Ultraviolet, or UV, radiation makes up part of the electromagnetic spectrum of light.
The human eye responds to light with wavelengths which are in the color portion of the spectrum.
Thus, we are able to see colors from red to violet. Light with wavelengths shorter than the
human eye can 'see' is called ultraviolet (beyond violet) light.
UV light is contained in the range of wavelengths that is produced
by the Sun. Most UV light is absorbed by the ozone layer which protects the Earth, or is reflected
back into space, so that only a small portion reaches the surface of the Earth. Sunlight is
received as direct rays and as diffuse light, i.e. skylight which has been scattered by
the atmosphere (skyshine). The sky is blue because air molecules scatter the shorter wavelength blue light more
than the red light (see the Science Corner discussion of "Why is the sky blue").
UV light is scattered even more than blue light, and thus, we actually receive more UV radiation
from this 'skyshine' than from direct sunshine! The large amount of ultraviolet in skyshine accounts
for the fact that it is possible to get sunburned on a beach on a cloudy day when there is no direct sunshine
from the Sun. The reflection of ultraviolet from the surface of water is twice as great as from a field
of grass, but only one-fourth as much as from freshly fallen snow. Thus, you will recieve eight times the amount
of UV radiation on a field covered with snow, as opposed to a field of grass, with all other things being equal.
Ordinary window glass in our homes, offices, and automobiles prevents the passage of most ultraviolet rays.
The UV spectrum of light can be further broken down into UV-A, UV-B, and UV-C. UV-C radiation
is extremely dangerous to plants and animals. Lucky for us, it is absorbed by the ozone layer
and does not reach the ground. Only 1% of solar radiation within the UV-B band reaches the earth's surface, with the other 99%
absorbed by the ozone layer. Nevertheless, UV-B rays are of particular interest because these
wavelengths can cause damage at the molecular level (in our DNA), and small changes in ozone can lead
to large changes in UV-B radiation reaching the earth's surface. UV-A rays are somewhat less dangerous, but
are important in the generation of smog and also in fading and damage to plastics, paints, and
So what affects the intensity of the UV radiation we receive at the surface?
The amount of UV radiation that reaches the earth's surface can vary greatly depending on the following
factors: time of year or season, time of day, weather conditions, surfaces, altitude, and latitude.
Season, Time of Day, and Weather Conditions
The amount of UV radiation that reaches the earths surface is higher during the summer months,
when the sun's rays are more direct. The middle of the day, when the sun is highest, has the highest
UV exposure. This period can vary from 10 am to 2 pm (or 11 am - 3 pm for Daylight Savings Time). Different types of
cloud cover may block or enhance UV rays. While thick, dark clouds can block UV radiation, puffy or thin-layered
clouds do not. Hazy days may see just as high UV values reaching the surface as on clear days. Some clouds may
even increase the radiation by reflecting and refracting the sun's rays or the skyshine back to the ground.
Elevation or Altitude
At higher altitudes, more UV can get reach the earth's surface, because the atmosphere tends to be
cleaner and thinner than at lower elevations.
Reflective Properties of Surfaces
Bright surfaces can reflect the sun's rays and increase the UV exposure. Snow, sand, water, and
concrete will reflect most UV radiation. Thus, your skin will receive a much higher dose of UV radiation
when near one of these materials. Contrary to expectations, a snow skiier can be sunburned during the
middle of winter while skiing on a mountain slope due to this factor.
Regions of the World, or Latitude
People living close to the equator, at low latitudes, experience extremely high levels of UV
radiation. Locations such as Australia, South America, and Africa all lie within this region. This plays a factor
for us here in Arizona, due to our far southern location with regard to much of the rest of the United
So how much more UV radiation am I receiving in Flagstaff, versus in Phoenix?
As mentioned earlier, UV radiation increases with altitude. This increase has been shown to
be between 4-5% for every 1000 feet ascended. Thus, for the same day in June, the amount of
UV radiation would be approximately 27% higher in Flagstaff than in Phoenix (about 6000 feet elevation
difference). If you were going from Phoenix to the top of the San Francisco Peaks (approximately 12,000 feet),
you would experience an increase in UV radiation of about 50%!
If you were to do this on a day in the winter with snowfall on the ground at Flagstaff, you would
also have to factor in the effect of the reflection of UV radiation on the snow, and your UV exposure
would go from 27% higher (just due to the elevation difference) to approximately 100% higher due to snow on the ground (snow is
approximately four times more reflective than desert).
While skiing up at 10,000 feet, you would receive about 200% higher levels of UV radiation than you would receive on the same
day in the winter in Phoenix.
This underscores the important fact that protection from the sun should be even more of a concern for anyone
who is planning activities at higher elevations, even in the winter.
How can I find out more about UV radiation and the UV Index?
There is a lot of information on the internet regarding UV radiation and its impacts on society. The link provided below will take you
to the official map produced by the National Weather Service which shows areas of high and low UV radiation potential (UV Index)
for the United States.