Why Do Planes Leave a "Tail" in the Sky? 

Source of pictures: pixabay

The sky in recent days was as beautiful as animation scenery. Daniel walked on the way, looking up the blue sky and white fluffy clouds. They were so close, as if he can touch them with his hands. Soon his neck got a little sore and he turned his head sideways. Then he saw a plane flying through the sky, leaving two long white "tails" behind. After a while, the "tails" faded away. To see the "tails" again, Daniel walked slowly and waited patiently. After a while, another plane flew by with its "tails" behind. Daniel looked up at it and guessed which part of the plane it had fallen from. In wondering, he arrived home.

As soon as he put down his schoolbag, he took his mother outside and said to show her something. Mom followed him and asked curiously, "What's it?"

Daniel looked at the sky and said, "Just a minute."

Mom was confused. A few minutes later, a plane appeared in the sky. Daniel excitedly pointed to the sky and said, "Look, mom! Tail of the plane."

Mom looked at where Daniel pointed to. In the clear blue sky, a plane was flying, followed by two white "cloudlike lines", which looked like a tail, but it was not a real tail.

Mom looked at it for a while, then she said, "It's not a tail. A plane has wings and empennage, but no tail."

Daniel was a little disappointed and asked, "Then what's it? Did the plane cut a hole in the blue sky? Is it the white bone of the sky?"

Mom was amused, "Of course not. It's just the contrail left by a plane flying across the sky. Contrails are mostly made up of water suspended in the air. Generally, there are two ways to form this 'tail'."

Daniel was curious and waited for his mom to continue.

"The first is that when a plane is flying in the air, it creates a low pressure area over its wings. The sudden drop in air pressure then causes the air temperature near the wings to drop. Water vapor therefore forms water droplets when the temperature drops to a certain degree. If the air temperature is cold enough, the droplets could freeze," Mom continued.

"The another way is more common. Planes fly in the sky, just like cars run on the road. They all need fuel. The difference is that cars run on gasoline and planes are powered by special jet fuel. Combustion of jet fuel results in low-temperature waste, usually including CO₂, water vapor and fuel that is not fully burned. Then water vapor condenses into drops of water. In this way, the plane discharges waste as it flies, thus forming white, tail-like trails."

"Does the 'tail' appear only in these two cases?" asked Daniel, "Are there any other conditions?" Why hadn't he noticed the 'tail' before?

Mom replied, "Generally, only when the air humidity reaches a certain level will there be contrails. If there isn't enough water vapor in the air, neither the plane waste nor the temperature drop caused by the air pressure can form water droplets. Contrails are also a predictor of weather. If it fades away soon, it's going to be sunny for the next few days. Otherwise, it means the air is very humid and there's a good chance of rain."

The "tail" was gradually disappearing in the sky. It seemed that it will be sunny tomorrow. Daniel concluded, "In both ways, the long 'tail', oh, contrail we see is formed by the condensation of water vapor."

Mom nodded her head in agreement and entered the house with Daniel until the contrail completely faded away. 

Author: Huang Jing