Ever wondered why aeroplanes leave behind tracks in the sky?

If you are someone who eagerly looks up at the sky to see an aeroplane flying by, then you are a plane spotter. One thing that fascinates plane spotters is the white streak these aircrafts leave behind across a clear blue sky. But have you ever wondered why they do so? After all, we never see a bike, car, or truck doing this.  Let’s clear the air for you. 

The science behind contrails  
The white lines left behind by planes are nothing but artificial clouds. Now, that is intriguing! Well, they are called contrail, a portmanteau of the words ‘condensation’ and ‘trail’. But why do contrails appear? 

Just like any other vehicle, an aeroplane engine produces exhaust. Now, these hot exhaust gases escape the plane’s power source, and the water vapour in these exhaust fumes comes in contact with the air outside, which is extremely cold. The reason for the cold surrounding air is the high altitude that airplanes fly at. As most of you know, they hover around at a height of 35,000 feet or above. Just eight kilometres above the ground, the temperature can get as cold as minus 40-degree Celsius. So, the height at which planes fly has the perfect weather condition for contrail formation. 

Let us apply some basic science knowledge to understand the temperature difference. It’s the same concept of water cycle we have studied applies here. The cold air forces the water vapour from the aeroplane engine to condense. This leads to the formation of tiny water droplets or ice crystals, which too evaporate eventually. But before they evaporate, the condensed water vapour and ice crystal mixture quickly takes the shape of cloud-like trails in the sky, leaving us amazed!  

While contrails result from hot exhaust gases coming in contact with cold air, you can use this phenomenon to understand the weather. The thickness of the contrail changes depending on altitude, humidity and temperature. If you notice a thin and short-lived contrail, know that it indicates clear weather and low humidity. In contrast, if there is a thick and long-lasting track, the weather is humid. Such contrails are also early indicators of a storm. 

The skywriting technique 
You may have seen air shows during occasions such as the Republic or Independence Day where aeroplanes create letters in the sky. You’ll be surprised to know that they aren’t contrails. So then, how do they do it?? The answer is skywriting technique. Only small-sized aeroplanes or helicopters are used to create such messages or unique patterns in the sky. It turns out they are equipped with special smoke machines that help create conspicuous designs or writings easily visible from the ground. But how exactly does this happen? There are pressurised oil containers placed inside smoke machines fitted in the plane. When instructed, the pilot quickly operates the smoke machine to spray a mixture of oil and smoke into the plane’s exhaust system. The mixture burns almost immediately, leaving a cloud-like dense white smoke. But not all pilots can do this. As a matter of fact, pilots need special training to create these unique patterns or messages in the sky.