the WEATHER CORNER

BY JAN NULL
Special to the Mercury News

Q. What is the wind chill factor, and how do you calculate it? Bill Terry - Palo Alto

A.  Wind chill is the cooling one feels from the combination of wind and temperature. To calculate wind chill, you need two factors: the wind speed in miles per hour and the air temperature in degrees Fahrenheit. The formula for wind chill is quite complex.

An easy way to figure wind chill is to use a table such as the one at http://ggweather.com/windchill.html on the Internet. The formula also is posted there.

I also created a spreadsheet with wind chill and a host of other meteorological calculations and conversions. Go to http://ggweather.com/wx_calc.htm to download it.

Q. We want to buy an accurate thermometer to hang outside our bedroom window. The few we have looked at appear cheaply made, and we question their accuracy. Any information or suggestions will be greatly appreciated. Liz Rhodes - Palo Alto

A. You really don't need an expensive thermometer to get fairly accurate readings. However, like real estate, the most important factor is location, location, location, because the siting of a thermometer can greatly influence the accuracy of your reading. Most importantly, the reading should be taken in the shade and in a well-ventilated area.

It also should be taken about five feet over a natural surface and as far from concrete or hard surfaces as possible. These objects reflect heat directly during the day and absorb heat at night, causing the thermometer to indicate warmer than the true air temperature. The bottom line is that you want the temperature of the well-ventilated air, unaffected by sunlight or warmth from other objects.

Q. How many fluid ounces of water are in an inch of rain? I have two plastic rain gauges that read very differently even though they have the same shape and entry throat diameter. I tried the fluid equivalent of a one-inch cube of water and a cylinder of water one-inch long with a one-inch diameter.  Neither came close to a one-inch reading in my rain gauges. I found a definition of one inch of rainfall as that amount of rain that collects on a U.S. Weather Service standard collection that fills up to the 1-inch mark.'' Is there a better definition of volume? Bill Terry - Palo Alto

A.  Your problem is that an inch of rain is the depth over any area, so the volume of the water would depend on the size of the area. For example, an inch of rain will be one-inch deep in a small plastic rain gauge, a coffee can or spread over a parking lot, so the volume will vary greatly. A standard Weather Service rain gauge is a cylinder eight inches in diameter, so one inch of rain would have a volume of about 28 ounces or about three cups.

Measurements of very large volumes of water, such as lakes or reservoirs, are expressed in acre-feet. This is the volume of water necessary to cover one acre (about 91 percent of the size of a football field) to a depth of one foot. It is equal to 43,560 cubic feet or 325,851 gallons.

Q. How do I set the barometer that I received for Christmas?   Many callers

A. Most barometers have a small screw on the back that can be turned to adjust the needle. The barometric pressure can be obtained from many places on the Web and is often on television weather reports. That's the easy part. However, there are a couple of other considerations.

Because a barometer is simply measuring the weight of the column of air above, the reading decreases as you go to a higher elevation. (An altimeter is nothing more than a barometer that is calibrated to read out the altitude.) Consequently, the pressure at 4,000 feet, near the top of Mount Hamilton, is only about 93 percent of sea-level pressure.

It's also important to realize that words such as ``fair'' or ``stormy,'' often printed on barometers, are only general guidelines. It's better to look at whether the pressure is rising or falling, and how rapidly the change is happening. That's why most barometers have a pointer that you can place over the current reading. The next time you look at the instrument, you can see how much the pressure has changed and whether it is rising or falling.

TOP 10 LIST: I received a number of reader nominations for additions to my list of the Top 10 Bay Area weather events of the past 50 years, published Jan. 4. Most often mentioned were the January 1950 tornado in Sunnyvale and the big freeze of 1972.

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