California experiences a variety of winds in September, in conjunction with worldwide wind motions seen throughout the Northern Hemisphere. One global phenomenon is the monsoon winds, when waves of tropical air surge north into the subtropical desert latitudes. The tropics right now are bustling with convective storm activity. The superheated deserts pull the moist air northward in strong bands of flows called tropical waves. Check out this picture of tropical waves marching across Africa. These waves, when they get out over the warm tropical Atlantic water, will form self-sustaining cyclonic storm engines known as "tropical storms", which max out, in perfect conditions, as Hurricanes (aka Typhoons/aka Cyclones).
Similar cloud plumes dance their way up Mexico's mountainous Sierra Madres every year at this time also, entering the desert southwest as an annual event called the monsoons. Similar wind patterns occur out at sea, too, in the case of Hawaii's Kona Wind.
That's the wet southern wind. Another wind affecting California in September is the bone-dry night-time offshore wind, which peak in November as the Santa Ana winds (aka Mono winds here in Central California). They are nasty, howling winds that travel downslope, downcanyon out of the Great Basin. They drive firefighters crazy because they create ideal firestorm condition and can only hope they die down and allow for a fireline to take hold. When these winds blow, the fire easily hops the lines.
The third wind is the the normal summer onshore wind, which at this time of year is about to end its summer run. The cold California Sea Current, which is cold because it upwells from the benthic (deep sea), cools the air above it, then that air and is pulled onshore by the hot interior, forming a layer of cool air along the California coast referred to as the "marine layer." When this wind is dormant, the marine layer gets thin and recedes to just along the beaches. When the offshore winds are active, the layer is blown away.
Another phenomenon this time of year is no wind activity at all, when a high pressure area settles over California, leading to smog emergencies. Right now conditions are of no winds at all, save for localized uphill/upcanyon winds by day and downhill/downcanyon winds by night. As such, the giant Station Fire in the San Gabriel Mountains has spared the urban centers below them a devastating downhill run.
It's really a good thing these hills near the San Fernando Valley are burning now. The burn area will serve as a firebreak now, as the Santa Ana winds will soon be arriving. When pacific storm season begins in the Pacific Northwest, displaced Great Basin air gets pushed downhill through California.
Photos stolen from wunderground.com
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