Published Tuesday, July 1, 2003 in the San Jose Mercury News

Weather Corner

Personal Beacons Can Save Stranded Individuals

Beginning today, the weather satellites that provide the pictures we all see on the 6 o'clock news may help save your life. In a joint program between the National Oceanic and Atmospheric Administration (NOAA) and the U.S. Air Force, individuals can use personal locator beacons that can send a digitally encoded distress signal with your location to rescue personnel.

The devices are similar to the emergency locator transmitters that are used to locate aircraft that have crashed or emergency position indicating radio beacons that are used on boats and ships.

Operating on a frequency of 406 megahertz, personal locator beacons must be registered by their owners with NOAA. Each has a unique identification number with the owner's name and emergency contact information.

The signal from a beacon, which uses global positioning satellite technology to determine location, is detected by both NOAA geostationary satellites in orbit 22,500 miles above the Equator and polar orbiting operational environmental satellites about 450 miles above Earth.

The signal, along with the beacon's identification number, is relayed to the Air Force Rescue Coordination Center at Langley Air Force Base in southeastern Virginia. The center verifies whether the beacon's owner is indeed lost or overdue and notifies the appropriate state search and rescue personnel.

Not only will the beacons save lives by the timelier rescue of people in distress, they will also conserve the resources of rescue agencies by allowing them to go directly to the point of trouble. A number of other countries have used the beacons since the 1990s, and there has been an experimental program in Alaska since 1994.

Personal locator beacons can be found at major sporting goods and marine supply stores such as REI and West Marine Products. They cost about $600 to $800 and are about the size of a video cassette, but both the cost and size are expected to drop as they become more readily available.


Q Why is it hotter in San Francisco around 2 p.m. than it is at noon? Al Williams - San Francisco

A This is not unique to San Francisco; many locations reach their maximum temperature even later in the afternoon. The reason is that even though the sun is at its highest at noon, very little of the warming of the air is from the sun directly. Instead, the air is warmed from below by heat given off from the ground and other objects that have been warmed by the sun. Air is a very poor conductor of heat, so this process is quite slow. This results in the lag between when the sun is at its maximum and the hottest time of the day.

Q I'm writing to you from Jacksonville, Fla., but we moved here 10 months ago from California. My question is this: What is the highest possible percentage of relative humidity? I also would like to know the record for the highest humidity recorded in Humboldt County. I have a friend who says that the relative humidity was more than 100 percent on a couple of occasions. Is this possible? Rocky Van Burn = Jacksonville, Fla.

A While it is possible to have a relative humidity greater than 100 percent, this occurs only briefly and under very specific circumstances. The air is saturated and the relative humidity equals 100 percent when the dew point temperature is the same as the air temperature. However, the temperature can briefly drop below the dew point temperature and the relative humidity can exceed 100 percent for a short time. When this occurs, the air is said to be super-saturated, but this usually occurs under laboratory conditions and I have never seen a weather observation with a relative humidity greater than 100 percent.

Q Is one safe in a car while in a thunderstorm? If the car gets hit with a lightning bolt, what would happen to the person inside? Tim Ransbottom - Santa Rosa

A Typically, electricity will take the path of least resistance and is carried on the skin or outside of a conducting object. This is known as the ``skin effect,'' and in the case of a car, the outer body carries most of the electrical charge. Consequently, a person inside a car can be relatively safe unless they are touching something metal that is connected to the car's body.

Q Are fish or whales killed when lightning hits the ocean or a lake? Tim Robinson - Redwood City

A Water is a good conductor of electricity so the electrical current from a lightning discharge travels through water quite well. It's unknown how far the charge can go and still be dangerous to marine life, but it is probably on the order of tens of yards. Within this zone, animals would probably be incapacitated, if not killed outright.

Q How safe is it to be on a cruise ship in the Caribbean during hurricane season? I've heard the seas can get very rough, but how do the ships avoid something that seems so unpredictable? Denise Olazar - Sonoma

A Compared with a lot of weather events, hurricanes are relatively predictable and slow-moving. Consequently, cruise ships can vary their itineraries to stay out of harm's way so that hurricanes are no more than an inconvenience because of the rough seas they may kick up. The greatest problems arise if they occur in port, meaning that airline flights and even an occasional cruise can be canceled.

Q How do forest fires create their own weather system? Andrew Cassetta - Novato

A Not only can wildland fires be affected by the weather, but they can indeed make their own weather. As a fire burns, strong updrafts are created and an area of low pressure develops. Air from surrounding areas of high pressure then rushes to fill the void, with strong gusty winds resulting. The ferocity of these winds can be enhanced as they are funneled through canyons and over ridge tops.

Another byproduct of the strong updrafts produced by a fire are clouds that even have their own name -- pyrocumulus or ``fire cumulus.'' These arise when moisture from trees and other burned objects is pulled high into the sky by updrafts and condenses into clouds. Under the right or wrong conditions, these clouds can produce rain to dampen a fire or lightning that can cause additional fires.

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 can telephone questions at (510) 657-2246, fax them to (510) 315-3015 or e-mail them to or fill out a form online at