Don't see very many contrails much in Hawaii--most of the trans-Pacific flights go farther North and we're out of the flight paths for Australia flights to the Continental U.S. So, have a question for all you aviation/weather buffs on what I suppose is a pretty elementary question.

So, I faintly remember Contrails in relation to whether there's more moisture in the atmosphere, but can't quite get it down. If the contrails are longer, is there more moisture in the atmosphere or less?

And, how does that affect weather?

This is a border-line TravelBuzz!/OMNI question given the relations of contrails to plane travel, though richard please feel free to move it if deemed necessary.

"If a contrail is short and dissipates quickly that tells us the upper atmosphere is relatively dry and that good weather, i.e. high pressure, is likely to remain for at least another 12 to 24 hours. . . . However, if the upper atmosphere is nearly saturated, then an addition of a little moisture from a jet's exhaust will bring the upper atmosphere to its dew point (100-percent saturation) and a long and lasting contrail will form."

http://www.sailnet.com/collections/a...eid=carrmi0045

It doesn't take much moisture to get 100% relative humidity at -60.

It is indeed marginal. Whether an aircraft contrails depends on a range of factors, relative humidity being among the most significant, but also altitude/air pressure, air temperature, temperature and pressure of the aircraft exhaust (they do vary), proportion of water vapour in the exhaust gases, and engine power setting. Dissipation is also dependent on winds and, to some extent, turbulence if any of the air.

It is calculable by meteorologists. Although not really of importance to civil airliners, it is very significant to the military who may not wish to draw attention to their presence, hence the work done to understand whether contrails will appear on any given day.

It's not just jets. WW2 piston-engined bombers over Europe, even at only 25,000 feet, certainly contrailed on occasions, which is why this analysis started. You can see it from B-17s in the 1943-original film "Memphis Belle" (not the more recent Hollywood travesty).

This reminds me.... According to the book "Window Seat" (Gregory Dicum), contrails can become cirrus clouds under the right conditions. In the days following September 11, skies were clearer and there was a greater variation in ground-temperature across the US due to the lack of contrails. I don't know if that's true but I thought it was interesting.

I think I remember hearing about atmospheric studies they were able to do w/ no planes in the air after 9/11. I suppose all of the flights (cumulatively) do a pretty good job disturbing the atmosphere

...and all this time I thought it was the government gassing us under some super-secret plan.

Our in-house meteorologist (I work in TV) went on and on when I showed his this thread.

What about the contrails formed at the wingtips? I've seen these very often.

These are very different. They're caused by local changes in temperature and pressure because of the aerodynamic effects of the relevant bits of the wing passing through the air at exactly that point. As soon as the disturbance has passed, the contrail will dissipate.

Those created by engines aren't the same, because they're largely made up of the huge amounts of water dumped into the air by the engine exhaust.

On a car message board I used to frequent, there was a woman who was convinced of this conspiracy theory. She said that contrails were made by military jets pumping gasses into the sky. No one could convince her otherwise. One person on the board was a meteorologist who explained why contrails really formed. Another person suggested she use binoculars or a telescope to look at the planes that were forming the contrails.

Very informative information on the contrails--thanks to everyone who has contributed!