There was an interesting post over on the New York SkyWatch blog, which raised many of the common questions that people have about various contrail anomalies. I’ve attempted to answer all of the questions here:

Question #1, why [do] jet contrails appear as if the jet engine is being deliberately turned on and off?
It’s because the jet is flying through uneven areas of temperature and humidity. Explained here

Question #2, Explain why jet aircraft are leaving persistent contrails in grid patterns?
Because some jets fly north-south, and some east-west on airways that are several miles wide, and where they cross you get a grid. Winds at altitude blow at around 90mph, and these blow the trails across the sky, increasing the size of the grid.

Question #3, Shouldn’t the entire sky be filled with contrails? Would we ever see any truly blue sky again if all jets left persistent contrails behind?
Sometimes there are a lot of contrails, and they do spread out and cover the sky. Sometimes there are only a few. Sometimes there are none. It depends on the weather. There are only a certain number of jets flying overhead every day. They have to be at the right altitude for contrails to form. In some weather conditions this is limited to only the highest jets, or sometimes just jets in a narrow band of humid air. On very cold humid days you will see a lot of contrails, but no more than the number of jet at altitude.

Question #4, I know that contrails are formed under certain weather conditions and altitudes. However, there are times when conditions do not exist for contrails to persist?
Then they will not persist. The problem here is that it is incredibly difficult to know what the humidity is at a specific time, altitude and location. Humidity measurements are done by sending up a balloon every 12 hours from weather stations 300 miles apart, the balloon can be blown hundreds of miles in a random direction during its ascent. How can you get an accurate local humidity reading at a specific time and place from a balloon reading 6 hours ago and 300 miles away, when humidity can vary enough for contrails to form or not form in as little as half a mile (as you can see from the broken contrails), and humidity can vary by 50% over the course of a day?

Question #5, Certain contrails even look like they are laden with chemicals. You know the ones I mean. The ones that appear to drip by the weight of their own substance, mushrooming along the bottom edge of the trail. C’mon what’s in this stuff. It doesn’t even look like ordinary condensation to me?

They are called “pendules”, as seen in this pre-1991 photo, and described by Schaefer and Day in 1981. When a plane flies through the air at 500mph it creates wake turbulance, which is made up of wake vortices, (whirlpools of air), at regular intervals. These vortices make the contrail clump up in areas of greater and lesser density. In a dense persistent contrail, the vortices will produce the clumping pendules seen in the first photo. If the contrail is thin or not persistent, then they can leave interesting patterns which can resemble smoke rings. The type and visibility of the vortices will depend on the the size, shape and speed of the jet, as well as the turbulence and density of the air it is moving through.

Question #6, Speaking of abnormal, I have seen jets emitting contrails that are dark in appearance. I assumed that it might be the lighting conditions until I saw the black and white contrails side by side - check out this video?
I covered this in the “dark lines” article. It is the lighting conditions, the dark contrail is in shadow, and when you see contrails “side by side”, the lighter contrail could well be ten miles behind (or a mile above) the darker contrail, and so not in shadow.

Question #7, Another interesting aspect to these dark trails is that the material drifts to the ground in clouds that resemble the black smoke from a nearby fire. I have observed this process several times in the fall. I haven’t seen them since the beginning of the year?
Precipitation falling from clouds will look dark if it is in shadow, which is probably will be if the shadow is caused at sunset by a hill or cloud bank.

Question #8, And then there’s the other trails that appear to swallow up other trails and clouds. One of my YouTube videos shows this quite clearly. My first impression was that someone had a HUGE washcloth and scrubbed a portion of the sky. And then I saw jets going through the cloud mass and the contrails that were left created lines in the mass that expanded and left the cloud in sections.
This effect was observed in 1944, it’s basically a distrail, which is the opposite of a contrail, the cloud contains moisture, and the added moisture, particulates and turbulence of the jet passing through makes the amount of moisture in the air too great for it to exist as a cloud, so it precipitates as snow or rain. This wipes away the cloud where the contrail has been. This can also happen with cloud layers that are below the contrail, as precipitation from the contrail falls on the cloud layer. This is also known as “hole punch clouds” and “fallstreak holes“. Distrails can combine with the vortices of question 5, as in the photo on the right.

I hope I’ve answered all these questions. If you feel there is still something unclear, or you have extra questions, then please leave a comment below.

*UPDATE* Some more questions were raised in the comments, and I’ll add them here, with answers.

Question #9, On your website you have a picture of one jet leaving a “chemtrail” with another jet at the same altitude in the background is not leaving one. I have seen jets side by side and one leaving a normal contrail and another leaving a chemtrail?
There are two possibilities. either the jets are not at the same altitude, and just look like they are because of the viewing angle, or they have different engines with different exhaust characteristics. The more efficient an engine is, the more likely it is to leave a lasting contrail, as there is more water in the exhaust. The photo on the right shows an Airbus A340 (maiden flight: 1991) on the left, leaving contrails, and a Boeing 707 (maiden flight: 1957) not leaving contrails. Both are flying at 33,000 feet (part of a German test to study contrail formation), but the newer engines of the A340 produce more water vapor.

Questions #10, How do you explain pictures of aircraft spraying chemtrails from ports other than the engines?
It’s an optical illusion. The contrails come from the engines. Engine exhausts contain a lot of water, which (combined with the water in the air), condenses, freezes and causing the contrail. Because it’s hot when it exits the engine, it takes a fraction of a second to condense and freeze (in 40 below temperatures). So it begins to freezes about 100 feet behind the engines, which makes it look like it’s coming from the ends of the tail section. This illusion is stronger on a shorter two-engine plane - look at the inner engines on the picture on the left. Since it’s a four engined 747 (240 foot long), the contrails form before the tail section, but with shorter planes such as a 767-300 (180 foot long) the contrails would be forming about at the tips of the tail section when viewed from below (although they are actually well underneath the tail, as you can see in the close-up).

In very humid conditions, the turbulence caused by the aircraft itself can cause moisture in the air to condense, and hence freeze. This happens in areas of low pressure above the wing, and in the wingtip vortices, so you can get what looks like a solid sheet of contrail from the wing, and thin streamers from the wing tips (and maybe the tail), combined a bit further back with the engine contrails, as in the photo on the right. These low-pressure wing contrails can form at any altitude, given the right humidity, and account for the tales of planes landing “still spraying”.

Question #11, How do you explain scientists testing the fallout and finding aluminum, barium, nano particles, fungus, molds, viruses, etc in the mix?
They did not test contrails, they just tested some stuff they found on the ground, with no indication that it was connected with contrails (it would take several days for aerosolized matter to reach the ground, and by that time it would have been spread hundreds of miles from the original site). Most of the things they claim to have found are things that naturally occur in air and dirt.