Forces of Change
Sky-High Observer
Aura Satellite
Photo courtesy of NASA/GSFC

The amount of UV that reaches the ground is mainly affected by:

Solar Elevation:

The higher the sun is in the sky, the greater the UV radiation that reaches the ground.


Clouds can block from 11 to 69 percent of UV depending on how much of the sky is cloudy.


Tropospheric ozone from photochemical smog and other sources traps some UV light but not nearly enough to compensate for the loss of ozone in the stratosphere.

Sky High Observer

The Aura Earth-observing satellite launched in July 2004 is the latest in a line of instruments and satellites that give us the ability to measure changes in climate and air quality and in the health of the ozone layer that protects living things from UV radiation.

AR-81 atop the SERC tower
Home Grown
Atop a 37 m (120 ft) tower, Dr. Patrick Neale, photobiologist at the Smithsonian Environmental Research Center near Annapolis, Maryland, is studying UV every day.
Photo Chip Clark © Smithsonian Institution