A Winter Storm

Published: January, 2009 

A column of air descending onto the Earth’s surface generates the High. This descending air, upon reaching the surface of the earth, spreads out in all directions; its effects are significant to the weather of both the Bay Area and the Hawaiian Islands.

The western outflow of the Pacific High creates the Trade Winds that are a dominant factor in Hawaiian weather, while the eastern outflow creates the prevailing Westerlies that are so common here in the Bay Area. In addition, the northern outflow of the Pacific High deflects storms away from the Bay Area. Storms generated far to the north—in the Gulf of Alaska and the Bering Sea—have no chance of reaching us here in the Bay Area as long as the Pacific High remains in place.

With the approach of winter, however, the picture changes. As the sun passes across the sky farther to the south, the center of the Pacific High follows it; hence, we lose its protection. Storms are then able to reach far enough south to become the dominant force during the Bay Area’s winter.

Often more than 1,000 miles in diameter, these storms move across the earth’s surface from west to east, bringing with them disturbed, sometimes even tumultuous, air. Cold polar air creates these storms; the cold air meets warmer air, slides under it, and forces it to rise. The rising air cools, and the moisture in it condenses to form clouds and rain. The rising air also causes the barometric pressure to drop, which in turn sucks in more air, driving the rising air farther up. Sometimes, this whole process will become self-sustaining, and a winter storm is born. These storms will often last a week or more before dissipating.

So, what do we here in the Bay Area experience as one of the winter storms moves over us?

We already have one clue. Remember—the air at the center of the storm is rising and the surrounding air is flowing in to replace it. As the storm center approaches us from the northwest, we in the Bay Area will feel the wind shift from our usual west or northwest direction to the south. The storm itself is to the north of us, but because the air ahead of it is rushing into it, the wind will be from the south.

A second strong indicator is a change in our cloud pattern. The typical daily fog cycle features overcast nights and mornings, with the fog burning back toward the coast as the day heats up, only to return in the late afternoon or early evening as the air cools down again. When a storm is approaching, we see something completely different. First, in a clear blue sky, very high, wispy, thin clouds form. Commonly called mare’s tails and officially called cirrus clouds, these are ice crystals 30,000 – 35,000 feet up. The wind most likely has not shifted to the south yet, and the rain could be 36 to 48 hours away. As we watch, the clouds become ever thicker and lower, until a low grey blanket covers the entire sky. The wind shifts to the south, the rain starts, and the storm is upon us.

As you ride the ferry, look out the window from time to time and notice the wind and the sky. The signs of what is coming are there to be read by those who know the language.