Cracking Hurricane Secrets

Improved satellite images, advanced computer design, and increasingly sophisticated models all helped atmospheric scientists learn how hurricanes came to be born, live, track across the oceans, and die—either a natural death at sea or a violent one when they went ashore. An early hurricane model (HURRAN) predicted a hurricane's potential track by comparing its current atmospheric situation with those of previous hurricanes. The model assumed that if the existing weather pattern had occurred before, the current hurricane would repeat the previous hurricane's path. HURRAN did not always provide useful information, but it was better than no guidance at all.

Similar hurricane patterns were not always available, so the meteorologist Charles J. Neumann developed the climatology and persistence (CLIPER) model in 1972. A climatological forecast bases its decision on averages, while a persistence model extends a storm's path along its existing route. This model combined the two to derive prediction information from both past and current situations. CLIPER was not perfect, but meteorologists tested new hurricane models against CLIPER. If model forecasts were more accurate than CLIPER, then they were "skilled." If not, they were modified until they became skilled.

Meteorologists also developed theoretical models to gain knowledge of hurricane struc ture. They abandoned 1960s era models because computers were not large enough to allow the hurricane to "interact" with its environment—an absolute necessity for scientists to determine the factors influencing hurricane intensification, dissipation, and movement. John B. Hovermale (1938-94) and Robert E. Livezey's (1920- ) 1976 Movable Fine Mesh (MFM) model had difficulties with short-range forecasts (12-24 hours) because of problems inputting the initial atmospheric state. The longer-range forecasts were more accurate. The MFM eventually transitioned from research to operational model. By 1980, the National Hurricane Center was using seven different models to predict the most likely hurricane track.

Other advances in hurricane understanding resulted from the hurricane modification project STORMFURY. The special aircraft purchased to probe hurricanes for STORMFURY were outfitted with special communications equipment that allowed technicians to send data directly to the National Hurricane Center. The real-time availability of this detailed information greatly assisted meteorologists in the forecast center to make sense of hurricane behavior. Since the late 1970s, hurricane forecasting has continued to improve, allowing ample warning time so people can escape from approaching tropical storm systems.

estimated that between 5 million and 10 million people were in danger of starving. In an area already known for political instability, the mass migration of millions of people looking for water and food to survive would further destabilize the region.

Early in the decade, some atmospheric scientists were convinced that a basic shift in weather patterns was the root cause of the drought and the resulting southward migration of the Sahara. In the more typical pattern, cold polar air receded northward during the spring and summer, allowing warmer—and moister—air masses to fill the void. The summer monsoons would produce large cumulus clouds that provided showery rains to the area. In the autumn, the cooler, dry air masses

Climate change can worsen the effects of desertification, as seen in this photograph taken in central Negev. (Photo by Annette Wefer-Roehl, Pedro Berliner, and Yoav Avni. Initiative for Collaboration to Control Natural Resource Degradation [Desertification] of Arid Lands in the Middle East)

would return and the skies would clear until the next spring and summer. Over the previous two decades, a cooler air pattern had begun to predominate. It appeared that the larger, cooler air mass to the north could no longer recede far enough to allow the monsoon rains into the region. If this were true, there would be no way to stop the desert's advance. The possibility of permanently reduced global grain harvests loomed when the same pattern occurred in South Asia and the U.S. Great Plains.

By 1977, the Sahel drought had ended, but the world's deserts were still growing by 14 million acres per year; 43 percent of Earth's surface was already desert or semidesert. Scientists estimated that if desertification were not slowed, by the end of the century at least one-third of the world's arable land would be lost as the population and demand for food grew. It was time to look at the causes of desertification.

The U.S. meteorologist Jule Charney thought scientists needed to consider the possibility that a biogeophysical feedback mechanism might be at work in desertification. In desert regions, the light-colored, sandy soil reflects back solar radiation, a process that leads to overall cooling of the upper atmosphere. Colder air sinks back to the surface, and as it does, it warms and dries. This sinking air reduces the possibility of precipitation and dries out what little moisture is available. Reduced moisture in turn leads to dying vegetation that generates exposed dry soil, higher albedo, and even less precipitation. This feedback pattern, Charney thought, would lead to recurrent drought in areas bordering deserts, which would slowly creep into neighboring semiarid regions.

Climate change can worsen the effects of desertification, as seen in this photograph taken in central Negev. (Photo by Annette Wefer-Roehl, Pedro Berliner, and Yoav Avni. Initiative for Collaboration to Control Natural Resource Degradation [Desertification] of Arid Lands in the Middle East)

As the soil dried, there would also be less evapotranspiration—the release of moisture into the air from both soil and plants. Charney surmised that only a shift in atmospheric circulations would break the pattern. If no shift occurred, the area's new climate would be drier. Numerical models were not producing conclusive results. Other options needed to be considered.

By the end of the decade, many scientists had abandoned climate-based drought ideas in favor of human-induced desertification concepts. As previously nomadic peoples settled in one place, overgrazing became a serious problem. Once the meager vegetation was gone and the moisture left with it, they were forced to migrate. Others had to migrate too and soon people and animals outstripped water and vegetation. Some areas had been forested, but the cutting of trees for shelter and fuel left the ground susceptible to erosion as the soil dried and the winds picked up the topsoil and blew it away. As the dust was blown into the upper atmosphere, it further blocked the incoming solar radiation, leading to more cooling, sinking air, and drying. Although all of these human impacts had been occurring over thousands of years, it was not until the late 1970s that the populations had exceeded the carrying capacity of the land.

The thought of mass starvation caused by a combination of climate change and human-induced desertification was frightening. As images of small children with swollen bellies standing in sparse landscapes littered with the desiccated bodies of livestock flickered across the world's television screens, people became increasingly concerned that their planet was in trouble.

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