National Capital Data Timelines

Visibility and Meteorological Data Collected at the NPS Park Police Headquarters Monitoring Site

Visibility -- Standard Visual Range

These data are raw and have not yet been validated using the standard procedures to guarantee their quality prior to their release for scientific and regulatory purposes. These data may be revised at a later time.

Particulate matter and gaseous air pollution affects visibility to some degree in every national park. Air pollution can create a white or brown haze that affects not only how far we can see but how well we are able to see the colors, forms, and textures of a scenic vista. The pollution causing this haze is often carried by the wind hundreds of miles from where it originated.

Visual range is one measure of visibility and is defined as the greatest distance at which a large black object can be seen and recognized against the background sky. It is not directly measured but rather calculated from a measurement of light extinction which includes the scattering and absorption of light by particles and gases. (These terms are explained in an Introduction to Visibility.) Extinction depends on the mass and chemical composition of the particles and gases and is a quantitative measure of how the passage of light from a scenic feature to an observer is affected by air pollutants. Extinction is monitored with transmissometers or nephelometers, or reconstructed from measurements of particle mass and chemical composition.

The larger the visual range the better the visibility. Average annual visual range in 2004 ranged from 30 to 75 miles in the eastern U.S. and 90 to 140 miles in the western U.S..

Visibility -- Haze Index

These data are raw and have not yet been validated using the standard procedures to guarantee their quality prior to their release for scientific and regulatory purposes. These data may be revised at a later time.

Haze Index, expressed in deciviews (dv), is also derived from light extinction measurements, and is designed in such a way that changes in it is linear with changes in human visual perception, similar to decibels. Unlike visual range, the deciview scale is near zero for a pristine clean atmosphere and increases as visibility is degraded.

Temperature

These data are raw and have not yet been validated using the standard procedures to guarantee their quality prior to their release for scientific and regulatory purposes. These data may be revised at a later time.

Relative Humidity

These data are raw and have not yet been validated using the standard procedures to guarantee their quality prior to their release for scientific and regulatory purposes. These data may be revised at a later time.