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The presence of jet aircraft in the upper atmosphere can be used as an indicator of the currents and flow patterns of the upper air. The exhaust of jet planes puts water vapor, aerosol hydrocarbon particulates and sulfur compounds in the upper air. These particles serve as the foundation for cloud building near 30,000 ft. In nature, microcrystalline salt crystals formed by breaking ocean waves as well as volcanic dust and other particulates are driven aloft by updrafts. These minute particles serve as the most common source of what are known as ice nuclei. Ice nuclei are an earth element in a super-cooled area of the atmosphere. Microscopic water droplets is attracted to the surface of the particles where it freezes, forming nuclei for raindrop formation. As the microscopic nuclei gather moisture on their surfaces they grow larger and heavier and turn into microscopic frozen water droplets. As the frozen droplets grow they fall through the atmosphere and attract more droplets. These droplets freeze together in clusters and the nuclei grow still larger and heavier.
Through this process, the growing ice crystal eventually becomes so large that it falls farther down into a much warmer area of the atmosphere. It then melts and becomes the basis for rain. This is the shortened version of a universal process by which raindrops form and fall from clouds. Most rain starts out as microscopic ice crystals that are formed around a seed of some form of particulate earth. We will return to this idea at the end of this article for some interesting considerations.
In the case of the exhaust trails of jet aircraft the water vapor and hydrocarbons from the unburned jet fuel serve as ice nuclei for high ice clouds in the cruising level of most commercial aircraft around 30,000 to 35,000 feet. Meteorologists know this area as the tropopause. Tropopause means the weather stops (pauses) here. The tropopause is the sensitive upper boundary where the weather patterns for the next few days, present themselves in a kind of symbolic script, for the knowledgeable observer. It is ironic that the most responsive and sensitive area in the atmosphere is just the level in which it is most efficient to fly jet aircraft.
When conditions in the highest levels of the atmosphere are ripe for the formation of clouds, the familiar and ubiquitous jet trails can be seen crossing the sky. It is possible to tell a great deal about the conditions at the tropopause level by watching the formations of jet trails. In the first image the jet trail can be seen isolated in the sky as it tapers away to nothing on both sides of the cloud formation. This is due to the fact that the aircraft that made the trail was traveling though an area that did not support the formation of the cloud and then suddenly traveled through a plume of air that supported condensation.
What is happening here is that warm air rising from the ground carries vast amounts of water vapor upwards into the highest levels of the atmosphere. This water vapor exists as massive, plumes of warm, moist air ascending to very high levels. When a rising plume reaches the upper atmosphere it condenses into high ice clouds known as cirrus or feather clouds. In the case of the jet contrail (condensation trail) in the first image, the condensing cloud formed where the aircraft passed through a rising plume of moist warm air. The air on either side of the plume was not sufficiently moist to support the formation of an enduring cloud. As a result the contrail only remained visible in the warmth plume. We will return to this later as a mode for reading the direction and intensity of the upper winds.
In image two it can be seen that the aircraft moved through successive warmth plumes as it passed from north to south (left to right) in its flight. Where the warmth plume supported the formation of clouds with extra moisture, the contrail appears. Where there was no plume of rising warm air there is no cloud formation.
In image three we see an intermittent contrail that has suddenly begun to form a stream of ice crystals that are blowing out of the cloud towards the right. This would tell us that the prevailing wind aloft was from the left and that the plumes of warm air were most likely orienting across the contrail as the ice crystals are dropping out of the forming cloud. The formation of ice crystals out of a growing cloud is called glaciation. It is a signal that the cloud is a mature cloud that has stopped growing and is now in the decay or precipitation phase of its existence.
In image 4 we can see the difference between newly formed contrail that is forming and disappearing a few seconds behind the aircraft that is forming it and an old contrail that has been in existence for a while and is now glaciating and spreading into a layer of cirrus clouds. This means that in the layer in which the aircraft is traveling the conditions for the persistence of the contrails in not positive. The older, glaciating contrail is at right angles to the new trail. This could mean that the aircraft is only forming a contrail where it is passing through the plume that is supporting the spreading, persistent contrail. In this case however the transiting aircraft was forming a disappearing contrail all the way through its transit of the area. This meant that the persistent contrail and the disappearing contrail were at different layers in the upper atmosphere.
In image 5 we can see a close-up of a glaciating contrail. The upper winds are blowing across the contrail from the top to the bottom of the photo. Notice that the puffs of ice clouds below the heavier band at the top are much larger than the small puffs of cloud in the top of the band. This tells you the wind direction at 30,000 feet and that the glaciation of the cloud has just begun.
In image 6 taken a few minutes later, all of the puffs are about the same size. This indicates that the plume that is supporting the formation of the clouds is providing warm moist air to the whole band of clouds and that the persistence of the band is likely to be strong since there is overall vertical movement in the cloud formations.
In image 7 taken about five minutes later, we see the characteristics of the puffy clouds have been severely altered. The puffy form signifies that the air in the clouds is moving upwards. This means that the cloud is building. In image seven, fine streamers of ice are being pushed out of the glaciating cloud from top to bottom in the photo. This streaming means that the cloud is no longer building upwards but the predominant force of the formal development of the cloud is horizontal within the layer in which it is forming. It also says that this cloud is losing contact with the warmth plume that formed it and it is moving away from the upward surge of warm air and is now in an environment where the surrounding air temperature is very cold and dry. As a result ice is forming.
Not all contrails are equal. In image 8 we see a new contrail that is disappearing behind the aircraft as it moves from below to above in the photo. Below the newly forming contrail we can see an old contrail that has been crossed by another aircraft. The relatively newer contrail is only forming in the zone where the old contrail had formed. This indicates that the warmth plume that is moving perpendicularly to the broad band of the old contrail is still supporting the growth of the older band of clouds but that the aircraft flying above this older band is in a layer that is not supportive of condensation.
By watching such sequences the wind direction aloft, and wind speed aloft as well as the prevailing condition of upper air moisture present in the tropopause can be perceived. This image shows that the large cloud-building plume of rising warm air has a limitation in height. Above it there would be no cloud building. This would indicate that even though the clouds are building in the lower band there would probably be no future developments out of these clouds regarding large- scale changes in the weather.
In image 9 the sky is filled with ice clouds that once were contrails. Given the right conditions, these aircraft seeded, high ice clouds can build into naturally occurring cirrus clouds that would normally form in advance of an approaching warm front. Some researchers believe that such high ice cloud cover induced by aircraft has dire influences on global weather by keeping warmth close to earth. This is due to the insulating properties of the high ice clouds.
In image 10 we can see a beautiful type of cloud known as virga. In essence, in this cloud the ice is falling out of the cloud as the cloud is being pushed to the right by the upper winds. The tail of the falling ice crystals is being blown in advance of the cloud itself. As they fall into warmer air they undergo a process of sublimation. That is, the ice crystals transform directly into a gaseous vapor rather than forming droplets of water. The warm vapor rises quickly and reforms at the height of the original cloud but downstream of it. This can be seen in the streaming cloud moving out to the right of the original virga. The sublimation process is actually a rain that never reaches the ground. It is caused by a superabundance of ice nuclei in the upper levels of the atmosphere. As beautiful as it is, the virga cloud is a sign of drought. The water in these types of clouds does not reach the earth but stays trapped in a cycle of precipitation and sublimation for cloud building cycles that lie far above the earth.
In image 11 a haunting angelic dance of late in the day virga tell of dry weather ahead even though their appearance delights the eye and stimulates the imagination. This type of cloud formation is the result of a busy day of aircraft contrails being laid down in the sky at the border of the tropopause the most sensitive boundary for weather in the atmosphere. The question regarding the influence of contrails on drought patterns is a deep one involving many variables. With contrails, the wonderful predictive images of the high ice clouds as harbingers of storms weaves into the more darkly etched human concerns of an economy based on fossil fuels.