Weather Watch

Why rocket's contrail ended abruptly

Special to The TribuneSeptember 1, 2013 

Chris Arndt’s photo shows where the contrail ended in relation to the Delta IV Heavy rocket, marked by the red arrow.

CHRIS ARNDT

Gravity is a force that most of us don’t think about. That is, until you try to launch several tons of satellites into orbit.

On Wednesday, the 23-story high Delta IV Heavy rocket blasted into space from Vandenberg Air Force Base. This rocket can place a payload of 13,800 pounds in geosynchronous orbit, 22,000 miles above sea level.

At launch time, Chris Arndt of SLOweather.com took a photo of the rocket from San Luis Obispo as it ascended into the atmosphere. He noticed the Delta IV rocket’s tail of white clouds abruptly stopped after a few seconds into its fiery flight into space.

Chris and a few other readers of this column wondered why and at what altitude did the plume of white narrow clouds stop. An insightful question indeed!

To answer this question, this starts with the Delta IV rocket engines. Inside the engines are combustion chambers where liquid hydrogen and liquid oxygen are burned, really more like “exploding.” This condition creates massive amounts of pressure against the upper inner-surface of the chamber. This pressure or force drives the rocket skyward. The exhaust, which is hot water vapor or steam, leaves the bottom of the rocket engine and condenses into brilliant white narrow clouds contrasted against the blue sky.

These white narrow clouds are called condensation trails or contrails. They can evaporate swiftly if the relative humidity of the surrounding air is low. However, if the relative humidity is high, contrails can last for many hours. Even if the day’s temperatures are mild but the relative humidity high, you can sometimes see your breath.

Before launch, the 30th Weather Squadron from Vandenberg launches weather balloons with a tiny transmitter called a radiosonde attached. As the weather balloon climbs through the atmosphere, its transmitter broadcasts back to the receiving station readings on temperature, pressure and humidity levels, as well as Global Positioning System coordinates for the winds.

At launch time, the relative humidity at the surface was about 93 percent and the rocket’s exhaust plumes were clearly visible. At about 2,000 feet, the rocket flew through a strong temperature inversion layer. The air temperatures quickly rose to 80 degrees, but the dew point remained at about 54 degrees. This caused the relative humidity level to drop to around 41 percent, still high enough to produce a visible rocket plume.

At about 12,000 feet of altitude, the air temperature gradually cooled to 72 degrees, but the dew point temperature plummeted to 37 degrees. The calculated relative humidity level dropped 28 percent and more than likely was the altitude where the plume disappeared from the sky.

The weather balloon soundings also revealed 65 knot southwesterly winds in the upper levels of the atmosphere. PG&E meteorologist Neil Flaiz, stationed in Northern California, told me, “At this time of the year, upper-level southwesterly flow is usually very dry with relative humidity levels in the single digits.”

If you looked to the sky last week, you probably noticed the lack of contrails from aircraft.

• • •

Did you know that PG&E delivers some of the nation’s cleanest power? More than 50 percent of the electricity the company provides to customers comes from sources that are renewable and/or emit no greenhouse gases.

John Lindsey is a media relations representative for PG&E and a longtime local meteorologist. He is president of the Point San Luis Lighthouse Keepers. If you have a question, send him an email at pgeweather@pge.com.

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