Contrails - Research, comments and links   (Index)

Contrails and Aviation-cirrus

Airpollution and Rainfall (01)

Smoggy Skies, Soggy Skies.
After a tough week at work or school, nothing sounds better than a weekend of fun in the sun. Unfortunately, that work week may be responsible for rained-out ball games and soggy picnics on Saturday and Sunday.

Two ASU climatologists have discovered a link between air pollution and weather patterns on the Atlantic coast of North America. It seems that car exhaust and factory smoke produced during the week may lead to a pattern of rainy weekends.

To get their results, Randy Cerveny and Robert Balling, Jr. studied daily carbon monoxide and ozone measurements collected at a Canadian monitoring station located on an island off the coast of Nova Scotia. Carbon monoxide and ozone are two common air pollutants.

They also studied rainfall data gathered by weather satellites in orbit over the Atlantic Ocean, and information stored in databases of Atlantic hurricane measurements. They found that weather on the East Coast of the United States is likely to be rainy on weekends and clear during the week. They also found that pollution hits its highest levels at the end of the week—possibly causing the weekend showers. “The dirt and dust, the solid parts of the pollution, tend to absorb heat. That makes the air around those parts warmer. Warm air rises. As warm air rises, it tends to cause clouds and precipitation,” says Cerveny, an ASU professor of geography.

The researchers believe the seven-day pollution pattern is caused by the human work week, because nothing in nature follows a seven-day cycle. The seven-day week is a purely human creation. Not surprisingly, activities that cause pollution tend to happen on weekdays, so it makes sense that pollution levels peak at the end of the week.

“[Pollution] primarily comes from auto exhaust and from factories. It’s the nasty stuff that helps to create the brown cloud we see over Phoenix,” Cerveny says. Balling and Cerveny also found a link between pollution patterns and hurricanes. Weekend hurricanes tend to be much weaker than storms that occur Monday through Friday. Hurricane wind speeds can drop by 10 miles-per-hour on the weekend, knocking them down a whole category on the Saffir Simpson Scale, a tool used by scientists to rate the strength of hurricanes and other cyclonic storms.

Cerveny and Balling only studied the Atlantic coast. As western cities grow, some people wonder whether or not their weekends might get wetter as well. Cerveny doubts it. “Winds flow from west to east,” he says. “While it is possible that pollution from areas in southern California may affect our weather here [in Arizona], it’s unlikely to affect the West Coast.”--Diane Boudreau

For more information, contact Randy Cerveny, Ph.D., or Robert Balling, Jr., Ph.D., ASU Office of Climatology, 480.965.6265. Send e-mail to Cerveny at or to Balling at

Publication date: Spring 1999
ASU Research Magazine

Studying Ship Tracks

Throughout most of the twentieth century, the scientific community was only certain about a few basic facts regarding cloud formation. They knew that in order to form, clouds require water vapor as well as tiny microscopic particles (aerosols) from the surface of the Earth. When water evaporates into the atmosphere, it spreads evenly throughout the surrounding air often to the point where the relative humidity is greater than 100 percent. Aerosols that dissolve easily in water, such as ammonium sulfate and sea salt, give the excess water molecules something to cling to. These aerosols act as the nuclei, or "seeds," around which cloud droplets take shape, and together these droplets form clouds. Were it not for particles in our atmosphere, the sky would almost always be clear and the air around us thick and humid (King et al., 1995). Many researchers suspected that as the concentration of these particles increases, the properties of the cloud could change. However, they remained unclear on exactly what these changes would be, the effects aerosols could have on rainfall, and most importantly where aerosol particles come from. Many believed that humans produce a large number through the burning of fossil fuels and plants (biomass). In the late 1980s and early 1990s, a series of investigations on the exhaust from ship’s smokestacks answered some of these questions. Not significant sources of pollution themselves, ships burn fossil fuels and release their exhaust in the form of sulfur dioxide, a gas that leads to the formation of sulfate aerosols in the atmosphere. The exhaust produces clouds that are relatively low in elevation and resemble larger versions of airplane contrails. Unlike contrails, these "ship tracks" are ideal for study since they remain in the air for many hours and are surrounded by relatively pristine marine air (King et al., 1993). Initially, the ship tracks were used to see if an increasing number of aerosols from pollutants would make the clouds brighter. The scientists believed the additional aerosols from the ships would give the water vapor more nuclei to cling to, so that a greater number of smaller drops would form in the cloud. These smaller drops, in turn, would make the cloud more reflective to sunlight. The same phenomenon can be seen when ice cubes are crushed. As the ice is broken up, the once smooth surface is shattered into many tiny surfaces at varying angles. These tiny surfaces reflect incoming light in all directions and cause the crushed ice to appear white and opaque. Water droplets do not contain all these ridged surfaces, but the researchers were fairly sure their fragmentation would have a similar effect. Through satellite observation and in situ aircraft measurements, scientists not only showed that their hypothesis was correct, but they also came across an effect that no one had foreseen. In addition to making the clouds more reflective, the aerosols were causing them to retain water and to stop drizzling. The cloud seeding by the ships’ exhaust made the droplets so small that they could no longer easily merge together to reach the size needed for gravity to pull them to the ground. Since no drizzle came out of the seeded clouds, the cloud water just kept building up (King et al. 1995).


A layer of haze blankets the city on a hot summer day, blurring the Tulsa skyline.   A trail of exhaust smoke spews out from the tailpipe of the car in front of you on your way to work.   Clouds of smoke billow from an industry.

What do these images have in common? Tiny particles, called "fine particulates." Soiling the air, spoiling the view and taking our breath away. Literally.

Ever since our early ancestors gathered around their first fire, humans have been releasing fine particulates into the air. Only recently have scientists discovered the bad news: fine particulates are so small that they can easily be inhaled into the deepest reaches of our lungs, causing serious lung and heart disease. Fine particulates are also a visual blight, capable of reducing visibility so much that beautiful views are blotted out.

Hold on a moment! Human beings have managed to live with fine particulates for thousands of years, haven't they? Why all the fuss now?

Well, the truth is that lung disease has always been a major health problem throughout the world, and a leading cause of death. Just as bacteria were causing serious illness long before they were discovered, it's likely that fine particulates have always contributed to lung disease. We just didn't know it.

Recent studies have found that fine particulates at current levels can pose a greater danger to our health than the better-known kinds of air pollution, such as smog, sulphur dioxide and carbon monoxide. Fine particulates are linked with all sorts of health problems — from a runny nose and coughing, to bronchitis, emphysema, asthma and even death.

It's time to take care of our air! To take action to control fine particulates, we need to understand the nature of the problem. This site will introduce you to fine particulates: what they are, where they come from and why they are so effective at impairing breathing and visibility. It will also explain what the government is doing to get fine particulates out of the air.

Global Problems Due to Particulates

Particulates also cause global problems such as suppression of rainfall and supplying transport for chemicals harmful to our atmosphere.  Particulates serve as cloud condensing nuclei (CCN), which means that they attract water in the atmosphere and help form clouds.  This may sound like a good thing but particulate CCN form clouds very inefficiently, they reduce the size of cloud droplets, but increase the density of droplets in the cloud.  The overall effect of these CCN particulates is a suppression of rainfall over regions of high pollution.  It is unknown at this time if these CCN particulates reduce the amount of rainfall on a global scale.

Our atmosphere is like a giant reactor where hundreds and hundreds of reactions occur every second.  Particulates carry chemicals that disrupt the balance of the atmospheric chemistry and chemicals that can block solar energy from entering or leaving the earth.  For example, the "hole" in the ozone layer isn't actually a hole, but a region over Antarctica where the ozone is greatly depleted due to aerosols formed in high clouds (aerosols are basically another name for particulates).  During the sunless winter, a giant vortex of air forms over the Antarctic, pollution from all over the globe is drawn towards the vortex.  Large concentrations of photo-chemical species build up in the vortex.  These chemical species are ozone depleting, but require sunlight to deplete  the ozone.  When sun starts to reach the Antarctic after winter, the large buildup of ozone-depleting species rapidly deplete the ozone causing the "hole" in our atmosphere.  This is just one example of how particulates affect our atmosphere.  The atmospheric chemistry is very complex and the long term effects of particulates are largely unknown, but we can see the short-term results everyday through smog and current health problems.

On a soggy morning this past April, Daniel Rosenfeld moved briskly through an overcast parking lot into one of many monolithic brick structures that dot NASA’s Goddard Space Flight Center. When he arrived at the conference room, he switched on a laptop and began bringing up satellite images of what looked like an apocalyptic vision of the East Coast. From Massachusetts to North Carolina, swirling red, orange and yellow clouds floated above a blue landscape. 

Peering over his glasses and intently at the screen, he pointed to an especially fiery spot on the map near the border of Pennsylvania and New Jersey. "The atmosphere here is fully polluted," he said. "Those colored streaks are mostly due to man-made aerosols."

Dirty Clouds Heat up the Atmosphere and Raise Objections (5/12/03)

Brown clouds of soot, apparently created by dried-dung fires on the Indian continent, have blown 1000 miles out over the Indian Ocean. Two miles thick, and dispersed over an area the size of the United States, this cloud is probably involved in global warming. The detailed effects of such clouds is uncertain; they may shade and cool the surface of the earth, but the soot particles absorb sunlight and radiate heat , thus warming the atmosphere. Millions of Indian housewives cook with this plentiful, cheap fuel. The Indian government has become touchy about this brown cloud and don’t want to fund further studies. The U.S. government wants to know more , because this situation adds to their objections to the 1997 Kyoto Protocol, which exempts developing countries such as India and China from cutting emissions of greenhouse gases. This treaty, which the U.S. did not ratify, does not mention soot as a greenhouse gas, but it surely is. Another plume of pollution over the ocean east of China contains of soot that is polluted with mercury from coal-burning power plants. It appears that this pollution cloud has affected weather over China, causing droughts in one area and heavy rainfall in another. As those who have read Chapter 8 know, global warming is complicated and political. (Wall Street Journal, 5/6/03, page 1)

Soot, an Important Greenhouse Gas According to a paper by Mark Z. Jacobson to the Dec. 11, 2001 meeting of the American Geophysical Union, in San Francisco, diesel soot contributes almost half as much as carbon dioxide to global warming. Carbon soot absorbs light energy at all wavelengths from the infrared through the visible into the ultraviolet and then emits this energy as heat in the infrared. Because soot has a rather short lifetime in the atmosphere (it rapidly reacts with oxygen in the air to form carbon dioxide), controlling soot should have a major cooling effect on the climate. This powerful green house gas from diesel vehicles was not considered in the 1997 Kyoto Protocol, nor have most computer models of global warming considered diesel soot. This is a major argument against diesel engines. The media seems to be ignorant of the role that soot has in global warming.

Improvements in Diesel Exhaust (7/1/03)

Diesel soot strongly contributes to global warming, but there is relief in sight.   A remarkable emission control system that cleans up diesel exhaust is being installed across Europe.   Although more efficient than their gasoline-fueled internal combustion cousins, diesel engines pollute the air with fine soot particles. style You have seen and smelled the diesel smoke while driving behind a bus.  These particulates are known to be a health hazard, but ordinary exhaust catalysts cannot remove this soot.   Many scientists and most environmentalists do not realize that this soot is also a powerful greenhouse aerosol.   Now the diesel pollution problem is improving in Europe because of this new CRTu’ process, invented at Johnson Matthey.   Although this new diesel exhaust catalyst was developed in 1989, it could not be used until sulfur-free diesel fuel became available, because sulfur inactivates (poisons) the platinum-based exhaust catalyst.   Now that sulfur free diesel fuel is available, the CRTu’ system is selling briskly in Europe: 5,000 systems were sold in 2002 and 500 a month are now being sold.   It is interesting that the CRTu’, system, which was awarded the UK’s biggest engineering innovation award, the MacRobert Award, makes use of a toxic component in the exhaust gas, nitric oxide, to convert soot particles into carbon dioxide.    You might remember from pages 137-8 in Chapter 6, that this molecule, nitric oxide, is a hormone that is made in many parts of our bodies.   The question is when will this diesel exhaust purifying system reach the United States; sooner if those people interested in global warming were aware of the ecological and health problems associated with diesel engines.

Acid Rain in Washington

The problem of urban pollution and acid precipitation (often called acid rain) is of increasing concern in the Washington metropolitan area. Pure, distilled water has a neutral pH of 7. Normal, unpolluted rain is slightly acid, with a pH of 5.6, because the carbon dioxide in air combines with water to form small amounts of a weak acid called carbonic acid. Rainfall in the Washington area has an average pH of 4.2 to 4.4, more than 10 times as acid as unpolluted rain. The main source of pollution here is exhaust from automobiles, trucks, and other forms of transportation. Vehicle exhaust contains nitrogen oxides and sulfur dioxide, which combine with water to form strong acids. 

Acid Rain-the Effect of Air Pollution
Carol Mahan  


Air pollution

The motor car is an important contributor to air pollution in South Africa. The transport sector (including aeroplanes, ships, trains and road vehicles) contributes 44% of the total national nitric oxide emissions and 45% of the total national volatile organic carbon emissions (VOC). Road vehicles, rather than trains, aircrafts and ships, contribute the most to the total carbon dioxide, nitric oxide and VOC emissions from the transport sector (94%, 53% and 89% respectively). Road vehicles also contribute to lead emissions, especially in the urban areas.

Particulate Air Pollurion with Emphasis on Traffic Generated Aerosols
The present thesis "Particulate Airpollution, with Emphasis on Traffic Generated Aerosols" has ... removed by e.g. rainfall or wind.

Effects of Air Pollution on local climate

Dust particles become condensation nuclei to facilitate rainfall downwind from the source of dust. Dust particles also cause an increase in the amount of visible light radiation which reflects back into space (= reduced albedo.)

y Ann Taviani on Monday, October 27, 2003 - 01:20 pm:

Topic 9 Question 1: VOCs

VOCs are volatile organic compounds. Hydrocarbons are often VOCs and they enter the atmosphere in automobile exhausts and play a major role in the formation of photochemical smog. For the most part, most VOCs are not pollutants themselves; however when they react with other sustances in the atmosphere, secondary air pollutants are created and these secondary air pollutants are associated with smog. VOCs can enter the environment through natural sources and anthropogenic sources as well.

The main source of anthropogenic hydrocarbons is the petroleum industry. This is because the petroleum industry produces gasoline and gasoline is a complex mixture of many volatile hydrocarbons and when in urban areas, the gasoline vapors escape into the atmosphere in several ways. Some of the ways include: when gas is pumped at gas stations, during filling of storage tanks, and as unburned gasoline in automobile exhaust.
In terms of natural sources and the natural world, the smell of pine, eucalyptus, and sandalwood trees is derived from the evaporation of VOCs called terpenes from these trees leaves. 85% of the total emissions of volatile hydrocarbons is from natural sources and 15% is from anthropogenic sources. It may seem like it's not a big deal because the anthropogenic sources are a lot lower; however, the emissions from anthropogenic sources are not evenly distributed, but are concentrated in urban areas, whereas natural emissions are evenly distributed.

Considering we cannot control VOCs from natural sources, we can work on controlling them from anthropogenic sources. One of the major ways of controlling VOCs and their hydrocarbons is the use of catalytic converters in automobiles. These catalytic converters which oxidize and reduce, remove about 98% of hydrocarbons emitted from auto exhaust systems. 

Air pollution can prevent rainfall, reported in the 10 March 2000 issue of Science [...]
Because urban and industrial air pollution is a significant problem in many regions of the world, Rosenfeld's findings suggest that human activity may be affecting rainfall patterns on a global scale. 

These data are the first direct evidence of how urban and industrial pollution affects rainfall levels, a question scientists have debated for several decades. In fact, some previous studies have concluded that air pollution might increase rainfall, but the debate has continued due to a lack of convincing data.
Contact: Heather Singmaster
American Association for the Advancement of Science

Environmental Effects of Urban Traffic - A case study of Jaipur City

Growing Urban centres necessitate the sprawling of transportation network, increasing distance between places of residence and work which needs to be covered in minimum time. The increased socio-economic status of urban population coupled with inadequacy of public transport has encouraged personalized means of transport. This craze for owning vehicles in urban centres, has led to considerable noise and air pollution, especially in big cities. (Table 1)

acid rain (deposition) result from both wet and dry acidic depositions that occur downwind of areas that are emitting SO2 and NOx from fossil fuel burning; other acids are involved to a lesser degree; e.g. HCl from coal burning power plants; where pH is < 5.6 all rainfall is normally slighly acidic - water reacts with CO2 to form weak H2CO3 (carbonic acid); pure rainfall has a pH about 5.6 pH involves a logrithmic scale not only isolated in mid-west; now found along both coasts of U.S.; internationally, developing countries tha rely on coal will face serious problems; e.g. China
causes of acid rain SO2 and NOx emissions have declined since 1970 and have generally leveled off at 20 M metric tons per year; in the 1990s, SO2 have decreased while NOx has increased SO2 and NOx are transformed by reactions with O2 and water vapor to form sulfuric and nitric acids travel long distances with prevailing winds to be deposited in the form of rainfall, fog, or snow; sulfate and nitrate particles may also be deposited directly on land and later activated by moisture to be acids SO2 - emitted by stationary sources: fossil fuel power plants
NOx - emitted by stationary and transport-related sources 

Air Pollution Linked to Drought

Source: Copyright 2002, Cosmiverse
Date: June 14, 2002

Since the 1960s, Sahel in northern Africa, which borders the Sahara, has experienced widespread drought. Rainfall was up to 49% lower than in the first half of the 20th century, causing widespread famine and death. Air pollution may have contributed to the Sahel drought, says Australian researcher Leon Rotstayn.

"Global climate change is not solely being caused by rising levels of greenhouse gases. Atmospheric pollution is also having an effect," says Rotstayn, who works at the Commonwealth Scientific and Industrial Research Organization (CSIRO). "The Sahelian drought may be due to a combination of natural variability and atmospheric aerosol," he added. "Cleaner air in the future will mean greater rainfall in this region."

According to Rotstayn, tiny atmospheric particles, known as sulfate aerosol, contribute to global climate shifts. The majority of sulfate aerosol comes from the burning of fossil fuels and metal smelting, while smaller amounts come from burning vegetation in the tropics, or natural sources such as marine plankton. Atmospheric aerosol concentrations are far greater in the northern hemisphere, cooling the atmosphere there more than in the southern hemisphere. It is this imbalance that affects the tropical rain belt.

The aerosol particles, according Rotstayn, make cloud droplets smaller which results in clouds that are brighter, last longer and reflect more sunlight back into space. The change in cloud structure has a cooling effect on the Earth's surface. Tropical rain belts, which typically migrate north and south following the sun's seasonal movements, are weakened in the northern hemisphere due to aerosol saturated clouds.

The weakened rain belts, not moving as far north, have impacted Sahel. Air pollution over China similarly affected their summer monsoon rainfall belt, for in northern China the summers of 1997 through 1999 were each plagued with droughts. 

What is Acid Rain : Air Pollution
of fuels (eg natural gas) and air pollution control technology. Rainfall is naturally acidic due to the presence of carbon dioxide in the atmosphere

Acid rain has been called the environmental issue of the 1980s and it continues to be a problem today. However, acid rain is not a new problem at all. It dates from the middle of the 19th century when a Scottish chemist, Robert Angus Smith, began to study the effect of air pollution in Manchester and used the term 'acid rain' to describe his findings. What is very new is the scale of the problem. In Smith's time, acid rain fell both in towns and cities and downwind from them, but now, the pollution is spread far and wide, within and between nations. It has now become an international problem.

Ship tracks

The track of large ships is sometimes visualised by a trail of shallow stratus clouds. These clouds, known as 'ship tracks', form in the wake of ships and are remarkably long-lived (Fig 1). They typically are between 0.5-5 km wide, i.e. wide enough to be seen in visible satellite imagery. Sometimes a ship track appears as a band of enhanced cloud thickness embedded in stratus. Ship tracks are due to cloud condensation nuclei (CCN) in the ship's exhaust (4). They are most likely in a near-saturated environment that is otherwise depleted of CCN. Such environment is very common in the marine boundary layer over the subtropical highs. Over these large, quasi-stationary highs, the boundary-layer air is divergent, making it unlikely to draw in CCN-rich continental air.

The nature and climatic effect of ship tracks was investigated in a field campaign labeled MAST (Monterey Area ShipTrack), which was conducted during June 1994 off the central California coast. Ship tracks increase the albedo, yet have very little effect on the long-wave radiation balance, because they are so shallow. Therefore ship-tracks tend to cool the global climate, although the magnitude of this effect is likely to be small.


Mims, F.M. and D.J. Travis, 1997. Aircraft contrails reduce solar irriadiance. EOS, 78, Oct 14. Travis, D.J. and S.A. Changnon, 1997. Evidence of jet contrail influences on regional-scale diurnal temperature range. J. Weather Mod., 29, 74-83. Sassen, K., 1997.Contrail-cirrus and their potetial for regional climate change. Bull. Amer. Meteor. Soc 78, 1885-903. Ferek, R. J., D. A. Hegg, P. V. Hobbs, P. Durkee and K. Nielsen, 1998. Measurements of Ship-Induced Tracks in Clouds Off the Washington Coast. J. Geophys. Res., 103, 23,199-23,206.  

Posted on Tue, Jul. 23, 2002
Contrails: New studies warn of global effects

Jet emissions of vapour, Gases may contribute to warming.
By Dan McKinney

On clear winter days, the sky can resemble an enormous Etch A Sketch. White lines left in the wake of jets crisscross each other, often stretching from one horizon to the other. But as ephemeral as they may seem, these cloudy trails could be contributing to global warming by trapping heat, according to a growing body of scientific research. If counted all over the globe, contrails -- the vapor trails left by aircraft -- have been found to cover a small but noticeable portion of the Earth's surface.

From 35,000 feet below, contrails may look like wispy lines. But they can grow into clouds many miles wide and hundreds of feet tall, and they often contain pollutants left by the burning of thousands of gallons of jet fuel. The trails and the clouds they create help to trap heat, contributing to the greenhouse effect, several recent studies contend.

To be sure, power plants, heavy industry and automobiles are now believed to be the largest sources of greenhouse gases in the world. The airline industry says there is not enough evidence to single out contrails as a significant environmental factor. But as aviation grows, some scientists are concerned that contrails will become a significant part of the global warming problem. ``Although aviation does not make a large contribution to the warming problem today, over the course of the next 50 to 100 years, it could become as important a part of the problem as passenger cars, according to some estimates,'' said Michael Oppenheimer, professor of geosciences and international affairs at Princeton University. 

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