WHY DO PLANES LEAVE WHITE TRAILS IN THE SKY? - With so many commercial jets flying routes all over the world, there probably isn’t a person on Earth who hasn’t noticed the white smoke coming from airliners. When flying at high altitudes, jet engines exhaust extremely hot air and water vapor, among other things. The cold temperatures and low pressures at these altitudes result in the water forming into visible ice crystals. The results are white clouds known as contrails, which is short for condensation trails. There are three components necessary for clouds to form in the atmosphere. There must be moisture present in the form of water vapor. The atmosphere must cool enough for that water vapor to condense into visible moisture (either as water droplets or ice crystals). There must be some form of condensation nuclei. The nuclei can be any bit of particulate in the atmosphere, like dust or pollution, that water molecules can hold onto. Most clouds you see are the result of moist warm air rising. As it gets higher, the air cools down to the temperature of the surrounding air. When it reaches the dew point, a visible cloud forms. Condensation trails behind aircraft are simply clouds. All three components are created, and when conditions are just right, these contrails will appear behind every plane in that part of the sky. Jet engines produce an incredible amount of heat, along with water vapor and other combustion exhaust particles like carbon dioxide.

An airliner leaves contrails as it passes the Moon

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Why do Planes Leave White Trails in the Sky?

BY MATT CLAIBORNE

 

 

With so many commercial jets flying routes all over the world, there probably isn’t a person on Earth who hasn’t noticed the white smoke coming from airliners.

But why do these streaks only occur behind some planes?

Are they dumping fuel?

Is it hazardous?

What exactly are these vapor trails?

Why Planes Leave Trails

When flying at high altitudes, jet engines exhaust extremely hot air and water vapor, among other things.

The cold temperatures and low pressures at these altitudes result in the water forming into visible ice crystals.

The results are white clouds known as contrails, which is short for condensation trails.

The Science Behind Contrails

There are three components necessary for clouds to form in the atmosphere.

There must be moisture present in the form of water vapor.

The atmosphere must cool enough for that water vapor to condense into visible moisture (either as water droplets or ice crystals).

There must be some form of condensation nuclei. The nuclei can be any bit of particulate in the atmosphere, like dust or pollution, that water molecules can hold onto.

Most clouds you see are the result of moist warm air rising.

As it gets higher, the air cools down to the temperature of the surrounding air. When it reaches the dew point, a visible cloud forms.

Condensation trails behind aircraft are simply clouds. All three components are created, and when conditions are just right, these contrails will appear behind every plane in that part of the sky.

Jet engines produce an incredible amount of heat, along with water vapor and other combustion exhaust particles like carbon dioxide. This exhaust is shot out the back of the jet engine into the surrounding cold air.

The atmosphere at high altitudes is frigid. Most airliners fly between 30,000 and 38,000 feet above sea level, where the temperatures range between -45 and -61 degrees Celsius.

At these temperatures, the water vapor will condense into water droplets, which will become ice crystals quickly.

Like the upper-level cirrus clouds at these altitudes, the ice crystals will blow and scatter with the wind conditions. If the upper-level winds are calm, the contrails may remain intact for a long time.

While it is true that the contrails contain pollutants, just as all combustion engines leave pollutants when operating, the contrails are not made of smoke, visible pollution, or chemicals.

They are clouds just like natural cirrus clouds, made of ice crystals suspended in the atmosphere.

Why Don’t All Planes Leave White Smoke?

A specific set of atmospheric circumstances have to line up to create an exhaust contrail.

It’s seldom cold enough at lower altitudes for this to happen, so the primary place they occur happens to be at the cruising altitude of jets.

You aren’t likely to see contrails behind propeller-driven aircraft or low-flying planes. It’s also why the presence of contrails varies from day to day, depending on the weather conditions present in the upper atmosphere.

Classifications of Exhaust Contrails

NASA’s Langley Research Center has studied contrails and their effects for decades. They divide contrails up based on how persistent they are in the atmosphere.

Contrails are ranked as short-lived, persistent, and persistent-spreading.

Aerodynamic Contrails

The process described above is known as an exhaust contrail since it is formed behind the engines. But the wings and control surfaces can cause a similar phenomenon, known as aerodynamic contrails.

Persistent spreading contrails

Aerodynamic contrails are more likely to be visible near the surface in areas with a lot of moisture already in the atmosphere.

When riding in a plane and looking out the window, you may see clouds form on top of the wing or in the vortices trailing behind it. They tend to happen quickly as the plane travels between different levels in the atmosphere.

As the air over the top of the wing accelerates, it also cools. If there’s enough water vapor in the atmosphere and the temperatures are close to the dew point, this cooling is enough to create a cloud.

These clouds take several forms, depending on how the aircraft is configured.

F/A-18 Hornet in flight with aerodynamic contrails
over the wings and in the wingtip vortices.

In some cases, a smooth cloud forms perfectly over the wings of the plane. In other cases, the accelerated air flowing behind the air in the wingtip vortices creates a cloud. 

Since the cloud shows the area of lower pressure over the wing, it changes as the pilot controls the lift made by that wing.

As the pilot changes the wing’s angle of attack, the aerodynamic contrails may grow larger or dissipate.

These contrails are especially neat to see since they visually show where the lift is being generated and where the wake turbulence lies. It’s a physical and visual manifestation of something you usually only learn about theoretically.

Any aircraft can leave aerodynamic contrails. They can form at any altitude as long as the atmospheric conditions are just right. They’re even often seen on the nose cones of rockets on liftoff!

Conspiracy Theories

Aviation, by its very nature, is a technical subject that involves a lot of science and technical jargon. Sometimes, when the non-flying public is unaware of the science behind it, conspiracy theories can pop up.

Why Do Planes Dump Fuel?

Some people believe that the white trails behind aircraft are fuel because they’ve heard of pilots “dumping fuel” in the movies.

Some aircraft are indeed equipped to be able to dump their jet fuel overboard. This is an emergency procedure designed to reduce the landing weight of an airplane.

Transport aircraft are often designed to take off at much higher weights than they can land. Since under normal conditions, the plane will burn thousands of pounds of fuel between its departure and destination, the plane will be well within limits before landing.

But what about in the case of an emergency, can the plane land safely if something goes wrong right after takeoff?

In some cases, the answer is no. The fuel needs to be burned off by remaining in the air or dumped overboard to enable the plane to land immediately.

Dumping of fuel is extremely rare and carefully controlled. It would only happen over unpopulated areas, and remaining aloft until a safe landing can be accomplished is always the preferable solution.

The vapor trails left by a plane at altitude have nothing to do with dumping fuel.

 “Chemtrails”

Another popular conspiracy theory is that aircraft are being used by the world’s governments to disperse chemicals into the atmosphere.

This theory has resulted in the term “chemtrails,” which is becoming more and more common.

Some believe they are related to a secret government atmospheric control program involving cloud seeding and weather manipulation.

Effects of Contrails on Climate

One area of valid research and study has been done on whether or not the presence of contrails in the atmosphere affects the climate.

Several hypotheses have been floated that the contrails, by increasing the amount and coverage of upper-level clouds, reduce sunlight penetration to the surface.

Theoretically, this could reduce surface temperatures. But they also trap in heat, so research needed to be done to find out whether the clouds have a net warming or cooling effect.

NASA has studied the effects carefully, and the results show a minor net warming effect.

Observationally, they’ve shown that warming is more pronounced in areas with the most flight activity, like over the North Atlantic flight corridor between America and Europe.

Matt Claiborne

Airline Transport Pilot. Certified Flight Instructor-Airplane, Single and Multiengine Instrument

https://www.aircraftcompare.com/blog/why-planes-leave-white-trails/

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TYPES OF CLOUDS - The 10 Basic Types of Clouds - The many variations of clouds can be grouped into one of 10 basic types depending on their general shape and height in the sky. Cumulonimbus tower across the low, middle, and upper atmosphere - Cumulus clouds are the clouds you learned to draw at an early age and that serve as the symbol of all clouds (much like the snowflake symbolizes winter). Stratus clouds hang low in the sky as a flat, featureless, uniform layer of grayish cloud. Stratus clouds are seen on dreary, overcast days and are associated with light mist or drizzle. Stratocumulus - When viewed from underneath, stratocumulus have a dark, honeycomb appearance. You're likely to see stratocumulus on mostly cloudy days. They form when there's weak convection in the atmosphere. Altocumulus clouds are the most common clouds in the middle atmosphere. Nimbostratus clouds cover the sky in a dark gray layer. Altostratus appear as gray or bluish-gray sheets of cloud that partially or totally cover the sky at mid-levels. Cirrus - Like their name suggests (which is Latin for "curl of hair"), cirrus are thin, white, wispy strands of clouds that streak across the sky. Cirrocumulus clouds are small, white patches of clouds often arranged in rows that live at high altitudes and are made of ice crystals. Cirrostratus clouds are transparent, whitish clouds that veil or cover nearly the entire sky. Cumulonimbus clouds are one of the few clouds that span the low, middle, and high layers.

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Types of Clouds

The 10 Basic Types of Clouds

Also learn what weather's coming based on the cloud type 

Tiffany Means

 

 

According to the World Meteorological Organization's International Cloud Atlas, more than 100 types of clouds exist.

The many variations, however, can be grouped into one of 10 basic types depending on their general shape and height in the sky.

Thus, the 10 types are:

·      Low-level clouds (cumulus, stratus, stratocumulus) that lie below 6,500 feet (1,981 m)

·      Middle clouds (altocumulus, nimbostratus, altostratus) that form between 6,500 and 20,000 feet (1981–6,096 m)

·      High-level clouds (cirrus, cirrocumulus, cirrostratus) that form above 20,000 feet (6,096 m)

·      Cumulonimbus, which tower across the low, middle, and upper atmosphere

Whether you're interested in cloud watching or are just curious to know what clouds are overhead, read on to find out how to recognize them and what type of weather you can expect from each.

01 Cumulus

Cumulus clouds are the clouds you learned to draw at an early age and that serve as the symbol of all clouds (much like the snowflake symbolizes winter).

Their tops are rounded, puffy, and a brilliant white when sunlit, while their bottoms are flat and relatively dark.

When You'll See Them

Cumulus clouds develop on clear, sunny days when the sun heats the ground directly below (diurnal convection).

This is where they get their nickname of "fair weather" clouds. They appear in the late morning, grow, and then disappear toward evening.

02 Stratus

Stratus clouds hang low in the sky as a flat, featureless, uniform layer of grayish cloud. They resemble fog that hugs the horizon (instead of the ground).

When You'll See Them

Stratus clouds are seen on dreary, overcast days and are associated with light mist or drizzle.

03 Stratocumulus

If you took an imaginary knife and spread cumulus clouds together across the sky but not into a smooth layer (like stratus), you'd get stratocumulus — these are low, puffy, grayish or whitish clouds that occur in patches with blue sky visible in between.

When viewed from underneath, stratocumulus have a dark, honeycomb appearance. 

When You'll See Them

You're likely to see stratocumulus on mostly cloudy days. They form when there's weak convection in the atmosphere.

04 Altocumulus

Altocumulus clouds are the most common clouds in the middle atmosphere.

You'll recognize them as white or gray patches that dot the sky in large, rounded masses or clouds that are aligned in parallel bands.

They look like the wool of sheep or scales of mackerel fish—hence their nicknames "sheep backs" and "mackerel skies."

Telling Altocumulus and Stratocumulus Apart

Altocumulus and stratocumulus are often mistaken.

Besides altocumulus being higher up in the sky, another way to tell them apart is by the size of their individual cloud mounds.

Place your hand up to the sky and in the direction of the cloud; if the mound is the size of your thumb, it's altocumulus. (If it's closer to fist-size, it's probably stratocumulus.)

When You'll See Them

Altocumulus are often spotted on warm and humid mornings, especially during summer.

They can signal thunderstorms to come later in the day. You may also see them out ahead of cold fronts, in which case they signal the onset of cooler temperatures.

05 Nimbostratus

Nimbostratus clouds cover the sky in a dark gray layer.

They can extend from the low and middle layers of the atmosphere and are thick enough to blot out the sun.

When You'll See Them

Nimbostratus are the quintessential rain cloud. You'll see them whenever steady rain or snow is falling (or is forecast to fall) over a widespread area.

06 Altostratus

Altostratus appear as gray or bluish-gray sheets of cloud that partially or totally cover the sky at mid-levels.

Even though they cover the sky, you can typically still see the sun as a dimly lit disk behind them, but not enough light shines through to cast shadows on the ground.

When You'll See Them

Altostratus tend to form ahead of a warm or occluded front. They can also occur together with cumulus at a cold front. 

07 Cirrus

Like their name suggests (which is Latin for "curl of hair"), cirrus are thin, white, wispy strands of clouds that streak across the sky.

Because cirrus clouds appear above 20,000 feet (6,096 m) — an altitude where low temperatures and low water vapor exist — they are made up of tiny ice crystals rather than water droplets.

When You'll See Them

Cirrus typically occur in fair weather. They can also form out ahead of warm fronts and large-scale storms like nor'easters and tropical cyclones, so seeing them can also indicate storms may be coming.

NASA's Earthdata site quotes a proverb that sailors learned to warn them of coming rainy weather, “Mares’ tails (cirrus) and mackerel scales (altocumulus) make lofty ships to carry low sails.”

08 Cirrocumulus

Cirrocumulus clouds are small, white patches of clouds often arranged in rows that live at high altitudes and are made of ice crystals.

Called "cloudlets," the individual cloud mounds of cirrocumulus are much smaller than that of altocumulus and stratocumulus and often look like grains.

When You'll See Them

Cirrocumulus clouds are rare and relatively short-lived, but you'll see them in winter or when it's cold but fair.

09 Cirrostratus

Cirrostratus clouds are transparent, whitish clouds that veil or cover nearly the entire sky. 

A dead giveaway to distinguishing cirrostratus is to look for a "halo" (a ring or circle of light) around the sun or moon.

The halo is formed by the refraction of the light on the ice crystals in the clouds, similarly to how sundogs form but in an entire circle rather than just on either side of the sun.

When You'll See Them

Cirrostratus indicate that a large amount of moisture is present in the upper atmosphere. They're also generally associated with approaching warm fronts.

10 Cumulonimbus

Cumulonimbus clouds are one of the few clouds that span the low, middle, and high layers.

They resemble the cumulus clouds from which they grow, except they rise into towers with bulging upper portions that look like cauliflower.

Cumulonimbus cloud tops are usually always flattened in the shape of an anvil or plume. Their bottoms are often hazy and dark. 

When You'll See Them

Cumulonimbus clouds are thunderstorm clouds, so if you see one you can be sure there's a nearby threat of severe weather (short but heavy periods of rainfall, hail, and possibly even tornadoes).

 

Tiffany Means

Meteorology Expert

Education

B.S., Atmospheric Sciences and Meteorology, University of North Carolina

Introduction

Studied atmospheric sciences and meteorology at the University of North Carolina

Former administrative assistant for the National Oceanic and Atmospheric Administration

Member of the American Meteorological Society

Experience

Tiffany Means is a former writer for ThoughtCo who contributed articles about weather for five years. She has interned with the domestic and international weather departments at CNN, written monthly climate reports for NOAA’s National Centers for Environmental Prediction, and participated in a number of science outreach events, including the Science Olympiad Competition. Means has personally experienced such weather greats as the Blizzard of 1993 and the floods of Hurricane Francis (2004) and Ivan (2004).

Education

Bachelor's degree in atmospheric sciences and meteorology from the University of North Carolina at Asheville

ThoughtCo and Dotdash

ThoughtCo is a premier reference site focusing on expert-created education content. We are one of the top-10 information sites in the world as rated by comScore, a leading Internet measurement company. Every month, more than 13 million readers seek answers to their questions on ThoughtCo.

For more than 20 years, Dotdash brands have been helping people find answers, solve problems, and get inspired. We are one of the top-20 largest content publishers on the Internet according to comScore, and reach more than 30% of the U.S. population monthly. Our brands collectively have won more than 20 industry awards in the last year alone, and recently Dotdash was named Publisher of the Year by Digiday, a leading industry publication.

https://www.thoughtco.com/types-of-clouds-recognize-in-the-sky-4025569

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