Page 108 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

72 part I The energy–atmosphere System
Distinct regions within the ionosphere, known as the D, E, F1, and F2 layers, are important for broadcast communications and GPS signals. These regions reflect certain radio wavelengths, including AM radio and other shortwave radio broadcasts, especially at night. Activity such as solar flares can trigger radio blackouts. This also affects airplanes that fly over the Arctic; over the North Pole, these aircraft lose contact with geosynchronous sat- ellites and must rely on radio communications, which can be disrupted in a blackout.
Before they reach ground, GPS satellite signals must first pass through the ionosphere, where gases bend and weaken radio waves. Solar and geomagnetic storms that disturb the ionosphere can cause GPS position errors as large as 100 m. (Find out how to fly through the ionosphere using NASA’s 4D Ionosphere program in conjunction with Google EarthTM at science.nasa.gov/science-news/science- at-nasa/2008/30apr_4dionosphere/.)
Ozonosphere That portion of the stratosphere that con- tains an increased level of ozone is the ozonosphere, or ozone layer. Ozone is a highly reactive oxygen molecule made up of three oxygen atoms (O3) instead of the usual two atoms (O2) that make up oxygen gas. Ozone absorbs the shorter wavelengths of ultraviolet (UV) radiation (prin- cipally, all the UVC, 100–290 nm, and some of the UVB, 290–320 nm).* In the process, UV energy is converted to heat energy, safeguarding life on Earth by “filtering” some of the Sun’s harmful rays. However, UVA, at 320–400 nm, is not absorbed by ozone and makes up about 95% of all UV radiation that reaches Earth’s surface.
Although UVA, with its longer wavelengths, is less intense than UVB, it penetrates skin more deeply. Stud- ies over the past 20 years have shown that UVA causes significant damage in the basal (lowest) part of the epi- dermis, the outer layer of skin, where most skin cancers
*Nanometre (nm) = one-billionth of a metre; 1 nm = 10–9 m. For comparison, a micrometre, or micron (mm) = one-millionth of a metre; 1 mm = 10–6 m. A millimetre (mm) = one-thousandth of a metre; 1 mm = 10–3.
occur. UVA levels are fairly constant throughout the year during daylight hours and can penetrate glass and clouds. In contrast, UVB intensity varies by latitude, season, and time of day. UVB damages the skin’s more superficial epidermal layers and is the chief cause of skin reddening and sunburn. It, too, can lead to skin cancer.
The total amount of ozone in the stratosphere is pre- sumed to have been relatively stable over the past several hundred million years (allowing for daily and seasonal fluctuations). Today, however, ozone is being depleted beyond the changes expected due to natural processes. Focus Study 3.1 presents an analysis of the crisis in this critical portion of our atmosphere.
CRiTiCALthinking 3.2 Finding Your Local Ozone
To determine the total column ozone amount at your pres- ent location, go to “What was the total ozone column at your house?” at ozoneaq.gsfc.nasa.gov/ozone_overhead_all_ v8.md. Total column ozone is the total amount of ozone in a column from the surface to the top of the atmosphere.
Select a point on the map or enter your latitude and lon- gitude and the date you want to check. The ozone column is currently measured by the Ozone Monitoring instrument (OMi) sensor aboard the Aqua satellite and is mainly sensi- tive to stratospheric ozone. (note also the limitations listed on the extent of data availability.) if you check for several dif- ferent dates, when do the lowest values occur? The highest values? Briefly explain and interpret the values you found. •
UV Index Helps Save Your Skin Weather reports regu- larly include the UV Index, or UVI, in daily forecasts to alert the public of the need to use sun protection, espe- cially for children (Table 3.2). The UV Index is a simple way of describing the daily danger of solar UV-radiation intensity, using a scale from 1 to 11+. A higher number indicates a greater risk of UV exposure; an index of 0 indicates no risk, such as at night. Higher risk means that
TABLE 3.2 UV Index*
Exposure Risk Category
UVi Range
Comments
low
High
extreme
less than 2
6–7
11+
low danger for average person. Wear sunglasses on bright days. Watch out for reflection off snow.
Use sunscreens with SPF of 15 or higher. reduce time in the Sun between 11 a.m. and 4 p.m. Use protections mentioned above.
Unprotected skin is at risk of burn. Sunscreen application every 2 hours if out-of-doors. avoid direct Sun exposure during midday hours. Use protections listed above.
Moderate
3–5
Take covering precautions, such as sunglasses, sunscreen, hats, and protective clothing, and stay in shade during midday hours.
Very high
8–10
Minimize Sun exposure 10 a.m. to 4 p.m. Use sunscreens with SPF ratings of over 15. Use protections mentioned above.
*The UV index was developed by scientists at environment Canada in 1992 and subsequently adopted by other countries. in 1994 it became the standard of the World Meteorological Organization (WMO) and World Health Organization (WHO), with revisions introduced in 2004. See: www.ec.gc.ca/uv/default.asp?lang=En&n=C74058DD-1/.