WEATHER CORNER

BY JAN NULL
Special to the Mercury News

Q. During winter, every so often, roofs and lawns around here are white in the morning, and people call it frost. Most of the time, the temperature is nowhere near 32 degrees Fahrenheit. How can frost form when it is above freezing? Russ Hugi - Belmont

A. The important thing to keep in mind is that the temperatures we typically measure here are for the air. The frost we see happens when moisture in the air freezes onto a surface 32 degrees or less. This process occurs when water vapor goes right to ice, without going through the liquid phase. It is called deposition.

The above of course begs the question, why aren't these surfaces the same temperature as the air around them?

The answer lies in the amount of heat lost by various objects at night. Surfaces good at absorbing heat during the day emit it at night and will cool more than things that don't absorb it well.

For example, a black car gets hotter than a white car when left in the sun and is a good absorber. Consequently, it will cool faster than a white car overnight.

Air does not warm or cool as fast as some of the surfaces you mention, meaning the air can be above freezing but frost can form on various surfaces. Typically, if the air temperature is below about 38 degrees and the night is clear, there will be some frost.

Q.  What is the most important thing to know to predict the weather?   Lauren Salazar - Menlo Park

A.  Knowledge of the local geography in the area for which you are forecasting.

It is absolutely necessary to have the various meteorological tools such as satellite imagery, weather radar and computer simulations of the atmosphere. But if you don't have a thorough knowledge of your local surroundings, especially in the Bay Area with all its microclimates, it is extremely hard to make a good forecast.

Q. Why don't as many tornadoes occur in California as in the Midwest? Joe Leoni - Portola Valley

A. The major factor is that wind patterns that bring warm humid air north form in the Gulf of Mexico and move into the Midwest. This provides the ``fuel'' needed for the massive thunderstorms that spawn tornadoes.

In the spring and early summer, winds from the Gulf of Mexico bring humid air into the Plains, where it clashes with cooler air
pushing south out of Canada and the Rockies.

This is not to say that California does not have significant numbers of tornadoes. As a matter of fact, in 1995, Fresno County had more verified tornadoes than any county in the United States.

Q. I am confused as to the difference between a lunar eclipse and a new moon. Aren't they both caused by Earth's shadow? Tom Taylor - San Jose

A. A lunar eclipse happens when the moon enters Earth's shadow. However, with the new moon, the sun and moon are on the same side of Earth, and only the ``back'' of the moon is illuminated. Consequently, the side facing us is in the moon's own shadow.

Q. Why do lightning bolts travel up instead of down? Neil Ferrari - Menlo Park

A. Actually, they travel up and down.  Usually, the lower portion of a thunderstorm develops a strong negative charge and an initial discharge, called a ``stepped leader,'' and moving about 240 miles per second, reaches from cloud to ground. When it nears the ground, it draws a ``streamer'' (a line of positive charges) upward to meet it. When they meet, an electrical connection is completed between the cloud and the ground, and the ionized air molecules become the path of the main bolt of lightning. As negative charges rush toward the ground they collide with the air, causing it to glow with a bluish-white color. The air
near the ground is first to start glowing, but even though the negative charges all move from cloud to ground, the bright
flash of lightning moves from the ground at 61,000 miles per second! This super-heats the air, which expands explosively
outward, producing the shock wave we hear as thunder. This bright flash from ground to cloud is called the return stroke.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them weathercorner@ggweather.com.