Clouds, Mixing Clouds, and Contrails.

Clouds

It's been known for a very long time, since before the invention of powered flight, that cloud formation can be enhanced by the presence of tiny particles in the atmosphere. Scientists call these particles Cloud Condensation Nuclei (CCN). In nature CCN come from many sources including dust and minerals blown up from the ground (e.g. deserts), smoke particles and volcanic ash, biological material (from leaf litter, lichens, and bacteria) and perhaps surprisingly, salt crystals. The CCN enhance cloud formation by providing a surface onto which water can condense as liquid or solid. This doesn’t mean that tiny cloud droplets and ice crystal can’t form with the presence of CCN. It's just more likely they will form with CCN present. They can and do form without CCN as well.

When the relative humidity of a parcel of air is 100% we call that air mass saturated. At saturation the equilibrium between condensation and evaporation of water and water vapour is equal in both directions. There are as many molecules evaporating as are condensing. Since water vapour is invisible to our eyes but water droplets or ice crystals are not, we see the clouds that form when air is saturated with water. Clouds can form any time the air becomes saturated with moisture.

The pressure this vapour exerts is called the saturation vapour pressure and is proportional to temperature. Cold air has a lower saturation pressure than warm air. Basically cold air can mix with a smaller amount of water vapour than warm air can at equilibrium. We say cold is drier. This is why homes (in many other parts of Canada, back east as it were) have humidifiers to be used in winter. The cold, dry outside air that is brought into the house dries even further as it is warmed to room temperature. The relationship between temperature and the saturation vapour pressure is not linear. That is, for each degree of additional warming the amount of additional water vapour that can be present in equilibrium increases even more than it did for the previous degree. The

Try it at home

You can easily demonstrate the effect of CCN on cloud formation yourself at home with a plastic two litre soda bottle. Here’s what to do. Put a small amount of water in the bottle (a “clear” bottle with no dye is needed). You really only need a few millilitres. Close the lid tightly and shake it around a bit. This will ensure that the air inside is saturated with water vapour.

Next, squeeze the bottle tightly with both hands for a few seconds. Squeezing increases the pressure in the bottle which increased the temperature. Increasing the temperature allows more water to evaporate. The system will come to a new equilibrium in a few seconds. After this brief pause release the pressure suddenly (cooling the system). You should see … nothing happen.

You just showed that you cannot form a cloud in a bottle using water and air alone. You need an extra ingredient, the cloud condensation nuclei (CCN) mentioned earlier are the answer. You can easily add CCN by dropping a lit match into the bottle and closing the lid. Now repeat the process. Squeeze, hold, release. A cloud will form suddenly*. You can make the cloud evaporate again by squeezing the bottle. It will reappear when you release the pressure. This process should repeat many times but eventually stops working since the CCN will get scrubbed out of the air by the tiny droplets that merge with the water in the bottom and on the sides of the bottle**.

The cloud droplets that you’ve made in this simple experiment are really cloud droplets. They are individually microscopic. You won’t be able to discern them. If you are outside in a fog you will have noticed that at least some of the droplets are larger, large enough to see. This is a normal part of cloud formation in the wild. There are a number of processes at play that can increase droplet size. These processes could also take place in your bottle but they would require more time and volume than is available to be significant. I’ll put some links to more information below.

*Obviously I can’t absolutely guarantee that the cloud will form. In my experience this is a very reliable experiment. Many of you will have seen this before in the form of the cloud that often forms in the neck of a newly opened, cold bottle of some pressurized beverage.

Mixing Clouds

Using the figure below, let’s consider what happens if you mix two air masses together. One of the air masses, call it A, is cold and dry (the stratosphere or upper troposphere) and the other, B, is hot and moist (jet engine exhaust).

A schematic of the processes that allow a mixing cloud to form. The mixture of air masses at point S and B' will not form a cloud but mixing S and B will. From: http://www.hko.gov.hk/en/education/edu06nature/ele_mixing.htm#

A schematic of the processes that allow a mixing cloud to form. The mixture of air masses at point A and B' will not form a cloud but mixing A and B will. From: here.

Neither of these parcels of air on their own has conditions suitable for water droplet or ice crystal formation. They are both below the Saturated curve. Mixing them together will create a new air parcel lying somewhere along the line between them. Or, even a changing air parcel moving along the line between them. To understand that this is so picture mixing up two equal quantities of water with different temperatures. The temperature of the mixture will be the average of the two starting values. If the two quantities are different at the start then the new temperature will be a volume weighted average of their temperatures.

When the new air mass, made of the combination of A and B, lies on or above the saturated line a cloud can form. When it lies below this is unlikely or impossible. If we consider jet exhaust with the properties of B" it's clear that a mixture of the ambient atmosphere and the exhaust will never produce a cloud.

The cloud that can form is called a mixing cloud and we see it happen frequently in the form of aircraft contrails. It’s also the phenomenon responsible for the visual appearance of steam above your kettle or seeing your breath on a cool day. You may have noticed that the presence or absence of a cloud in front of your face when you exhale is determined by the outside temperature and humidity. Since your body temperature and the humidity of your breath is (mostly) constant the cloud will only form if the mixture of air from your breath and the atmosphere around your face becomes saturated with respect to water vapour.

Contrails

When a mixing cloud forms as a result of jet engine exhaust we call it a contrail. The first known description of an aircraft contrail is in a letter written in the fall of 1918 and published in Scientific American in June 1919.

“never before had I seen a plane writing in white upon the blue slate of the sky”

There are basically four possible outcomes when engine exhaust is mixed with ambient air during the passage an aircraft.

No contrail forms,
a contrail forms but quickly evaporates (sublimates) again,
a persistent contrail is formed because the saturation pressure of the mixture is just balanced at the equilibrium point, and
a contrail forms and seeds additional crystal growth resulting in a spreading cloud.

As we saw in our diagram above which of these occurs depends on the position of the mixed air along the line connecting the ambient atmosphere and the exhaust. Because mixing continues until the properties of the mixed air are very similar to the environment it's possible for the contrail to change as time goes on.

Which of these four possibilities occurs or persists depends on the properties of the atmosphere around the aircraft but also on things as difficult to determine (from the ground) as the type of engine, the fuel used, and engine power settings. In the third and fourth cases a contrail can, in addition, become distorted by the relative motions of the air at slightly different altitudes or along the path of the aircraft. If ice crystals (in the fourth case) can grow large enough they may begin to fall and can be quickly blown out into stretched fall streaks. If the air mass below is saturated or supersaturated new cloud crystals can grow and the cloud will increase in thickness and extent. It’s also possible for small crystals to be lifted up by rising air.

Some examples ...

The photos I’ve attached show an almost persistent contrail that I observed this morning (2014-03-11). In fact, in one patch of air that the jet flew through the contrail did seem to be quite stable and persistent though the ends of the trail evaporated within a few minutes of the aircraft’s passing.

A contrail forming behind an aircraft with jet engines.

The appearance of contrails is affected, like all clouds, by the position and colour of the sunlight and by perspective, your viewing position and angle.

Contrails are just cirrus clouds and do have an effect on climate by way of that similarity. They are high cold clouds that tend to let the sun’s energy pass through to the surface but provide some infrared radiation to the atmosphere below and act as a cold radiation source to outer space. This means they tend to be a source of additional warming.

The Wikipedia page for Cloud Physics will introduce to some terms and concepts you can read more about.

Cirrus clouds.

A research lab exploring the formation of ice crystals in air at low temperature

A nice page describing mixing clouds (I borrowed their figure).

Finally, here are some photos that are illustrative and, perhaps, just cool!

A distrail.

A distrail is created when the enhanced cloud particle formation from the engine exhaust results in air too dry for cloud to form.

Cirrus for certain but was it originally a contrail?

Is the relative straight cirrus (image top) with a long series of fall streaks grown from a contrail or natural? I'm not sure but know that relative straight cloud features do occur because sometimes the boundaries in the atmosphere where cloud can form are indeed long straight lines.

Now a series of photos taken over a 17 second long time span of a contrail evaporating.

The first of a sequence of four photos showing a contrail evaporating again. At the same time some other interesting features are apparent. The presence of two strong vortexes and a weaker cloud structure. Time and date: 2013-06-13 13:34:06

The vortexes decay away through a process called Crow Instability. Time and date: 2013-06-13 13:34:08

Loops and ring-like structures can from as the vortexes break apart and decay away into turbulence. Time and date: 2013-06-13 13:34:11

The contrail has almost evaporated away. Time and date: 2013-06-13 13:34:23 Note: this photo is was taken 17 seconds after the first one in the series.