Weather Watch

Air density is a heavy matter in flight

Special to The TribuneSeptember 1, 2012 

When I was stationed on the USS McCandless (FF-1084), the officer in charge of our Navy SH-2F Seasprite helicopter crew, Lt. Cmdr. Chuck Taylor, always briefed us before we flew. During the briefing, the subject of air density was often discussed. Helicopters fly by chopping the air; the denser the air, the more efficient the main rotors become and the less power they need from their jet engines.

If we ever had an unexpected loss of energy from even one of the two engines that supplied power to the helicopter’s rotors during periods of low air density, the consequences would have been tragic.

You see, the greatest amount of power required from the helicopter’s engines occur at takeoff, landing and especially in a hover. During landings, particularly at night, our pilots would often struggle to put the helicopter in a stable hover over the flight deck as it pitched and rolled with the changing seas. At times like that, we would move left and right, forward and backward, in an attempt to center the helicopter’s main landing gear over a frightfully small flight deck safety circle that was painted white on a deck coated with black nonskid material.

If the helicopter’s landing gear was too far forward of the white circle, you ran the risk of the main rotors striking the ship’s hangar. If you were too far back of the circle, the rear landing wheel on the helicopter’s tail pylon could end up in the flight deck safety net hanging over the water.

At times like this, I often thought about the loss of an engine. Even if you did an emergency jettison of the helicopter’s sonobuoys, external fuel tanks and other equipment that would literally shed hundreds of pounds of weight from the helicopter in a split second, you may still not have enough power to safely land on board the ship when the air was less dense.

From my backseat in the helicopter, I had a clear view of the landing gear and the white circle. When the landing gear was finally centered in the circle, I would call out, “Clear to land!” and the pilot would abruptly slam the helicopter on the steel deck. Shortly afterward, ship crewmembers ran out from the safety of the aircraft hangar and chocked and chained the helicopter to the vessel. Of course, it was easier when we landed on enormous aircraft carriers.

Not only are helicopters affected by air density, but the fixed-wing aircraft are as well. A few years ago at the San Luis Obispo County Airport on a scheduled flight to Phoenix, the flight attendants asked whether two passengers would volunteer to get off the plane because it was too heavy to make a safe takeoff because of low air-density conditions.

Pilots calculate air density from charts, calculators or weather monitoring stations, such as the Diablo Canyon nuclear power plant’s meteorological tower.

The four factors that most affect air density are altitude, atmospheric pressure, air temperature and humidity.

Generally, the higher your altitude, the less dense the air will become. One day while sailing across the Indian Ocean, we flew our H-2 helicopter to nearly 17,000 feet. At this altitude, the helicopter’s flight controls became mushy and the engine temperatures heated up as they worked overtime to chop the rapidly thinning air.

Because of changing weather conditions, atmospheric pressure fluctuates continuously. Increasing air pressure raises air density.

On the other hand, increasing the air temperature will decrease its density. Think about a hot-air balloon; as the air inside the balloon is heated by a gas or propane torch, it becomes less dense. Because it’s less dense or lighter, the hot-air balloon rises through the cooler and heavier atmosphere that surrounds it.

It may seem counter-intuitive, but humid air is lighter, or less dense, than dry air. Most of the water that is in the atmosphere is in the form of a gas or water vapor. Water vapor weighs less than nitrogen or oxygen. Because nitrogen and oxygen make up 99 percent of the atmosphere, increasing the amount of water vapor or relative humidity makes the air less dense or heavy.

If you would like to learn more about air density and density altitude calculations, log into Richard Shelquist’s website at: http://wahiduddin.net/calc/density_altitude.htm.

Today’s weather report

Light to gentle (4- to 12-mph) northeasterly (offshore) winds will develop this morning as a high-pressure system moves into the Central Coast from the east.

The onset of the offshore winds will warm the beaches, coastal valleys and North County by as much as 7 and 14 degrees compared with Saturday’s high temperatures. The offshore winds will also keep most of the coastal low clouds out to sea.

The marine layer will redevelop tonight along the beaches and spread into the coastal valleys by Monday morning.

However, it will clear rapidly by the late-morning hours. Further warming is expected Labor Day afternoon, with temperatures ranging from the low to mid-70s at the Pops by the Sea concert at Avila Beach under clear and sunny skies.

Overall, the rest of the county will see spectacular weather through Labor Day with temperatures in the 80s in the coastal valleys and the mid- to high 90s in the North County.

For the rest of the week, an upper-level ridge of high pressure will build in from the southwest, which will raise temperatures in the coastal valleys and North County.

As the week progresses, temperatures will be above-normal for early September; however, extreme heat is not expected.

Along the beaches, a pattern of night and morning low clouds and fog, clearing by the afternoon hours with increasing northwesterly winds, will start Tuesday and continue through next weekend.

There may be an increase in monsoon moisture from the south that may produce variable mid- to high-level clouds along with spectacular sunrises and sunsets.

Today’s surf report

Today’s 3- to 5-foot northwesterly (300-degree deep-water) swell (with a 7- to 9-second period) will continue at this height and period through Wednesday.

Increasing northwesterly winds will generate a 3- to 5-foot northwesterly (320-degree deep-water) sea and swell (with a 5- to 9-second period) Thursday through next Sunday.

Arriving from the Southern Hemisphere, today’s 2- to 3-foot (195-degree deep-water) swell (with a 15- to 17-second period) will remain at this height, but with a gradually shorter period through tonight. Wave heights in Southern California could reach well over 8 feet.

Another Southern Hemisphere (195-degree deep-water) swell will arrive along the Pecho Coast on Monday and will keep the swell height at 1 to 3 feet (with a 14- to 16-second period) through Wednesday.

Note: A medium-period swell from Hurricane Ileana will come from 160 degrees. Because of the southeasterly incoming direction, the swell will have little effect along the Pecho Coast.

Seawater temperatures

Seawater temperatures will range between 53 and 56 degrees through Monday, increasing to between 54 and 57 degrees Tuesday, and will remain at this level through Friday.

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John Lindsey’s column is special to The Tribune. He is a media relations representative for PG&E and longtime local meteorologist. If you have a question, send him an email at pgeweather@pge.com.

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