US Forest Service Interpreting Visibility Data

Real Time display panels for visibility data show the air pollution and meteorological conditions associated with each image. These data are collected at either the site of the camera or another location within the scene of the photograph.

As you look at the scene and compare it to the visibility data, you may wonder if the scene is as clear as it could be, or if haze or fog may be 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 photo to the small photo taken on an unpolluted day (Pristine). If today's photo is not as crisp as the "pristine" photo, then there may be haze, black carbon or fog obscuring the view.
visibility data

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 man-made 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. Brown clouds tend to occur on calm winter mornings during rush hour traffic. 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 there has been precipitation in 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. However, fine particles and black carbon could be low, medium or high. Fog is a natural condition.

To learn more about pollutants and meteorological conditions, please read on. Every type of data and its relation to visibility is described in detail below. Note that most Forest Service sites do not measure all of these variables.


Fine Particles

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 gasses such as nitrogen oxides (NOx) and volatile organic compounds (VOCs) react and condense in the atmosphere. These gasses originate in the same sources as above. They also come from the evaporation of fuels and household and industrial solvents.

This variety of gasses and direct particle emissions results in a mixture of fine particles with different sizes, chemical properties and impacts on health and the environment. 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. It can be caused by diesel engines in urban areas or by coal-fired power plant emissions. 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, 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. It is similar to soot and 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 waste. Black carbon absorbs light and contributes substantially to the low-altitude, brown clouds sometimes seen over urban areas and valleys in the winter.

If black carbon concentrations are high when visibility is poor, then the visibility 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. However, most of the poor visibility will be due to the fog itself. When this happens, relative humidity will be near 100 percent and precipitation may be evident from the image, or indicated in the Real Time visibility information panel.


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, busses, power plants, fuel and paint vapors and other sources. Ozone is good up high, but bad nearby. Concentrations may reach unhealthy levels on warm, sunny days. During the summer, ozone can be unhealthy for several days in a row.

Because it is an invisible gas, ozone 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.

Condition Indications that poor visibility is caused by pollution Indications that poor visibility is not caused by pollution
High ozone X  
Low ozone    
High PM X  
Low PM   X
High BC X  
Low BC   X



Temperature is directly related to the 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 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, 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. 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, 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 man-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, such as 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 FSVISIMAGES 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 FSVISIMAGES display panel shows the direction that the wind is coming from. For example, a wind direction of NE would be blowing from the Northeast.


If measured amounts of precipitation are shown on the visibility information panel, poor visibility conditions are likely due to natural phenomena, especially if the precipitation has been measured within the past hour.


Visual range is defined as “the distance at which a black object can be discerned from a white background on the horizon”. However, visual range is measured 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 ranges are shown as "less than" a particular distance on the FSVISIMAGES Web site.