Ship tracks are cloud

Cloud

A cloud is a visible mass of liquid droplets or frozen crystals made of water and/or various chemicals suspended in the atmosphere above the surface of a planetary body. They are also known as aerosols. Clouds in Earth's atmosphere are studied in the cloud physics branch of meteorology...

s that form around the exhaust

Exhaust gas

Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...

 released by ship

Ship

Since the end of the age of sail a ship has been any large buoyant marine vessel. Ships are generally distinguished from boats based on size and cargo or passenger capacity. Ships are used on lakes, seas, and rivers for a variety of activities, such as the transport of people or goods, fishing,...

s into the still ocean

Ocean

An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

 air. Water molecule

Molecule

A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...

s collect around the tiny particles (aerosols) from exhaust to form a cloud seed. More and more water accumulates on the seed until a visible cloud is formed. In the case of ship tracks, the cloud seeds are stretched over a long narrow path where the wind has blown the ship's exhaust, so the resulting clouds resemble long strings over the ocean.

Ship tracks study

In 1965, the first "anomalous cloud lines" were observed in images from the TIROS

TIROS

TIROS, or Television Infrared Observation Satellite, is a series of early weather satellites launched by NASA, beginning with TIROS-1 in 1960. TIROS was the first satellite that was capable of remote sensing of the Earth. This initial remote-sensing effort was significant because it enabled Earth...

 VII satellite. It was hypothesized that the most likely cause was from the exhaust from ships. Many studies since have confirmed the cause, and they are now referred to as ship tracks. Scientists soon realized that the climatic impacts from aerosols could have a large effect on climate through the indirect effect

Twomey effect

Twomey effect — describes how cloud condensation nuclei , possibly from anthropogenic pollution, may increase the amount of solar radiation reflected by clouds. This is an indirect effect....

, and that ship tracks provided an excellent laboratory for their studies.

Scientists who study how human-produced aerosols affect cloud formation observe ship tracks because in most urban areas, they are unable to discern exactly how pollutant

Pollutant

A pollutant is a waste material that pollutes air, water or soil, and is the cause of pollution.Three factors determine the severity of a pollutant: its chemical nature, its concentration and its persistence. Some pollutants are biodegradable and therefore will not persist in the environment in the...

s contribute to forming clouds because the atmosphere over the land is too tumultuous. In contrast, ships release their exhaust into the relatively clean and still marine air, where the scientists have an easier time of measuring the effects of fossil fuel

Fossil fuel

Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

 emissions on cloud formation.

In general, the air above the oceans suffers from less turbulence

Turbulence

In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...

 and convection

Convection

Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....

 than the air above land. The lower atmosphere is especially calm over the eastern Pacific

Pacific Ocean

The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...

 in the summertime due to a layer of hot air that settles in 500 to 700 meters above that region of the ocean. This effect creates a temperature inversion, placing a cap on the cooler air below, trapping pollutants and water vapor. While the inversion is responsible for the smog that reduces air quality in Los Angeles

Los Ángeles

Los Ángeles is the capital of the province of Biobío, in the commune of the same name, in Region VIII , in the center-south of Chile. It is located between the Laja and Biobío rivers. The population is 123,445 inhabitants...

, it also allows for the formation of long lasting ship tracks. The particles billowing from ships' smokestacks enter the air above the eastern Pacific and create long, thin clouds that remain there for days.

Findings

It is theorised that sulfur dioxide

Sulfur dioxide

Sulfur dioxide is the chemical compound with the formula . It is released by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide unless the sulfur compounds are removed before burning the fuel...

 released from ships' smokestacks could be forming sulfate

Sulfate

In inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...

 aerosol particles in the atmosphere, which cause the clouds to be more reflective, carry more water and possibly stop precipitating

Precipitation (meteorology)

In meteorology, precipitation In meteorology, precipitation In meteorology, precipitation (also known as one of the classes of hydrometeors, which are atmospheric water phenomena is any product of the condensation of atmospheric water vapor that falls under gravity. The main forms of precipitation...

. This is regarded as proof that humans have been creating and modifying clouds for generations through the combustion

Combustion

Combustion or burning is the sequence of exothermic chemical reactions between a fuel and an oxidant accompanied by the production of heat and conversion of chemical species. The release of heat can result in the production of light in the form of either glowing or a flame...

 of fossil fuels.

Although ship tracks can sometimes be visible, researchers usually scan the near-infrared

Infrared

Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...

 light coming off the clouds. At this wavelength many ship tracks appear as bright lines that can be distinguished from the surrounding, uncontaminated clouds. On average, polluted clouds reflect more sunlight than their unaffected counterparts.

When compared to normal clouds, the number of water droplets per volume of air in ship tracks are more than doubled, the radius of the drops is decreased by roughly six percent and the total volume of liquid water per volume of air is increased twofold. In other words, this excessive cloud seeding from the ship causes the clouds to retain more water. Normally, rain forms when cloud drops coagulate and reach a size where gravity can pull them to the ground. Yet, in ship tracks, the cloud seeding made the drops so small that they could no longer easily merge together to reach the size needed to escape. Since no drizzle

Drizzle

Drizzle is a light rain precipitation consisting of liquid water drops smaller than those of rain, and generally smaller than 0.5 mm in diameter. Drizzle is normally produced by low stratiform clouds and stratocumulus clouds. Precipitation rates due to drizzle are on the order of a millimetre...

comes out of the seeded clouds, the liquid water just keeps building in the cloud. This makes the cloud brighter and more reflective to incoming sunlight, especially in the near-infrared part of the spectrum.