BY JAN NULL
Special to the Mercury News

Summer officially ended Saturday with the autumnal equinox, but most people consider the season to run from Memorial Day to Labor Day, and meteorological records are kept on a monthly basis. So I have put together a quick snapshot of temperatures for June, July and August.

It may surprise many that statistically San Jose ended up with about normal 30-year average temperatures. The average high temperature was just four-tenths of a degree above normal for the three-month period. Likewise, there were 13 days when the highs topped out at 90 degrees or more -- just one day more than normal.

However, there were 490 cooling degree-days -- 104 more than normal in that period. Cooling degree-days are a key index of energy consumption for air conditioner use. Heating and cooling engineers calculate degree-days to determine energy needs based upon temperature.

If the daily mean temperature is above 65, buildings will require air conditioning to maintain a comfortable temperature. The daily mean temperature is simply the average of the day's high and low temperatures. Each degree above 65 is counted as one cooling degree-day. For example, if the average temperature is 70, that adds up to 5 cooling degree-days.

The cooling degree-day anomaly this summer was mostly caused by the many nights when low clouds kept temperatures warmer than usual.

Elsewhere across the state, Sacramento had a coolish summer with only 49 days over 90 degrees compared to an average of 53, and an average maximum temperature 1.6 degrees below normal.

The largest statewide departures from normal were in the coastal sections of Southern California, where the usual June Gloom predominated through much of July and August as well. This dipped the average high temperature at Los Angeles Civic Center to 2 degrees below normal, and there was only one day when the thermometer reached 90 degrees, compared with the normal eight days.

Most striking was Los Angeles' cooling degree-days total -- 189 below normal. And in the densely populated Southern California coastal plain, this really helped take the edge off energy use.

The reason, in general, for the cool summer was a persistent trough of low pressure aloft that caused a continuous sea breeze and thick marine layer. Although we had several warm spells when high pressure aloft moved over the state from the desert Southwest, there were no prolonged heat waves with strong offshore flow and significant warm-up in the more populated coastal sections.

Q  What effects did the plume of smoke and debris from the tragic events at the World Trade Center have on air pollution in the New York City area? Or farther downwind? On the global climate? Adrienne Parcher -  Fremont

A  From most accounts the air quality impact was fairly localized to within about a mile of the World Trade Center, where the heavier particles such as concrete dust settled, creating inhalation problems for the rescuers. The lighter particulates, mostly burned materials, rose several thousand feet and were carried away by low-level winds.

Fortunately, the winds immediately following the disaster were from the north and carried the plume south over the New York Harbor and out over the Atlantic where it has dissipated. This plume even showed up dramatically on weather satellite imagery -- one is available at www.osei.noaa.gov/Events/Fires/US_MidAtl/FSMusNY254_N4.jpg

However, the amounts of material involved are too small to have a significant long-term impact. To put it in perspective, even the more than 500 oil well fires that burned for several months after the 1991 Gulf War apparently had only a regional impact and did not affect the global climate. A NASA study found that the plumes did not reach into the stratosphere, above 40,000 feet, where they could have spread globally.

In contrast, the volcanic eruptions of Mexico's El Chichón in 1983 and the Philippines' Mount Pinatubo in 1991 ejected three and six cubic kilometers, respectively, of material into the stratosphere. Both eruptions included large quantities of sulfur dioxide that helped form droplets of sulfuric acid. These in turn are highly reflective and contributed to global cooling on the order of 0.5 to 2 degrees.

Q  In the summer, we often see the Midwest and East being cooked by high heat and humidity, as we did recently. Why is the inland West fairly dry and the majority of the country east of the Rockies humid? Is it something to do with the air off the Gulf of Mexico that hits the East? Whereas the West gets air off the Pacific? But how about Toronto or Boston -- they're humid, and they're not close to the Gulf of Mexico. And why is Denver dry and cities just a little east, like Kansas City and St. Louis, very humid?   Gordon Kass - Los Gatos

A  A look at a surface weather map of North America in the summer usually shows two predominant features -- the Pacific High and the Bermuda High -- with clockwise circulation around each. The flow around the Bermuda High brings warm, humid air north from the Gulf of Mexico over the eastern half of the country, including cities in the Northeast and around the Great Lakes. Conversely, the flow into the Pacific states comes from the northwest over the cooler waters of the north Pacific. The rest of the West, including Denver, remains warm and dry in the region between these two circulations.