Discuss several ways that humans attempt to modify the weather.

People modify the weather deliberately as well as unintentionally. Efforts to enhance precipitation at ski resorts and to dissipate fog at some airports illustrate planned intervention. One example of inadvertent weather modification is the increase in cloudiness from condensation trails produce by jet aircraft.

Planned Weather Modification

Planned weather modification is deliberate human intervention to influence atmospheric processes that constitute the weather—that is, to alter the weather for human purposes. The desire to change or enhance certain weather phenomena dates back to ancient history, when people used prayer, wizardry, dances, and even black magic in attempts to alter the weather.

Snow and Rain Making

The first breakthrough in weather modification came in 1946, when Vincent J. Schaefer discovered that dry ice, dropped into a supercooled cloud, spurred the growth of ice crystals. Once ice crystals form in a cloud of supercooled droplets, they grow larger (at the expense of the remaining liquid cloud droplets) and, upon reaching a sufficient size, fall as precipitation.

Scientists later learned that silver iodide crystals could also be used for cloud seeding. Unlike dry ice, which simply chills the air, silver iodide crystals act as freezing nuclei for the supercooled droplets—that is, liquid droplets with temperatures below 0°C (32°F). Because silver iodide can be easily delivered to clouds from burners on the ground or from aircraft, it is a more cost-effective alternative than dry ice (Figure 5.28).

Seeding of winter clouds that form along mountain barriers (orographic clouds) has been attempted on numerous occasions. Since 1977, Colorado’s Vail and Beaver Creek ski areas have used this method to increase winter snows. An additional benefit of cloud seeding is that the increased precipitation, which melts and runs off during spring and summer months, can be collected in reservoirs for irrigation and hydroelectric power generation.

In recent years, the seeding of warm convective clouds with hygroscopic (water-seeking) particles has received renewed attention. The interest in this technique arose when it was discovered that a pollution-belching paper mill near Nelspruit, South Africa, seemed to be triggering precipitation. Research aircraft flying through clouds near the paper mill collected samples of the particulate matter emitted from the mill. It turned out that the mill was emitting tiny salt crystals (potassium chloride and sodium chloride), which rose into the clouds. Because these salts attract moisture, they quickly form large cloud droplets, which grow into raindrops through the collision–coalescence process. Thus, seeding of warm clouds using hygroscopic particles seems to show promise for accelerating the precipitation process.

Researchers estimate a 10 percent increase in snowfall from clouds seeded with silver iodide compared to those left unseeded. Because cloud seeding has shown some promising results and is relatively inexpensive, it has been a primary focus of modern weather-modification technology.

Fog and Stratus Cloud Dispersal

One of the most successful applications of cloud seeding involves spreading dry ice (solid carbon dioxide) into layers of supercooled fog or stratus clouds to disperse them and thereby improve visibility. Airports, harbors, and foggy stretches of interstate highway are obvious candidates. Such applications trigger a transformation in cloud composition from supercooled water droplets to ice crystals. The ice crystals then settle out, leaving an opening in the cloud or fog. Commercial airlines have used this method at selected foggy airports in the western United States.

Unfortunately, most fog does not consist of supercooled water droplets. The more common “warm fogs” are more expensive to combat because seeding will not diminish them. Successful attempts at dispersing warm fogs have involved mixing drier air from above into the fog. When the layer of fog is very shallow, helicopters have been used. By flying just above the fog, the helicopter creates a strong downdraft that forces drier air toward the surface, where it mixes with the saturated foggy air.

Hail Suppression

Each year, hailstorms inflict on average $500 million in property damage and crop loss in the United States (Figure 5.29). Occasionally a single severe hailstorm can produce damages that exceed that amount. As a result, some of history’s most interesting efforts at weather modification have focused on hail suppression.

Farmers desperate to find ways to save their crops have long believed that strong noises—explosions, cannon shots, or ringing church bells—can help reduce the amount of hail produced during a thunderstorm. In Europe, it was once common practice for village priests to ring church bells to shield nearby farms from hail.

Modern attempts at hail suppression use various methods of cloud seeding using silver iodide crystals to disrupt the growth of hailstones. In an effort to verify the effectiveness of cloud seeding as a means to suppress hail, the U.S. government established the National Hail Research Experiment in northeastern Colorado. This effort included several randomized cloud-seeding experiments. An analysis of the data collected after 3 years revealed no statistically significant difference in the occurrence of hail between the seeded and unseeded clouds, so the planned 5-year experiment was abandoned. Nevertheless, cloud seeding is still employed today in an attempt to prevent hail, and research on hail suppression continues.

Frost Prevention

A frost, or freeze hazard, is a strictly temperature-dependent phenomenon that occurs when the air temperature falls to 0°C (32°F) or below, killing flowers and produce. The word frost is commonly used for ice crystals that form on surfaces near the ground during the night. According to the World Meteorological Organization, the correct term for the deposits of ice crystals are hoar frost or white frost, which form only when air becomes saturated at subfreezing temperatures.

A frost, or freeze hazard, can be generated in two ways: when a cold air mass moves into a region or when sufficient radiation cooling occurs on a clear night. Frost associated with an invasion of cold air, which can produce widespread crop damage, is characterized by low daytime temperatures and long periods of freezing conditions. By contrast, frost induced by radiation cooling is strictly a nighttime phenomenon that tends to be confined to low-lying areas. Obviously, the latter phenomenon is much easier to combat.

Several methods of frost prevention are being used with varying success. They either conserve heat (reduce heat loss at night) or add heat to warm the lowermost layer of air.

Heat-conservation methods include covering plants with insulating material, such as paper or cloth, and generating particles that, when suspended in air, reduce the rate of radiation cooling. Warming methods employ water sprinklers, air-mixing techniques, and/or orchard heaters. Water sprinklers (Figure 5.30) add heat in two ways: first, from the warmth of the water and, more importantly, from the latent heat of fusion released when the water freezes. As long as an ice–water mixture remains on the plants, the latent heat released will keep the temperature from dropping below 0°C (32°F).

Air mixing works best when the air temperature 15 meters (50 feet) above the ground is at least 5°C (9°F) warmer than the surface temperature. Through use of a wind machine, the warmer air aloft is mixed with the colder surface air (Figure 5.31).

Orchard heaters probably produce the most successful results. However, because as many as 30 to 40 heaters are required per acre, fuel costs and pollution emissions are significant.

Inadvertent Weather Modification

The effects of inadvertent, or unintentional, weather modification are numerous, and scientists are beginning to understand them better. Unintentional weather modifications affect cloud formation and precipitation patterns. Changes in air quality, visibility, and the formation of acid rain caused by human activity are covered in Chapter 13, while human impacts on global climate are a major topic in Chapter 14. In addition, human effects on city temperatures, known as the “urban heat island,” are discussed in Chapter 3.

Effects of Large Transportation Corridors

You have undoubtedly seen contrails (from condensation trail) in the wake of an aircraft flying on a clear day. Contrails, which are simply human-made clouds, form because jet aircraft engines expel large quantities of hot, moist air. Upon mixing with the frigid air aloft, the moist air cools sufficiently to reach saturation.

Contrails typically form above 9 kilometers (6 miles), where air temperatures are a frigid −50°C (−58°F) or colder. Thus, it is not surprising that contrails are composed of minute ice crystals. Most contrails have a very short life span. Once formed, these streamlined clouds mix with surrounding cold, dry air and ultimately sublimate. However, if the air aloft is near saturation, contrails may survive for long periods. Under these conditions, the upper airflow usually spreads these narrow clouds into broad bands of cirrus or cirrostratus clouds. Similar increases in cloud cover have also been detected recently along major air transport routes.

With the increase in air traffic over the past few decades, an overall increase in cloudiness has been recorded, particularly near major transportation hubs (Figure 5.32). It appears these human-made clouds reduce the amount of incoming solar radiation reaching the ground and can lead to lower daytime maximum temperatures. Because cloud cover also reduces the amount of radiation cooling that occurs at night, additional research is needed to accurately determine the impact of contrails on climate change.

Urban Effects

Major population centers enhance the formation of warm clouds and increase precipitation in these urban areas by as much as 20 percent. This effect does not appear to alter winter precipitation patterns.

Increased industrialization associated with urbanization also influences weather conditions downwind of major cities. The added particulates and aerosols from industry and transportation sectors reduce air quality and visibility and also contribute to increased cloudiness and precipitation. In addition, large cooling lakes adjacent to industrial facilities—used to dispose of hot water that was heated during manufacturing—have been shown to cause localized fog, low clouds, and even icing in the winter.

• cloud seeding: The introduction into clouds of particles (most commonly dry ice or silver iodide) for the purpose of altering the cloud’s natural development.

• frost hazard (freeze hazard): Weather conditions in which ice crystals form when the temperature falls to 0°C or below, killing flowers and produce. Also called a freeze hazard.

• Planned weather modification is deliberate human intervention to influence atmospheric processes that constitute the weather.

• The effects of inadvertent or unintentional weather modification include increased cloudiness, which may lower surface temperatures, and an increase in precipitation.