Contrails 'breaking up' clouds

ki_cz

Member
I've been trying to find the scientific reason behind contrails 'breaking up' thin layers of cloud cover and so far haven't managed to find anything. It's something that I don't see too often, but attached is an example, I don't want to speculate regarding the cause, but have some ideas. If anybody can post some actual information about the cause of this it would be greatly appreciated:
 

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Trailblazer

Moderator
Staff member
They're called distrails.

http://contrailscience.com/contrail-to-distrail/

They can form in one of two ways: either the heat of the engine causes water droplets in relatively low clouds to evaporate, or by triggering glaciation (freezing) of supercooled droplets, which then fall out of the cloud. Your picture looks to me like the former type, but I could be wrong.

DISSIPATION TRAILS (DISTRAILS)
The term dissipation trail applies to a rift in clouds caused by the heat of exhaust gases from an aircraft flying in a thin cloud layer. The exhaust gases sometimes warm the air to the extent that it is no longer saturated, and the affected part of the cloud evaporates. The cloud must be both thin and relatively warm for a distrail to exist; therefore, they are not common.
Content from External Source
https://www.aviationweather.ws/080_Condensation_Trails.php


There is some debate regarding the mechanism behind the formation of a distrail.




One possible explanation is that the engine exhaust gasses, emitted by an aircraft flying through a thin veil of high level cloud such as Cirrus, warm the air causing the ice crystals in the cloud to turn to vapour and disappear, creating a clear passage behind the aircraft.

An alternative explanation is that when an aircraft flies through a super cooled cloud, this cloud freezes due to the disturbing airflow created by the aircraft, as well as by the addition of plenty of freezing nuclei due to the aircraft's exhaust gases. Some of the particles act as the nuclei onto which the droplets can start freezing. As the ice-crystals form, they grow in size and fall below, this is sometimes observed as fall streak clouds below the actual distrait, or they evaporate instantaneously when entering the lower layers.
Content from External Source
https://www.skybrary.aero/index.php/Distrail


Distrails, short for "dissipation trails," are evaporation paths cut through cloud layers by the heat of a jet airplane's exhaust. Their counterparts, contrails, are more commonly seen cloud streaks left in the wake of planes by cooling of water vapor in their exhaust plumes. Unlike contrails, which require the bitterly cold temperatures of the upper troposphere, distrails can form in one of two ways: in relatively warm, low clouds made of water droplets, or as in this case, from glaciation (icing) of a thin supercooled middle to high level clouds.
Content from External Source
http://www.spc.noaa.gov/coolimg/distrail/
 

Hama Neggs

Senior Member.
Content from external source There is some debate regarding the mechanism behind the formation of a distrail.




One possible explanation is that the engine exhaust gasses, emitted by an aircraft flying through a thin veil of high level cloud such as Cirrus, warm the air causing the ice crystals in the cloud to turn to vapour and disappear, creating a clear passage behind the aircraft.

An alternative explanation is that when an aircraft flies through a super cooled cloud, this cloud freezes due to the disturbing airflow created by the aircraft, as well as by the addition of plenty of freezing nuclei due to the aircraft's exhaust gases. Some of the particles act as the nuclei onto which the droplets can start freezing. As the ice-crystals form, they grow in size and fall below, this is sometimes observed as fall streak clouds below the actual distrait, or they evaporate instantaneously when entering the lower layers.


Would this mean that there are areas of atmosphere which are devoid of condensation nuclei?
 

Trailblazer

Moderator
Staff member
Would this mean that there are areas of atmosphere which are devoid of condensation nuclei?
Freezing nuclei aren't the same as condensation nuclei. There are condensation nuclei everywhere in the atmosphere, but not ice nuclei. That's why you get ice-supersaturated regions, where RHi is above 100#, which are where persistent contrails form.
 

Hama Neggs

Senior Member.
Freezing nuclei aren't the same as condensation nuclei. There are condensation nuclei everywhere in the atmosphere, but not ice nuclei. That's why you get ice-supersaturated regions, where RHi is above 100#, which are where persistent contrails form.

I'm not sure I fully understand that. Could you elaborate?
 

Spectrar Ghost

Senior Member.
Freezing nuclei need the right structure to act as seeds for ice crystals. CCN simply need a surface for water to condense into and a small enough size that their terminal velocity is low and they don't just fall out of the atmosphere.
 

Trailblazer

Moderator
Staff member
Freezing nuclei need the right structure to act as seeds for ice crystals. CCN simply need a surface for water to condense into and a small enough size that their terminal velocity is low and they don't just fall out of the atmosphere.
Yes, vapour will condense onto just about any particles in the air if the humidity is above 100% relative to water. But if the humidity is below 100% relative to water but above 100% relative to ice (usually about 60-65% relative to water at the altitudes we are talking about) then it will only form ice crystals if there are suitable freezing nuclei available.

In very simple terms, it's harder for ice crystals to form, because they have to form in a certain alignment (hexagonal crystals), whereas water droplets can form in any random configuration.

If there are such freezing nuclei present, you get cirrus clouds. If there aren't, you get areas that are supersaturated relative to ice, just waiting for some suitable nuclei, such as those produced by aircraft exhaust.
 

Hama Neggs

Senior Member.
If there are such freezing nuclei present, you get cirrus clouds. If there aren't, you get areas that are supersaturated relative to ice, just waiting for some suitable nuclei, such as those produced by aircraft exhaust.

OK, but what has to be present for natural cirrus clouds to form? Where does the initial nucleus come from?
 

Trailblazer

Moderator
Staff member
OK, but what has to be present for natural cirrus clouds to form? Where does the initial nucleus come from?
Mineral dust, for the most part. It seems to be not totally understood as yet.

The exact properties required for an aerosol particle to act as an ice nuclei are not fully known and may well be different for each of the modes above. Also it is likely that the temperature at which each nucleation mode will become active is different even for the same type of particle. This is an area of considerable research effort in laboratory ice cloud studies. In general it is believed that ice nuclei are quite different to droplet nuclei, while droplet nuclei are soluble, ice nuclei are generally insoluble. Factors such as size shape and crystal structure are believed to be important for ice nuclei. Aerosol such as mineral particles (e.g. desert dust etc.), soot and certain bacteria have been observed to act as ice nuclei and these all have very different characteristics. Additionally droplets with certain alcohol monolayer coatings have been found to freeze at much warmer temperatures than would be expected for homogeneous nucleation. Measurements of ice nuclei concentrations in the atmosphere are rather difficult, but there is evidence that only a small fraction of the aerosol population are able to act as ice nuclei and that this fraction increases with increasing particle size. This fraction is much lower than the fraction of particles which are able to act as droplet nuclei, with typical ice nuclei concentrations being around 1-10/litre.
Content from External Source
http://www.cas.manchester.ac.uk/resactivities/cloudphysics/background/ice/

A recent study found that ice nuclei are mostly mineral dust and metallic particles. It's not clear from the abstract where the metallic particles come from, but I would assume human activity, and probably jet exhaust (which contains metal particles due to engine wear).

Formation of cirrus clouds depends upon the availability of ice nuclei to begin condensation of atmospheric water vapor. While it is known that only a small fraction of atmospheric aerosols are efficient ice nuclei, the critical ingredients that make those aerosols so effective has not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, while sulfate/organic particles are underrepresented and elemental carbon and biological material are essentially absent.
Content from External Source
http://science.sciencemag.org/conte...4145?sid=3452d17d-c6da-4124-8363-a77b28dc9102
 
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