Contrails are clouds made from water vapor that condenses then freezes behind a plane engine. Since the engines are on constantly, it seems a bit odd when you see contrails with gaps in them, or even contrails that stop and start. If the engine is pumping out a constant amount of water, then why is the trail not constant? This puzzle is sometimes seized on by people who think that persistent contrails are actually “chemtrails”, or some kind of deliberate spraying operation.
But it’s actually very simply. The amount of water in the exhaust is pretty constant, but the amount of moisture in the air is not. The humidity varies with altitude, and a layer of low humidity can be sandwiched between two layers of high humidity. As a plane climbs or descends through this layer, then the trail will only form in the areas of high humidity, and so look like it was “switched off” in the area of low humidity.
You can get the same effect with temperature. A warm layer of air can actually lay on top of a colder layer in what is called an “inversion” (you’ll hear this on the weather sometimes, referred to as an “inversion layer”). When a plane flies through this inversion layer, the trail can be “broken”.
It’s not just climbing or descending flights either. The boundaries between these layers is not flat, and gravity waves or rising convection currents of air can create large volume of air that differ in temperature and/or humidity from the neighboring air, and so can break (or make) the contrail when then plane flies through them. You can think of these regions as invisible clouds:
This can be see in this 1944 footage (about 70 years old) from the WWII film Memphis Bell:
Don’t take my word for it though. Do a Google search for “broken contrail” and you’ll find lots of examples, and similar explanations (Click the photos above for two).
Occasionally a jet plane, especially if ascending or descending, will pass through a much drier or more moist layer of atmosphere which may result in a broken pattern to the contrail, with it appearing in segments rather than in one continuous plume.
If the contrail stops, then starts up again, creating a broken line, chances are the airplane flew through a dry patch.
What is happening here is that warm air rising from the ground carries vast amounts of water vapor upwards into the highest levels of the atmosphere. This water vapor exists as massive, plumes of warm, moist air ascending to very high levels. When a rising plume reaches the upper atmosphere it condenses into high ice clouds known as cirrus or feather clouds. In the case of the jet contrail (condensation trail) in the first image, the condensing cloud formed where the aircraft passed through a rising plume of moist warm air. The air on either side of the plume was not sufficiently moist to support the formation of an enduring cloud. As a result the contrail only remained visible in the warmth plume.
This uneven humidity does not always result in an instant on/off for the contrail, but can result instead in some regions of the contrail being more persistent than others. This can lead to a gap forming a minute or more afte the contrail originally formed: