Methods of weather modification

1. The first scientific attempt at coaxing moisture from a cloud was in 1946, when scientist Vincent Schaefer dropped 3 pounds of dry ice from an airplane into a cloud and, to his delight, produced snow. The success of the experiment was modest, but it spawned optimism among farmers and ranchers around the country. It seemed to them that science had finally triumphed over weather.

2. Unfortunately, it didn’t work out that way. Although there were many cloud-seeding operations during the late 1940s and the 1950s, no one could say whether they had any effect on precipitation. Cloud seeding, or weather modification as it came to be called, was clearly more complicated than had been thought. It was not until the early 1970s that enough experiments had been done to understand the processes involved. What these studies indicated was that only certain types of clouds are amenable to seeding. One of the most responsive is the winter orographic cloud, formed when air currents encounter a mountain slope and rise. If the temperature in such a cloud is right, seeding can increase snow yield by 10 to 20 per cent.

3. There are two major methods of weather modification. In one method, silver iodide is burned in propane-fired ground generators. The smoke rises into the clouds where the tiny silver-iodide particles act as nuclei for the formation of ice crystals. The alternate system uses airplanes to deliver dry-ice pellets. Dry ice does not provide ice-forming nuclei. Instead, it lowers the temperature near the water droplets in the clouds so that they freeze instantly – a process called spontaneous nucleation. Seeding from aircraft is more efficient but also more expensive.

4. About 75 per cent of all weather modification in the United States takes place in the Western states. With the population of the West growing rapidly, few regions of the world require more water. About 85 per cent of the waters in the rivers of the West comes from melted snow. As one expert put it, the water problems of the future may make the energy problems of the 70s seem like child’s play to solve. That’s why the U.S. bureau of Reclamation, along with state governments, municipal water districts, and private interests such as ski areas and agricultural cooperatives, is putting increased effort into cloud-seeding efforts. Without consistent and heavy snowfalls in the Rockies and Sierras, the west would literally dry up. The most intensive efforts to produce precipitation were during the West’s disastrous snow drought of 1976-1977. It is impossible to judge the efficiency of weather modification based on one crash program, but most experts think that such hurry-up programs are not very effective.

1. What is the main subject of the text?

a) the scientific contributions of Vincent Schaefer

b) developments in methods of increasing precipitation

c) the process by which snow crystals form

d) the effects of cloud seeding

2. The word spawned (given in the text in Italics) is closest in meaning to

a) intensified

b) reduced

c) preceded

d) created

3. After the cloud-seeding operations of the late 1940s and the 1950s, the farmers and ranchers mentioned in the first paragraph probably felt

a) triumphant

b) modest

c) disappointed

d) optimistic

4. Which of the following can be inferred from the text about the term weather modification?

a) It is not as old as the term cloud seeding.

b) It has been in use since at least 1946.

c) It refers to only one type of cloud seeding.

d) It was first used by Vincent Schaefer.

5. According to the passage, winter orographic clouds are formed

a) on relatively warm winter days

b) over large bodies of water

c) during intense snow storms

d) when air currents rise over mountains

6. To which of the following does the underlined word they (given in the text in Italics) refer?

a) water droplets

b) clouds

c) ice-forming nuclei

d) airplanes

7. When clouds are seeded from the ground, what actually causes ice crystals to form?

a) propane

b) silver-iodide smoke

c) dry-ice pellets

d) nuclear radiation

8. Clouds would most likely be seeded from airplanes when

a) it is important to save money.

b) the process of spontaneous nucleation cannot be employed.

c) the production of precipitation must be efficient.

d) temperatures are lower than usual.

9. About what percentage of the western United States’ water supply comes from run-off from melted snow?

a) 10 percent

b) 20 percent

c) 75 percent

d) 85 percent

10. What does the author imply about the energy problems of the 1970s?

a) They were caused by a lack of water.

b) They took attention away from water problems.

c) They may not be as critical as water problems will be in the future.

d) They were thought to be minor at the time but turned out to be serious.

11. The author mentions agricultural cooperatives as an example of

a) state government agencies

b) private interests

c) organizations that compete with ski areas for water

d) municipal water districts

12. It can be inferred from the passage that the weather-modification project of 1976-77 was

a) put together quickly

b) a complete failure

c) not necessary

d)easy to evaluate

Climatic Change

Weather we expect to vary, both from day to day and from season to season. Nor are we surprised when one year has a colder winter or a drier summer than the one before. Less familiar are changes in climate. Even though climate represents averages in weather conditions over periods of, say, 20 or 30 years, there is abundant evidence that it, too, is not constant but instead undergoes quite marked fluctuations over long spans of time. The most dramatic such fluctuations were the ice ages of the past.

The last ice age reached its peak about 20,000 years ago when huge ice sheets hundreds of meters thick in places covered much of Europe and North America. Then the ice began to retreat and climates became progressively less severe; in a period of 12,000 years the average annual temperature of central Europe rose from -4°C to +9°C (24°F to 48°F). By about 6,000 years ago average temperatures were a few degrees higher than those of today. A time of declining temperatures then set in, reaching a minimum in Europe between 2,500 and 3,000 years ago.

A gradual warming up followed that came to a peak between 1,200 and 800 years ago; so generally fine were climatic conditions then that the Vikings established flourishing colonies in Iceland and Greenland from which they went on to visit North America. The subsequent deterioration led to cool summers, exceptionally cold winters, and extensive freezing of the Arctic Sea from 700 to 300 years ago. So extreme was the weather about 350 years ago that it has been called the "Little Ice Age." Greenland became a much less attractive place than formerly and the colony there disappeared, the coast of Iceland was surrounded by ice for several months per year (in contrast to a few weeks per year today), and glaciers advanced farther across alpine landscapes than ever before or since in recorded history.

During the last century a trend toward higher temperatures became evident which has led to a marked shrinkage of the world's glaciers. In the first half of this century especially pronounced temperature increases took place whose most noticeable consequences were milder winters in the higher latitudes. In Spitzbergen, for instance, January temperatures averaged from 1920 to 1940 were nearly 8°C (14°F) higher than those averaged from 1900 to 1920, and Greenland became less inhospitable than before. \ Alas, these balmy conditions seem to have peaked about 1945, and since then the worldwide average annual temperature has been falling steadily (Fig. 5.13). The total drop in the past 30 years has been less than 0.5°C, which does not seem like very much, but the effects have been dramatic. What has happened has been a shift toward the equator of the various wind and climatic zones. In the Northern Hemisphere this shift has had a variety of effects. Siberia is growing colder as the polar front moves south. The northern rim of Africa, formerly in the dry zone of the horse latitudes, now receives unaccustomed rain as the cyclonic weather systems of the westerlies sometimes sweep over it. The horse latitudes have moved farther south, depriving vast areas of sub-Sahara Africa, the Middle East, India, and southern Asia of the moist tropical air that formerly brought them abundant rain. Famines have been the result. In North America, the pattern of air flow has changed so as to bring colder winters and more precipitation to western states.