Here they are, the wonderful fallstreak
clouds. This picture was taken in early September, 1999 outside my
apartment complex in State College, Pa.
Picture Description
The photograph above is a picture
of a flock of clouds called fallstreaks. This picture was
taken in early September on an unseasonably cool, early afternoon in State
College, Pa. Although you can not tell from this picture, it was
bright, sunny day with a cool breeze blowing at the surface making it seem
like an autumn day. It is said that fallstreaks are a direct link
with approaching warm fronts but fallstreaks can be found in a variety
of weather situations. Fallstreaks or cirrus uncinus are commonly
described as being shaped like hooks and looking almost cotton candy like
in appearance. Shown above are some very well defined fallstreaks
with a main cloud body and ice crystals trailing from the main cloud.
This is seen in the right half of the picture. However, there are
also some fallstreaks shown here that are dissipating shown in the left
portion of the photograph. There seems to be a trail of ice crystals
but the main body of the fallstreak cloud is not present. It is my
estimation that dry air has entrained into the main head of the fallstreak.
Thus, the rate of evaporation of the ice crystals increases and the cloud
dissipates with a trail being the only evidence of a once well defined
fallstreak. Fallstreaks are part of the cirrus family which are usually
located around 20,000 to 25,000 ft in the sky, but in reality fallstreaks
can form below or above the altitude range given. As long as water
is in a suitable environment for crystal formation, fallstreaks can exist.
Formation
Ice clouds, such as cirrus
uncinus have a unique way of forming. Their beginning is like
any other cloud. Air is lifted to the lifting condensation level
via updrafts, upsloping over a mountain or by any other means of lifting.
This is the level at which air becomes saturated. In order for fallstreaks
to occur, ice crystals must form and so it is known that saturation occurs
above the freezing level. At this point, vapor is able to condense
and become a liquid due to the condensation nuclei that is present.
Due to the location above the freezing level where this process takes place,
the liquid solidifies into ice. At this point, the formation of the
fallstreak becomes somewhat complex. What happens next is that the
newly formed ice grows in size. Evaporation occurs at a slower rate
from an ice crystal compared to its surrounding neighbors of water droplets
that haven't quite froze yet. This is because of the less tightly
bound state of the liquid; evaporation occurs more readily in situations
when it takes less energy for a water molecule to escape from its bond.
So, in the end an ice crystal grows when at the same time the surrounding
water droplets are decreasing in size. Water vapor around the ice
crystals condense onto the ice much more frequently than onto the neighboring
water droplets which in some cases are actually becoming smaller.
As the growth continues, the crystals weight increases and the crystal
finally drops from the cloud becoming part of the fallstreak's tail.
What produces the streak anyway?
As time goes by, the crystals
grow larger and heavier and because of this they begin to fall from the
mother cloud at a rate of 2 to 3 feet/second or 0.6 to 0.9 meters/second.
Notice the
magnitudes of the wind vectors. However, the velocity magnitudes
do not have to increase with height as shown above. What is
needed is a constant wind shear for this type of a fallstreak to be produced.
Many times as the ice crystals
descend, they travel through levels of constant wind shear.
In the diagram above, the fallstreak has a parabolic tail.
The parabolic tail comes about because of this constant wind shear which
is caused by the presence of different magnitudes of wind velocities at
different altitudes. The photograph of the fallstreaks at the top
of the page is an example of fallen ice crystals being effected by the
constant wind shear. As a result, a parabolic tail extends from the
main body or head of the fallstreak. In some parabolic cases, the
trail can be as a long as 4 to 5 miles. There is another case in
which fallstreaks can occur without shear being a factor. Ice crystals
can fall out of the main cloud and yet have no interaction with wind shear.
The result is a fallstreak with a vertical tail instead of a parabolic
tail.
Predicting Weather from Fallstreaks
A weather prediction that you
can make from a fallstreak is the forecast of whether cold or warm air
will enter the altitude level where the fallstreaks are located.
Since, as an observer, we know that a fallstreak is good indicator of wind
shear as stated above then we can expand on that knowledge. The wind
shear vector points away from the lagging tail towards the mother cloud.
The shear vector can sometimes be a rough but good estimate of the thermal
wind vector. This vector is an imaginary vector which separates the
cool air from the warm air at any certain layer of the atmosphere..
So, with this rough estimation it is possible to say that the shear vector
divides the layer, where the fallstreaks are positioned, into an area of
cool air placed left of the vector and warm air placed to the right of
the shear vector. You can see this in the diagram below.
Along with the shear
vector is the wind vector or the direction of the wind. The wind
vector (u) does not necessarily have to go in the same direction as the
shear vector and can be determined by you the observer or by the direction
of movement of any nearby clouds. The wind in the diagram above is
moving from cooler air towards warmer air. This is also what was
happening in the photograph. With the shear vector pointing to the
top right of the photograph and a wind vector pointing at an angle to the
right of the shear vector it is possible to say that cold air advection
is occurring at the level of the fallstreaks.
Conclusion
The nature of the fallstreak
cloud is a good example of showing the complexity of the ice crystal behavior
compared with regular water droplet clouds. Also an observer can
determine with help from the formation of the fallstreak what is actually
happening, in terms of cold or warm air advection, at the altitude of the
fallstreak cloud.
References
Demark, Tony. "Fallstreaks." 11 pars. 29 Sept 1999
Ludlam, F.H., and R.S. Scorer. Cloud Study. London: John
Murray (Publishers) Ltd, 1966.
Scorer, Richard, and Harry Wexler. A Color Guide to Clouds.
New York: Oxford, 1963.
-----. Cloud Studies in Color. New York: Pergamon
Press, 1967.
Scorer, Richard, and Arjen Verkaik. Spacious Skies.
London: David and Charles Publishers, 1989.
Hey.....fallstreak photograph is a copyright of Timothy Ballisty.
It shouldn't be copied and/or used without permission.
Back
to Cloud Observations Page
Back
to My Homepage
PICTURES
Part 1 PICTURES
Part 2
PICTURES
Part 3 PICTURES
Part 4
My
Links
Recent
Events