When a layer of cool air is trapped by a layer of warm air above it illustrates?

The greenhouse effect is a process that occurs when gases in Earth's atmosphere trap the Sun's heat. This process makes Earth much warmer than it would be without an atmosphere. The greenhouse effect is one of the things that makes Earth a comfortable place to live.

Watch this video to learn about the greenhouse effect! Click here to download this video (1920x1080, 105 MB, video/mp4).

How does the greenhouse effect work?

As you might expect from the name, the greenhouse effect works … like a greenhouse! A greenhouse is a building with glass walls and a glass roof. Greenhouses are used to grow plants, such as tomatoes and tropical flowers.

A greenhouse stays warm inside, even during the winter. In the daytime, sunlight shines into the greenhouse and warms the plants and air inside. At nighttime, it's colder outside, but the greenhouse stays pretty warm inside. That's because the glass walls of the greenhouse trap the Sun's heat.

A greenhouse captures heat from the Sun during the day. Its glass walls trap the Sun's heat, which keeps plants inside the greenhouse warm — even on cold nights. Credit: NASA/JPL-Caltech

The greenhouse effect works much the same way on Earth. Gases in the atmosphere, such as carbon dioxide, trap heat similar to the glass roof of a greenhouse. These heat-trapping gases are called greenhouse gases.

During the day, the Sun shines through the atmosphere. Earth's surface warms up in the sunlight. At night, Earth's surface cools, releasing heat back into the air. But some of the heat is trapped by the greenhouse gases in the atmosphere. That's what keeps our Earth a warm and cozy 58 degrees Fahrenheit (14 degrees Celsius), on average.

Earth's atmosphere traps some of the Sun's heat, preventing it from escaping back into space at night. Credit: NASA/JPL-Caltech

How are humans impacting the greenhouse effect?

Human activities are changing Earth's natural greenhouse effect. Burning fossil fuels like coal and oil puts more carbon dioxide into our atmosphere.

NASA has observed increases in the amount of carbon dioxide and some other greenhouse gases in our atmosphere. Too much of these greenhouse gases can cause Earth's atmosphere to trap more and more heat. This causes Earth to warm up.

What reduces the greenhouse effect on Earth?

Just like a glass greenhouse, Earth's greenhouse is also full of plants! Plants can help to balance the greenhouse effect on Earth. All plants — from giant trees to tiny phytoplankton in the ocean — take in carbon dioxide and give off oxygen.

The ocean also absorbs a lot of excess carbon dioxide in the air. Unfortunately, the increased carbon dioxide in the ocean changes the water, making it more acidic. This is called ocean acidification.

More acidic water can be harmful to many ocean creatures, such as certain shellfish and coral. Warming oceans — from too many greenhouse gases in the atmosphere — can also be harmful to these organisms. Warmer waters are a main cause of coral bleaching.

This photograph shows a bleached brain coral. A main cause of coral bleaching is warming oceans. Ocean acidification also stresses coral reef communities. Credit: NOAA

Particle pollution, also known as particulate matter or PM, is a general term for a mixture of solid and liquid droplets suspended in the air. Particle pollution comes in many sizes and shapes and can be made up of a number of different components, including acids (such as sulfuric acid), inorganic compounds (such as ammonium sulfate, ammonium nitrate, and sodium chloride), organic chemicals, soot, metals, soil or dust particles, and biological materials (such as pollen and mold spores).

The air we breathe indoors and outdoors always contains particle pollution. Some particles, such as dust, dirt, soot, or smoke, are large enough to be seen with the naked eye. Others are so small they can only be detected using an electron microscope (Figure 1).

Figure 1. How big is particle pollution?

Particles that are 10 micrometers (µm) in diameter or smaller pose the greatest problems. These smaller particles generally pass through the nose and throat and enter the lungs. Once inhaled, these particles can affect the lungs and heart and cause serious health effects in individuals at greatest risk, such as people with heart or lung disease, people with diabetes, older adults and children (up to 18 years of age). Larger particles (> 10 µm) are generally of less concern because they usually do not enter the lungs, although they can still irritate the eyes, nose, and throat.

Particles of concern can be grouped into two main categories:

• Coarse particles (also known as PM10-2.5): particles with diameters generally larger than 2.5 µm and smaller than, or equal to, 10 µm in diameter. Note that the term large coarse particles in this course refers to particles greater than 10 µm in diameter.
• Fine particles (also known as PM2.5): particles generally 2.5 µm in diameter or smaller. This group of particles also encompasses ultrafine and nanoparticles which are generally classified as having diameters less than 0.1 µm.

Note that PM10 is a term that encompasses coarse, fine and ultrafine particle fractions.  

Fine and coarse particles differ by their sources, composition, dosimetry (deposition and retention in the respiratory system), and health effects as observed in scientific studies. Though it is often hypothesized that specific components or sources may be responsible for particle pollution-related mortality and morbidity, the available evidence is not sufficient to allow differentiation of those components or sources that are more closely related to specific health outcomes. Rather, the overall evidence indicates that many particle pollution components can be linked with such effects. This course will mainly focus on the health effects of fine particles because the scientific evidence of health effects is much stronger than for other size fractions.

Where does particle pollution come from?

Some particles, known as primary particles, are emitted directly from a source, such as construction sites, unpaved roads, smokestacks or fires. Other particles, known as secondary particles, form in complicated atmospheric reactions involving chemicals such as sulfur dioxides and nitrogen oxides that are emitted from power plants, industries and automobiles. Secondary particles make up most of the fine particle pollution in the United States.

Cooking, smoking, dusting, and vacuuming can also produce particle pollution, particularly in indoor settings. Particles produced by combustion are more likely to be fine particles, while particles of crustal (earth) and biological origin are more likely to be coarse particles.

Where and when is particle pollution a problem?

Particle pollution is found everywhere - not just in haze, smoke, and dust, but also in air that looks clean. Particle pollution can occur year-round and presents air quality problems at concentrations found in many major cities throughout the United States.

Some particles can remain in the atmosphere for days to weeks. Consequently, particle pollution generated in one area can travel hundreds or thousands of miles and influence the air quality of regions far from the original source.  

Particle pollution levels can be especially high in the following circumstances:

• Near busy roads, in urban areas (especially during rush hour), and in industrial areas.
• When there is smoke in the air from wood stoves, fireplaces, campfires, or wildfires. 
• When the weather is calm, allowing air pollution to build up. For example, hot humid days with stagnant air have much higher particle concentrations than days with air partially “scrubbed” by rain or snow.

Because of their small size, fine particles outdoors can penetrate into homes and buildings. Therefore, high outdoor particle pollution levels can elevate indoor particle pollution concentrations. 

Based on 2015 data, Figure 2 and 3 show how likely it may be for a particular area to experience air quality advisories for fine particle pollution based on short-term (Figure 2) and annual (Figure 3) averaging.

Figure 2. U.S counties with high 24-hour average particle pollution concentrations in 2015. This map depicts fine particle pollution concentrations by U.S. county for 2015 based on short-term (24-hour) average concentrations.  The map’s color key is based on categories of the Air Quality Index (AQI) (see Patient Exposure and the Air Quality Index).  All orange and red areas exceeded the 24-hour ambient air quality standards for fine particle pollution during 2015.  The map illustrates how likely it may be for a particular area to experience air quality advisories for particle pollution based on short-term averaging.

Figure 3. U.S counties with high annual mean particle pollution concentrations in 2015. This map depicts fine particle pollution concentrations by U.S. county for 2015 based on long-term (annual) average concentrations.  The map’s color key is based on categories of the Air Quality Index (AQI) (see Patient Exposure and the Air Quality Index).  All orange and red areas exceeded the annual ambient air quality standards for fine particle pollution during 2015.  The map illustrates how likely it may be for a particular area to experience air quality advisories for particle pollution based on annual averaging.

Fine particle pollution often has a seasonal pattern. Fine particle concentrations in the eastern half of the United States are typically higher from July through September, when sulfates are more readily formed from sulfur dioxide (SO2) emissions from power plants in that region and contribute to the formation of fine particles.

Fine particle concentrations tend to be higher from October through December in many areas of the West, in part because fine particle nitrates are more readily formed in cooler weather and due to wood stove and fireplace use.

In some locations, such as mountainous areas where wood is burned for heat, particle pollution levels can be especially high during wintertime inversions (Figure 4). An inversion traps the smoke close to the ground, allowing particle pollution levels to increase before the inversion lifts.

Figure 4. Inversions. Sometimes a layer of cooler air is trapped near the ground by a layer of warmer air above. This is called an inversion and can last all day, or even for several days. When the air cannot rise, pollution at the surface also is trapped and can accumulate, leading to higher concentrations of ozone and particle pollution.  A variety of conditions can cause inversions to form. The most common is a nighttime inversion, in which clear skies allow air at the surface to cool faster than the air above.

This situation is exacerbated by complex terrain. Inversions are more likely to occur in valleys where pollution is trapped both vertically (by the warmer air above) and horizontally (by the valley walls).

Which forms when a layer of cooler air is trapped by a warmer layer above it?

When a layer of cold air is trapped beneath warm air the result is?

What is an occurrence where a warm layer of air traps a cool layer of air close to the ground preventing air pollutants from dispersing?

Is a layer of cool air trapped under a layer of warm air that keeps pollutants near the ground?