INTERPRETING VISIBILITY DATA

The Live Site display panel for visibility data shows the air quality and meteorological conditions associated with each image. These data are collected at the site of the camera or at another location within the scene of the image.

As you look at the scene and check it against the visibility data, you may wonder if the scene is as clear as it could be or if haze or fog is affecting the view. Here are some tips to help you interpret what you are seeing.

1. Look again; does the picture really seem clear? On clear days, the features on the horizon appear crisp. These days have low pollution levels and low relative humidity. Compare today’s image to the small image taken on an unpolluted (ideal conditions) day. If today's image is not as crisp as the ideal conditions image, there may be haze, black carbon, or fog obscuring the view.

2. Is it a hazy day? Haze is relatively uniform at the horizon but tends to diminish slightly at higher elevations. Look at the levels of human-caused pollutants including fine particles, black carbon, and ozone. Also note the relative humidity. Haze often occurs on hot, humid, summer days with medium or high levels of fine particles, ozone, and sometimes black carbon. Relative humidity tends to be medium to high.

3. Is it a brown cloud day? A brown cloud appears to envelop the scene but quickly thins out at higher elevations. Look at the particle and black carbon levels -- they are usually high. Ozone will be low and relative humidity may vary.

4. Is it a foggy day? Look at the relative humidity and precipitation levels. If the relative humidity is close to 100% and precipitation has occurred within the past 24 hours, then you are probably looking at fog. Fog tends to be gray while haze is generally white. It does not thin out at the top of the picture and is most common in the fall and spring. Ozone levels will be low. Fine particles and black carbon levels, however, could be low, medium, or high. Fog is a natural condition.

To learn more about the pollutants and meteorological conditions, please read on. Each of the data types and its relation to visibility are described further below.

Note that most Wyoming Department of Environmental Quality sites do not measure all of these parameters.

POLLUTANTS

Particulate

Particulate matter, or PM, is the term for particles in the air, including dust, dirt, smoke, and liquid droplets. They can be suspended in the air for lengthy periods and come in a variety of sizes. Particles larger than 2.5 micrometers are referred to as “coarse” particles. Particles less than 2.5 micrometers in diameter are referred to as “fine” particles.

Coarse Particles (PM10)
Coarse particles tend to have a natural origin and deposit close to their sources. These particles may be released from wind-blown dust, vehicles traveling on unpaved roads, materials handling, and crushing and grinding operations. PM10 is regulated by the Air Quality Division and the EPA under the Air Quality Division’s Ambient Air Quality Standards. Particles less than 10 micrometers in diameter pose the greatest health concern because they can be inhaled into and accumulate in the respiratory system.

Fine Particles (PM2.5)
Fine particles are a mix of microscopic solids and liquids. They can be emitted directly into the air during any process involving burning or combustion, including activities around the home (e.g. cooking, smoking, space heating, and open burning) and those involving motor vehicles, various engines, power plants, and other such sources. Fine particles also form when gases such as nitrogen oxides (NOx ) and volatile organic compounds (VOCs) react and condense in the atmosphere. These gases come from the same sources noted above. They also come from the evaporation of fuels and household and industrial solvents. PM2.5 is regulated by the Air Quality Division and the EPA under the Air Quality Division’s Ambient Air Quality Standards

Grand Teton National Park

This variety of gases and direct particle emissions results in a mixture of fine particles with different sizes, chemical properties, and health and environmental impacts. Fine particle levels are highest on warm, sunny days and on clear, calm winter mornings.

Fine particles affect visibility in two ways -- by absorbing light and by scattering light. Light absorption causes a brownness or blackness in the air. This is most obvious over urban areas and valleys during calm mornings, especially in winter. Light scattering causes a whitish haze, which is most obvious in the summer over widespread urban and rural areas.

Medium and high levels of fine particle concentrations are a strong indication that poor visibility is due in large part to pollution. If fine particle concentrations are low, then any visibility impairment is not likely due to pollution, but to natural causes. Click here to learn more about fine particles and the Causes of Poor Visibility.

Black Carbon
Black carbon is one of the many components of fine particles. Similar to soot, black carbon is emitted directly into the air from virtually all combustion activities. It is especially prevalent in diesel exhaust and smoke from the burning of wood and wastes. Black carbon absorbs light and contributes substantially to the low-altitude brown clouds sometimes seen during the winter over urban areas and valleys.

If black carbon concentrations are high when visibility is poor, then the impairment is probably due to air pollution. An exception to this rule would be during periods of fog. Under these conditions, black carbon will tend to stagnate in local areas, hence raising their concentrations. Most of the poor visibility, however, will be due to the fog itself; relative humidity will be near 100 percent and precipitation may be evident from the image or indicated in the Live Sites visibility information panel.

Gaseous

Nitrogen Dioxide (NO2)
Nitrogen dioxide is a reddish brown, highly reactive gas formed when another pollutant (nitric oxide) combines with oxygen in the atmosphere. It can often be seen as a layer over many urban areas. Once it has formed, nitrogen dioxide reacts with other pollutants. Eventually these reactions result in the formation of ground-level ozone. Major sources include automobiles and power plants.

Nitrogen dioxide can cause respiratory problems in animals and humans, and can also contribute to acid rain, water pollution, and visibility impairment. NO2 is regulated by the Air Quality Division and the EPA under the Air Quality Division’s Ambient Air Quality Standards.

Ozone (O3)
Ozone is a colorless, odorless gas. It occurs naturally in the upper atmosphere, where it absorbs harmful ultraviolet rays. But at ground level, it forms as result of air pollution from cars, trucks, buses, power plants, fuel and paint vapors, and other sources. Ozone is good up high, but bad nearby. O3 is regulated by the Air Quality Division and the EPA under the Air Quality Division’s Ambient Air Quality Standards. Concentrations may reach unhealthy levels on warm, sunny days. In urban areas during the summer, ozone can be unhealthy for several days in a row.

Because ozone is an invisible gas, it does not directly affect visibility. But in the summer it is usually associated with pollution episodes involving haze and participates in chemical reactions that lead to haze-forming particles. Medium or high concentrations of ozone are a good indicator that poor visibility conditions are due to pollution; however, pollution can cause poor visibility without necessarily leading to high ozone concentrations.

Use the following table to determine when poor visibility may be due to pollution or to natural conditions, such as fog. Note that "PM" and "BC" stand for particulate matter and black carbon, respectively.

METEOROLOGICAL CONDITIONS

Temperature
Temperature is directly related to concentrations of ozone, fine particles, and black carbon. Temperatures at or above the mid-80s (Fahrenheit) favor the evaporation and emission of volatile organic compounds (VOCs) and nitrogen oxides (NOx) and increase the speed of chemical reactions leading to ozone and fine particles. Temperatures below 40 degrees (Fahrenheit) may enhance the condensation of some fine particulate matter. High fine particle and black carbon concentrations may also occur at temperatures between 40 and 80 degrees (Fahrenheit), especially in the presence of low clouds, which can limit the dispersion of pollutants and concentrate them near the ground.

Relative Humidity
High levels of relative humidity are often associated with high levels of ozone and fine particles. In the case of fine particles, high humidity can lead directly to increases in the size and concentration of fine particles. This occurs because certain types of fine particles, especially sulfates and nitrates, are capable of absorbing water vapor. Once hydrated and enlarged, these particles cause light to scatter, which results in a whitish, regional haze. Sometimes, visibility is limited mostly by low clouds, fog, or rain, and not by air pollution. These situations can be visually distinguished from human-made haze after learning how they differ in grayness, thickness, and homogeneity. They are also characterized by extremely high levels of relative humidity (in the 90s).

Wind Speed
Wind speed affects air pollution and visibility through its dispersive effects on pollutants. When winds are calm or light (0 to 5 mph), pollutants can accumulate and reach unhealthy levels. Light to moderate winds (5 to 10 mph) sometimes increase pollutants by mixing emissions from various sources, urban centers, and transportation corridors. These winds also transport pollutants further downwind and may therefore raise concentrations in places that might otherwise be clean. High wind speeds (15 mph or greater) tend to disperse pollutants and prevent their accumulation. At these speeds, the amount of dispersion outweighs the transport effects, so high concentrations are unlikely to occur anywhere.

Wind speeds reported on WYVISNET.com are strictly in the horizontal direction. Air, however, also travels in vertical directions. When vertical wind speeds (or venting) are high, pollutants are dispersed vertically and do not become concentrated at the ground. Venting is strongest during clear, daylight hours.

Wind Direction
Wind direction determines where pollutants are going, and where they are coming from. The wind direction shown on the WYVISNET.com display panel shows the direction that the wind is coming from. For example, a wind direction of NE would be blowing from the northeast.

VISUAL RANGE

Visual range is defined as the distance at which a black object can be discerned from a white background on the horizon. Visual range is measured, however, by instruments that capture small amounts of air at a fixed location on the earth's surface. This method sometimes overestimates the visual range, especially in foggy or rainy conditions. This is the reason the visual range is shown as an “approximate” distance on the WYVISNET.com display.