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Not Just For Kids
Weather and Weathering In the study of earth science, weathering is “the breakup of rocks due to exposure to the atmosphere.” While weathering is the means by which rocks are broken down, erosion is the process that picks up the weathered pieces and moves them to another location. Erosion is defined as “the removal and transport of earth materials by natural processes.” This can be done by wind, water, mud flows, glaciers, avalanches, earthquakes, volcanoes, tornadoes, and hurricanes. The technical definition of weathering can also be expanded to include other materials. As such, weathering is the adverse response of any material to atmosphere and climate. Since weather and climate vary widely over the earth’s surface, the factors that cause weathering are just as diverse and complex. Different climates will result in varying amounts of weathering based on local conditions, global and geographical weather patterns, and latitude. Weathering is also dependent on day/night cycles, seasonal variations, the age of the material, and the length of time that a material is exposed to weathering processes. The primary environmental factors that lead to material deterioration are sunlight, temperature, and moisture, along with wind, pollution, and acid rain. Other variables include blowing dust, manmade pollutants, changes in ozone concentration, and acts of nature such as El Nino/La Nina, hurricanes, and volcanoes. The severity of weathering is further affected by the number and type of plants and animals present, topographic factors such as slope, and the nature of the material. Different materials are sensitive to specific environmental parameters. For example, metals are prone to oxidation and corrosion by humidity, rain, and salt spray. Corrosion typically results in a refined metal returning to its natural state (steel to iron oxide, copper to copper chloride, etc.). Plastics are degraded by UV radiation. Surface coatings are affected by temperature and moisture. End results of weathering may include: discoloration, cracking, fading, rust, flaking, peeling, brittleness, a reduction in the useful service life of a material, or actual product failure. Manufacturers must determine the objectives of their products before deciding what materials they are going to be made of. The major marketing area of the product must also be considered. Question to Consider: Can you think of any applications in which it would actually be beneficial for a product to decompose rapidly under outdoor weather conditions? Weathering processes are divided into three categories: physical, chemical, and biological. In many cases these processes act together, each enhancing the others’ work. Physical weathering is when a material is broken down by mechanical means without changing its composition. Examples would be frost action, freeze and thaw cycles, thermal expansion and contraction, wetting and drying, crystallization, pressure, salt decay, wind and water abrasion. These processes can reduce larger particles into smaller particles, thus giving water and oxygen greater access to its surface, and thereby increasing the effectiveness of chemical weathering. Chemical weathering, also known as decomposition, involves complex chemical reactions that change the material into a different substance by removing and/or adding elements. Two common chemical weathering processes are acidification and oxidation (rust). Biological weathering may be physical or chemical. It can be the result of tree root wedging, the pressure of plant roots, decomposition by lichens, mold, mildew, and microbiological organisms. Sunlight Sunlight influences our weather and sustains all forms of life on earth. Even as the sun nurtures us and lights up our world, it has a destructive side as well. All organic materials are damaged by too much sun. Skin cancer is directly attributable to the popular habit of sun tanning, which is really overexposure of the skin to solar radiation. Sunlight consists of 5% UV radiation, 45% visible light, and 50% infrared energy. UV contains the most energy, making it the primary cause of material degradation. Heat The temperature of materials exposed to sunlight has an influence on the effect of the solar radiation. Photochemical reactions are usually accelerated at elevated temperatures. Ambient air temperature, evaporation rates, and the color of the material all play a role. Various colors absorb radiation differently. Light colors and metallic finishes reflect light while dark colors absorb more wavelengths. This affects the surface temperature and subsequent degradation. In addition, metal surfaces remain hot for a longer period of time than other materials such as plastics. Moisture Moisture in the form of rain, fog, dew, water vapor, humidity, etc. affects materials in several ways. A material may absorb the water and expand, then as it dries out and contracts this fluctuation can stress the material. A similar effect happens during freeze-thaw cycles. Thermal shock from daytime rain showers can cause chemical and physical failures. Rain can reduce chemical deterioration by washing off pollution, or it can hasten deterioration by rinsing off dirt and dust accumulation which often serve as a protective coating against UV radiation. Rain may also physically deteriorate a surface. Water acts as a solvent, which means it can dissolve minerals. Acid rain is particularly damaging. Contact with water in any form can accelerate oxidation. The total duration that a surface is wet also has an effect. The weathering result of a drizzle lasting several hours is much greater than if a lot of rain falls in just a few minutes. Acid Rain Acid weathering and etching of materials has become a problem in recent years due to acid rain. This is because of factories putting pollution into the air, which then combines with water to fall down to earth as acid rain. The Midwest is the worst producer of acid rain, but that area isn't affected because most of the pollution is blown to the Eastern U.S., Northeastern U.S., or Canada. Carbonic acid is one of the usual acids formed when carbon dioxide combines with water. The most affected mineral is calcite, which is found in marble and limestone. Consequently, many buildings, statues and monuments made of these materials are slowly, but surely, weathering away. (Did you know that carbonic acid is an ingredient in carbonated soft drinks? People drink the same product that weathers rocks!) New River’s Weathering Test Lab In the weathering industry, the two major benchmark climates for weathering tests are a subtropical climate (southern Florida), and a desert environment (central Arizona). These sites are recognized around the world as the most extreme climates for materials exposed outdoors. Central Arizona provides high levels of UV solar radiation, elevated temperatures, and an arid climate, all of which are crucial factors in the weathering process. It also has a wide range of temperatures, which may fluctuate from 30 - 50 degrees C between day and night. Here, automotive interior materials experience the highest stress compared to other climates. An unpolluted desert environment is ideal for testing materials under these conditions. Most people don’t realize that New River is home to Arizona’s premier weathering testing laboratory. Have you ever noticed that large building surrounded by what looks like a bunch of solar collectors up on a ridge off New River Road just west of I-17? A sign by the road to the 40-acre material testing and evaluation site simply says “DSET.” My husband, Rich Olsen, is the Weathering Manager at DSET, leading a staff of specially trained scientists, engineers, and technicians. DSET offers desert exposure testing for a wide range of industries. DSET weathering clients include major manufacturers of paints and coatings, automotive products (GM, Ford, etc.), building materials, consumer products, household fabrics such as upholsteries and drapes, plastics (PVC, styrofoam, etc.), and any other companies who want to make sure their products will withstand the tests of time and the environment. This can include such diverse products as golf balls, printing inks, eyeglass lenses, street light poles, porch awnings, and dyed cotton fabrics. DSET weathering services include natural outdoor weathering, outdoor accelerated weathering, artificial indoor weathering, direct weathering, indirect weathering, and other special projects. They have an under-glass testing device for complete automotive assemblies such as door panels, steering wheels, and instrument panels. They even have full car enclosures and life-size test houses. DSET utilizes the latest technology in weather instrumentation for monitoring ambient temperature, humidity, rainfall, total wet-time and wind speed. DSET was originally founded in 1948. Their first test field was located in Wickenburg. In 1950, the company moved to Phoenix. The growth of Phoenix and subsequent air pollution forced the company to move to New River in 1969. DSET Laboratories, Inc. pioneered accelerated outdoor weathering in the 1960’s with its Fresnel-reflecting devices, and currently operates over 500 machines. Sunlight is concentrated onto specimens using ten mirrors with an intensity of approximately eight suns. The device tracks the sun and exposes specimens to the full spectrum of sunlight, making it one of the most realistic and widely used accelerated test methods in the world today. One of these solar concentrators is also the largest in the world, designed and constructed in 1977 to support a Department of Energy Silicon Solar Cell Test Program, enabling testing of a complete component rather than small specimens. DSET was incorporated in 1973 as Desert Sunshine Exposure Test, Inc. The name was changed to DSET Laboratories in 1980. For a short time, DSET was owned by a German company called Heraeus. Then DSET was acquired by Atlas Material Testing company in 1995, becoming part of the Atlas Weathering Services Group. DSET has continued to maintain its own name recognition, even though it is now part of the largest weathering testing organization in the world. Early in the 20th century, Atlas was a supplier of stage lighting for movie photography. This was back when Hollywood didn’t yet exist and Chicago was the motion picture capital of the world. When it was found that high intensity carbon arc lamps faded certain textile materials and garment fabrics worn by actors, the weathering industry was born. Paint manufacturers got involved with outdoor weathering testing in the 1920’s and 1930’s, after finding that the hot, humid subtropical environment of south Florida degraded paints quickly. DSET became a leader in the weathering industry when Arizona became an international reference site for hot arid desert climates in the early 1950’s. Atlas Weathering Services Group has 5 laboratories and 23 outdoor sites around the world with a variety of climates and environments, including desert, tropical, subtropical, temperate European, marine, Mediterranean, high-altitude, and corrosive industrial. While the primary benchmark sites are located here in New River, Arizona and in Miami, Florida, they also have a site in Prescott, Arizona for high altitude testing, a site in Chicago, Illinois for freeze-thaw cycles, a site in Louisville, Kentucky for acid rain compounds, and additional sites in Florida. Affiliated sites are located in Canada, the Netherlands, France, Columbia, Saudi Arabia, Russia, Singapore, Australia, Japan and China, providing exposures in a wide range of climatic and environmental conditions. Atlas Weathering Services Group has become the largest network of independent outdoor weathering laboratories in the world. New River has been an ideal weathering test location for more than three decades due to the significant number of annual days with adequate sunshine and clean air. Unfortunately, DSET again finds itself in the middle of a path of urban growth. Their weathering tests are conducted using extremely sensitive sun monitoring equipment. Any change in the ambient conditions caused by increased humidity, pollution, shadows, smoke particulates, dust from nearby construction, etc. that result in a reduction of viable testing conditions, would be a significant detriment to DSET’s ability to conduct its tests in New River. DSET is a notable exception to the general trend of businesses along the Interstate 17 corridor. It deserves to be locally recognized as the world-renowned weathering reference site that it is. New River Climate Data Average annual UV radiation = 350 MJ/m2. Average annual rainfall = 10 inches. Average summer high temperature = 102 degrees F. Average winter low temperature = 68 degrees F. Average relative humidity = 32%. (This is steadily on the rise due to the influx of green landscaping, pools, ponds, and golf courses. The humidity was only 24% in the 1980’s.) References Atlas Weathering Testing Guidebook and Fundamentals of Weathering Course.
www.atlaswsg.com (Atlas Material Testing Weathering Services Group: worldwide exposure network, climate data, and complete descriptions of testing methods and equipment, with photos.)
www.dc.peachnet.edu/~pgore/geology/geo101/weather.htm (Illustrated introduction to weathering.)
www.geog.ouc.bc.ca/physgeog/contents/11b.html (Virtual Textbook on the Fundamentals of Physical Geography - Weathering.)
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