Timothy Ballisty
Meteo 471 W
Dr. Fraser
This photograph was taken in early November 1999. I was standing in the parking lot outside the Walker Building at Penn State's University Park Campus. These white streaks in the sky are called contrails. Their consistent position in the sky point out a flight pattern that jets tend to take over the State College, PA area.
Description of Photograph
Above is a picture of a certain
type of mixing cloud called contrails created by jets soaring through the
sky. As you can see, it was busy day in the sky. The contrails
or condensation trails depict in this picture the several flight paths
(about six) that were created throughout the day over a small area of the
town of State College. There seems to be a certain flight path that
jets follow on a consistent basis. This is shown from the five parallel
lines directed from the upper-right to the lower-left center portion of
the photograph. There is, however, one exception to this assumption
with one contrail crossing right through the parallel contrails.
The weather conditions were
much like the conditions in my assignment two paper. A high pressure
system was moving in from the central plain states. There was a chill
to the air with the air temperature at 40ºF to 45ºF and the winds
were coming out of Canada from the north-northwest. This day happened
to be a very dry day with the measurement of relative humidity at 35% to
45%.
Mixing Cloud Physics
So what produces these beautiful
streaks in the sky? Factors such as vapor pressure and temperature
must be taken into account when trying to describe the processes that take
place when condensation trails are formed. As I stated before, contrails
are mixing clouds. Certain air masses with different attributes and
qualities are mixed together to form a cloud. These particular attributes
are best explained by explaining water molecules in its liquid and vapor
form and its relationship with the atmosphere.
The atmospheric pressure comes
about by the many molecules of gases in the atmosphere that exert a certain
partial pressure on its environment. These partial pressures contribute
to the total atmospheric pressure. One such gas is water vapor.
The pressure that it is exerts is called the vapor pressure. When
determining whether a mixing cloud will form, the equilibrium vapor pressure
must be taken into consideration. Equilibrium in an air mass is reached
when the rate of evaporation of water molecules is equal to the rate of
condensation. The pressure exerted by the water vapor at this point
is called the equilibrium vapor pressure. Bringing these ideas together,
one can better understand what happens when a mixing cloud forms.
It has the potential to form when the vapor pressure of water vapor in
an air mass surpasses the equilibrium vapor pressure.
Below is a vapor pressure
vs. temperature diagram. The orange curve represents the equilibrium
vapor pressure at a certain temperature. This curve separates all
subsaturated air from supersaturated air. The curve itself represents
saturation. Above the curve denotes supersaturation as is pointed
out in the graph. Supersaturation means that the air mass has a vapor
pressure at a certain temperature above the equilibrium vapor pressure.
At this point, the supersaturated air can form a mixing cloud if condensation
nuclei are present.
Figure
1. Vapor pressure vs. Temperature
In the case of the contrails, however, there is not one air mass contributing to the formation of a mixing cloud but two air masses that are mixed together. The points A and B represent two different masses of air. One air mass comes from what is released by the jet engines and the other air mass is the surrounding air outside the jet. Point A represents the very hot and moist air that is exiting the jet. This engine air has a high vapor pressure. Point B on the other hand represents the cooler and drier environmental air that the jet's air is emptying into. This environmental air has a low vapor pressure. The result of these two air masses mixing, with different temperatures and vapor pressures, is a linear combination. This linear combination is depicted by the blue line connecting points A and B. This line lies above the concave equilibrium vapor pressure curve. Hence, the mixing results in a supersaturated air mass. Again, if condensation nuclei are present, the water vapor in the supersaturated air condenses onto the tiny particles. In the situation of the contrails, the particles in the jet exhaust make up the nuclei.
Longevity
Longevity of a contrail depends
mainly on the temperature and pressure of the environmental air.
Although a cold, dry environmental air is needed for a contrail to form,
the air cannot be too dry. This will allow a long lasting contrail
to exist. If it is too dry, the warm and moist air from the jet will
mix readily with the environment and dissipation of the mixing cloud occurs
quickly.
In some instances, a contrail
may not form at all. The environmental air may be drier, cooler,
and lower in vapor pressure than the jet engine's air, however, the linear
combination of the two air masses may never cross over the equilibrium
vapor pressure curve. This can be seen by the red line in Figure
1. Air masses A and C, representing the previously mentioned situation,
form a line that lies below the equilibrium curve thus cloud formation
is not possible.
Looking at photograph 1, it
seems there is just the right ingredients in the atmosphere so that a long
lasting contrail can form. This favorable environment for contrails
existed that whole day. Contrails continued to form throughout the
day and were still present when photograph 2 was taken in the late afternoon
into evening hours.
Contrails still streaking through the sky as dusk settles in.
Looking back at photograph 1, two different types of contrails can be seen. The condensation trails on the left side of the picture are much more ragged looking than the contrails on the right side. Some dissipation has occurred to these ragged streaks that are also wind blown. The wind appears to have stretched out the contrails and have made them wider. But the sharper, more distinct contrails on the right, most likely have just been created by passing jets. There hasn't been enough time so to allow other elements like the wind to affect the shape of the contrail.
Comments
Contrails add to an already chaotic yet beautiful sky. They occur at any time of the year in any part of the world. The age of contrails can be found by the actual appearance of the contrail. A thinner and distinct contrail designates youth while a ragged and stretched contrail designates a contrail that has been around for some time. Finally, the amount of air traffic directly relates to the occurrence of contrails.
References
Bohren, Craig. Atmospheric Thermodynamics. New York: Oxford University Press, 1998.
Knopf, Alfred. The Audubon Society Field Guide to North American Weather.
New York:
Random House, 1992.
Hobbs, Peter, and Wallace, John. Atmospheric Science: An Introductory
Survey. San Diego:
Academic Press, 1977.
I believe, if I am not mistaken, that the two photographs above of contrails are copyright of Timothy D. Ballisty and no one else. Thank you for your time.
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