California. Colder air is denser than warmer air, so it tends to stay closer to the surface. Now, picture the Great Basin area as a huge reservoir of water. It's denser than air and flows downhill. Essentially, that's what is happening with the cooler air – it is flowing downhill toward Southern California. As it flows downhill, it compresses and heats up at a rate of 5.5° F for every 1,000 feet of descension. It is further warmed by daytime heating. When the air hits the blockage of the San Gabriel and San Bernardino Mountains, it, like water, will find the easiest pathway downhill. That brings into play both the Cajon and San Gorgonio passes, which offer the descending air the easiest route to flow through. Because the passes are relatively narrow, the air is forced to speed up, much the way you can increase the flow of water out of your garden hose by putting your finger on the end of the hose nozzle to cover part of it. You can get water to shoot a good distance by doing that. Likewise, the passes cause the air to accelerate. One other factor comes into play in determining the strength of the Santa Ana winds: the jet stream. In some situations, the strongest upper level airflows, known as the jet stream, which can easily exceed 120+ mph, come out of the north and over Southern California, aligning with the surface flow. In these situations, there is a transfer of energy from the air aloft, and the Santa Ana winds can blow stronger than the definition of hurricane force winds (i.e., 75 mph), especially in the typically most wind-prone areas of Southern California. And these conditions have been observed to persist for several days. Now, since this is dry air to begin within the Great Basin, there is no added moisture to the air as it flows south, and since the air temperature increases, the relative humidity decreases accordingly, sometimes into the single digits. Thus, already dried vegetation from the hot summer, coupled with extreme drying conditions and high wind speeds, creates the "perfect storm" for severe wildfires. Also, because the coastal regions are lower than the Inland Empire, the air continues to warm, and many times the hottest temperatures in Southern California are near the coast. Wildfires near the coast are fairly common during these events. Recognize though, that not all Santa Ana wind events are of the severe type. Sometimes, weak Santa Ana weather patterns develop, and winds are only moderate. And no two events will ever be identical. But the nature of these events, especially in the fall months, can wreak havoc on the region. One last topic: how did these winds come to be known as "Santa Ana" winds? There is no clear indication of how or when the term came to be, but some historians attribute it to the early Spanish settlers who may have used the term "caliente aliento de Santanás," which means "hot breath of Satan." More recent accounts attribute it to reporters around 1900 who observed strong, hot winds in the Santa Ana Canyon. Either way, they will be with us every year. If you have a question about any weather topic, please feel free to email me at mzeldin45@gmail.com. Periodically, I will devote a column to answer your questions. FOUR SEASONS BREEZE | SEPTEMBER 2020 15 Santa Ana Winds: Fast, Furious, & Fires

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