Published Tuesday, December 16, 2003 in the San Jose Mercury News
Planning for the solstice? Don't use your calendar
By Jan Null
Special to the Mercury News
Does your calendar say that Monday is the first day of winter? Well, it's wrong -- at least here, where we follow Pacific Standard Time.
It is true in Greenwich, England, which serves as the world's time-keeping reference point. Winter officially starts at 7:04 a.m. Greenwich Mean Time on Dec. 22.
But 7:04 a.m. in Greenwich is 11:04 p.m. in California -- a day earlier, Dec. 21. Farther west, it's even earlier.
And to further confuse the issue, here are some more facts to consider about the winter solstice, which is derived from the Latin word to "stop'' or "stand still.'' To the ancients who watched the sky, the winter solstice was marked as the day when the sun's arc through the sky reached its southernmost point.
On Sunday, Earth will be closer to the sun than it will be in July. On that day, we are about 91 million miles from it -- compared with July, when we're about 94 million miles away. This seems counterintuitive, but remember that Earth's axis is tilted away from the sun this time of year. This causes the sun's rays to spread across the globe from a low angle in the northern hemisphere. Think of a flashlight shining on a desktop. If the beam points down, the light is concentrated into a circle; if it shines at an angle, the same amount of light is spread over a larger surface. When the sun's energy is spread over a larger area, it is less effective at warming the planet.
Also surprisingly, the winter solstice is not the day of the latest sunrise or earliest sunset. The latest sunrise will be Jan. 5. And the earliest sunset has already happened, on Dec. 8. Why? The short answer to this apparent paradox has to do with the Earth's slightly eccentric orbit and a complicated formula -- conceptually and mathematically -- called the Equation of Time. A detailed explanation from the U.S. Naval Observatory can be found at http://aa.usno.navy.mil/faq/docs/dark_days.html.
Finally, there is the issue of the winter solstice being the official beginning of winter. The solstice is an astronomical event -- but from a meteorological point of view, winter in the Bay Area should probably be defined as the three coldest and wettest months: December, January and February.
Q. Have any hurricanes been reported in the North Atlantic well after the traditional hurricane season of September through November? A.J. Helton - Roseville
A. Yes. The latest hurricane to be observed in the Atlantic was Alice, on Dec. 31, 1954, and it persisted until Jan. 5. This was the second storm of that season named Alice -- the first happened June 22. The latest in the year that a hurricane made landfall was Nov. 20, 1925, near Tampa, Fla.
Q. How can two cars that are parked across the street from each other have different amounts of frost on them on the same morning (none on my car and a layer that needed to be scraped off my roommate's car)? Rachel Laubert - Petaluma
A. There are several factors that affect how frost forms on cars or other objects. The most likely explanation is that the car without frost was parked under a tree. This limits the amount of heat that is lost to space, and consequently the surfaces upon which frost might form do not reach the freezing point. (The same thing happens on an overcast night; of course, then both cars would be affected.) Conversely, a vehicle parked in the open cools more rapidly, especially on a clear, calm night. A second and lesser factor might be the car's color. A dark-color car loses and gains heat faster than a light-color car does.
Q. Are there different types of humidity? Northern California's high humidity is nothing like the humidity in Washington, D.C., the Midwest or the South? Why is that? Kathleen O'Looney - Santa Rosa
A. No. There is only one type of humidity. What makes people feel uncomfortable is the combination of high temperatures and high humidity. For example, the average July afternoon humidity of 65 percent in San Francisco is higher than that of Washington, D.C., at 55 percent. But the average July highs for San Francisco and Washington are 68 degrees and 88 degrees, respectively. People feel more uncomfortable when it is humid because our bodies cool by evaporating sweat -- and when humidity is high, there is less evaporation and hence less cooling. This matters less when the temperature is only 68 degrees -- but it's mighty uncomfortable when temperatures reach 88 degrees!
Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail email@example.com, or fill out a form online at http://ggweather.com/questions.htm. Please indicate in your e-mail what city you live in.