Blast of particles from our sun creates colorful northern lights

BY JAN NULL
Special to the Mercury News

The recent appearance of the northern lights over the Bay Area was quite a sight for those who managed to see them.

The northern lights, also known as the aurora borealis and in the Southern Hemisphere as the southern lights, or aurora australis, are caused by the interaction of Earth's magnetic field with charged particles from the sun. This mix of electromagnetic particles is called the solar wind and it varies in intensity, partly because of sunspot activity, which can blast huge quantities of these solar particles into space.

This was the case in the widely visible aurora on Nov. 5 and 6, after a huge eruption from the sun called a coronal mass ejection. As these solar particles collide with gases in Earth's upper atmosphere, brilliant colors are given off. These colorful collisions usually occur 60 to 150 miles above Earth in portions of the atmosphere known as the ionosphere and magnetosphere.

The northern lights are not the result of light being reflected off the polar ice caps, a common misperception. Auroral color is similar to light from a television. In a TV picture tube, a beam of electrons controlled by electric and magnetic fields hits the screen, making it glow in different colors, depending on the type of chemicals that coat the screen. The colors of the aurora depend on the atmospheric gases that interact with the aurora.

Auroras farthest from Earth's surface are yellow-green because of interaction with transient atomic oxygen high in the atmosphere, while those in the lower atmosphere are red and blue from the interaction with nitrogen and oxygen.

While not unheard of in this part of California, the aurora is probably visible in the Bay Area only about once a year, and even then only during ideal viewing conditions. Viewing is also tricky unless you get out of urban areas and away from light pollution -- city lights that obscure the ability to see starlight and other celestial phenomena.

The aurora event this month was captured on film by many people around the world. To see spectacular photography of this event, including a beautiful picture from the San Jose area, go to: www.spaceweather.com/aurora/gallery_06nov01.html.

Q You've used the term ``pineapple express'' numerous times now in a recent Weather Corner, but maybe you don't realize that most of us have no clue what it means. Strata Rose Chalup -
Sunnyvale

A My apologies for assuming that everyone was familiar with the term ``pineapple express,'' which is also sometimes referred to as the ``pineapple connection.'' These terms refer to storm systems that derive their moisture from near the Hawaiian Islands and then are brought northeastward to the West Coast on a strong jet stream.

Because of the warm moist subtropical air masses from which the pineapple express gathers its moisture, copious amounts of rain can result for California. This is why many of the flood events in California have at least a partial pineapple connection.

Q Would you please tell me why weather forecasters do not tell the humidity with the temperatures of cities across the country when they are showing weather maps on TV and in newspapers? Viewers or readers could judge comfort if they could relate high humidity with high temperatures. For example, a 90-degree temperature with 90 percent humidity in Chicago is more uncomfortable than a 100-degree temperature with 33 percent humidity in Las Vegas. If forecasters are going to give wind chill factor, they should also give humidity. Shirley Neusel - San Jose

A There are actually several issues involved here. First, it's a pretty tricky item for many to grab onto, but in general it refers to the amount of moisture in the air. This can be expressed in a number of ways, but the most common is relative humidity, which gives the percentage of possible moisture in the air. A relative humidity of 100 percent means the air is totally saturated, while 50 percent means the air is holding half its possible water vapor.

Further complicating the issue is that humidity usually changes dramatically during a day.

For example, many cool mornings have a humidity of near 100 percent, yet when the air warms, it will drop to 30 percent or 40 percent without gain or loss of moisture in the air. This is why it's called relative humidity. This, of course, raises the question of which relative humidity should be used by a weathercaster.

Second, there is a summertime counterpart to the wind chill temperature called the heat index, which depends on the temperature and the humidity. It relates to human comfort and health by how well the body can cool itself in warm, humid conditions.

The heat index is often used in the eastern parts of the country where there are higher values of heat and humidity, but it's less common here in the West where the heat index is very close to the air temperature. For more information on the heat index, see http://ggweather.com/101/hi.htm.

Finally, I must once more try to dispel the urban myth of 90 degrees with 90 percent humidity. These conditions just do not occur naturally. For this to happen, the dew point -- the temperature to which moist air must be cooled for condensation to occur -- would have to be at least 87 degrees. The highest that I have ever seen in the United States is about 82 degrees. If the temperature were 90 degrees and the dew point 82, the relative humidity would be only 77 percent.

This would translate to a heat index of 111. And if it were possible to get to 90 degrees and 90 percent relative humidity, the heat index would be 122 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 them to weathercorner@ggweather.com. Please indicate in your e-mail what city you live in.