Contrails are a direct indicator of present and future weather. Why? Because contrails, which are manmade clouds, show us just how much moisture there is in the upper atmosphere, and in what direction the upper air winds are moving.
Why do we care about moisture in the upper atmosphere and direction of upper airflow? Because moisture represents energy and upper airflow controls surface weather development and movement. Why is moisture equivalent to energy? Because it takes heat to put moisture into the atmosphere, and when that moisture condenses it forms clouds. When a cloud forms it releases the heat that was used to evaporate it originally and that heat re-warms the atmosphere.
Yes, this is an endless, repetitive loop, but it keeps our atmosphere in balance by distributing and re-distributing energy. So back to our original statement concerning contrails: a contrail is formed when moisture from the exhaust of a plane's jet engine condenses in the cold upper atmosphere and forms a cloud.
If a contrail is short and dissipates quickly that tells us the upper atmosphere is relatively dry and that good weather, i.e. high pressure, is likely to remain for at least another 12 to 24 hours. Now why is that, you might ask? Well, if the upper atmosphere is dry to begin with and a jet engine then adds some moisture, that small addition will be quickly absorbed.
However, if the upper atmosphere is nearly saturated, then an addition of a little moisture from a jet's exhaust will bring the upper atmosphere to its dew point (100-percent saturation) and a long and lasting contrail will form.
If on a clear day you see long and persistent contrails, then inclement weather is on the way. Within six to 12 hours cloud cover will increase, thicken, and almost always move lower. What's happening is that most likely a low-pressure system is moving into your region.
A classic example of this process is shown by the weather charts for March 21 and 22 2002. At 1800 UTC (1800 Greenwich time is 1300 East Coast time) on March 21, a low-pressure system can be seen developing over the eastern Great Lakes (44N 77W), with its associated cold front dropping southwest into the Midwest and Texas.
On the morning of March 21, at my location near Annapolis, MD, I noticed long and persistent contrails, even though the day had begun with clear skies and not a single cloud was in sight. Why such dominant contrails on this day? Because the counterclockwise wind flow around the developing low was drawing up warm moist air from the Gulf of Mexico. This warm, moist air was rising and cooling, causing the upper atmosphere to become saturated.
These contrails I was seeing told me that the upper atmosphere was being filled with moisture, and that inclement weather was on its way. Was I correct? Well, look at the weather chart for 00 UTC on March 22 (00 UTC is 7:00 p.m. East Coast time on the 21st). The cold front associated with the developing low can be seen stretching across the northern Chesapeake Bay. The skies by then were overcast and rain had begun to fall.
Behind this cold front is cool and dry air, and on March 23 skies cleared and high pressure filled back in. With this high came dry, cool air and contrails were short and dissipated quickly. I looked at those and knew instantly that we'd have good weather for several days to come!
The motion of a contrail is another helpful indicator regarding approaching weather. If a contrail moves quickly across the sky then upper level winds are strong and the approaching weather system will be upon you very soon. In fact if you can detect the motion of contrails without the need of a fixed reference (such as the top of your mast), then upper-level winds are most likely 100 knots or greater. On an average, the movement of surface weather systems is 50 percent slower than that of upper-level systems, so when upper-level winds are 100 knots, surface weather systems move at speeds up to 50 knots. (Please note, I am referring here to the speed of a system's overall movement across the earth's surface, not the internal wind speeds of that system. Internal wind speed is determined by isobar spacing.)
Additionally, if contrails are perpendicular to upper level winds then the contrails will be degraded quickly, you will see them being shredded by the upper winds. But if a contrail retains its integrity, it holds together and keeps its shape, then it is oriented parallel to the upper-level winds, either that or the upper level winds are weak, less than 30 to 50 knots. Note that if upper-level winds are strong, 100 knots or greater, and a contrail is parallel to this flow you will see roll clouds form within the contrail. Roll clouds are clouds that form perpendicular to strong upper level winds as these winds attempt to drag the contrail along. In a sense the contrail begins tripping over itself in an attempt to stay up with the accelerated wind flow, and like a breaking wave, falls over itself.
Contrails essentially represent moisture being injected into the upper atmosphere. What happens to that moisture (is it easily absorbed or does it bring the upper air to its dew point?) provides us with a visible indication of present and future weather. When you see long and persistent contrails you should start tracking barometric pressure if you're not already doing so. You will soon see a drop in pressure, and the nature of that drop will assist you in determining the strength of the approaching front or low.
Additionally, you should make use of the upper air 500-mb chart to track upper level winds. You will see a very satisfying correlation between the formation and movement of contrails and the flow of upper level winds. In fact with a little practice you can analyze the 500-mb flow over you by just tracking contrail formation, dissipation, and movement. So, watch the sky and keep an eye on contrails!