MadSci Network: Earth Sciences

Re: How small can pockets of dry air be in the atmosphere

Date: Wed Feb 8 13:38:54 2006
Posted By: Stephanie Shaw, Post-doc/Fellow, UC Berkeley Environmental Science, Policy, and Management, UC Berkeley
Area of science: Earth Sciences
ID: 1139062121.Es

Hi, and thanks for your question!

Let’s start with some background… the word “contrail” is an abbreviation for "condensation trail". These are man-made clouds that result from condensation of moisture in an aircraft's exhaust. Burning fuel in the engine creates carbon dioxide and water. These exhaust gases are released at the high altitudes of plane flight, where the air temperature is usually below freezing. Contrail formation is dependent on the temperature, pressure, and humidity of the atmosphere, as well as the ratio of moisture to heat produced by the engine. Specifically, if the water added to the atmosphere from the exhaust causes the atmosphere to become saturated or supersaturated when it was not before, the freezing temperatures convert the water into many tiny ice crystals, and a contrail will form. Contrails can also be formed near an airplane's wings. Drops in air pressure occur near the wing, usually with a resulting temperature drop, which can cause water to condense and form a contrail.

The phenomenon you are referring to in the photo is called a "broken" contrail, and is indeed usually caused by differences in ambient temperature and moisture as the plane is traveling. Check out the following websites for more information and photos: NASA Contrail FAQ

National Weather Service Contrail Information

In the photo it looks like a very abrupt switch of the contrail "off" and "on", which leads many people to believe the engine was probably turned off for a short while. However, you may also notice no change in the trajectory of the contrail, which would be likely if the aircraft’s engines were temporarily shut. In addition, air parcels of varying temperature and humidity can indeed have rather small dimensions. No quantitative definition of an air parcel or patch is possible, but certainly dimensions as small as a cubic foot are reasonable when discussing vertical air motions. Similarly, vertical velocities of air within updrafts is commonly several meters per second; in thunderstorms this can increase to tens of meters per second (30-50 meters per second is common). Hopefully this gives you a qualitative sense of the scale and movement of such air parcels.

Even if the atmosphere in the region of the plane's path is quite well- mixed in the horizontal, also remember that there are very often large temperature or moisture changes in the vertical. For example, the plane causing the contrail in your photo could have been rapidly ascending or descending, and passed through a thin layer of air with different properties from the surrounding air.

Thanks again for your question!

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