Climate depends on a number of factors. As explained in an earlier column, we are a desert mostly because of our latitude (which is 34 degrees north) and the "rain shadow." These are the overall factors that determine our climate, which is classified semi-arid.


Other "controls," most of them seasonal, determine our weather. These include high and low pressure areas, weather fronts, and wind patterns.


Highs (not temperatures, but atmospheric) rotate clock-wise (in the northern hemisphere), and from top to bottom. This compresses the air mass and warms it, which explains why, when under its influence, we experience fair weather.


Lows turn counter-clockwise (in the northern hemisphere), and from top to bottom. This cools the air mass, as explained in an earlier column, causing any moisture to condense out of it as rain. Also, a high pressure area, also know as a "ridge," is shaped roughly like a pyramid and the base of it cannot easily cross mountain ranges. The air mass is "bottled up," and this produces our infamous "Santa Anas." Lows are roughly the inverse, with the smallest area near the surface, and cross mountain ranges more easily. Lows are also referred to as "troughs."


Fronts come in two basic types: the warm and the cold. A front is an area where two unlike air masses collide. "Unlike," as in one warm and the other cooler, or one is moist and is drier. Ordinarily, a warm air mass rides up over a cold air mass, since warm air (and moist air) is more buoyant than cold air (and dry air). When cold and warm air masses collide, but neither is strong enough to push the other out of the way, we have a "stationary front." This can bring on a period of very wet weather that may last several days. And if the reverse happens, and the cold air mass rides up and over the warm air mass, its call an "occluded front." This, too, can bring about very unstable weather.


Steering the fronts, are wind currents. Ordinarily, a warm air mass moves south to north and west to east - the latter due mostly to the rotation of the Earth. A cold air mass generally moves north to south and west to east. Helping the west to east movement is one of the global wind belts, called the "westerlies." The westerlies move further northward and southward with the seasons and directed by the sun. June 21 is the first day of summer this year, also known as the solstice. The Sun is highest in the sky at midday and will remain there for a few weeks, then begin to slide further south again. Happy summer!