The Telltale Cloud

Deep beneath the earth's surface, a muffled roar and waves of vibrations signal the release of a radioactive cloud into the air. Using atmospheric modeling, scientists know not only exactly where the cloud is, but also where it is going.

The ability to perform such sophisticated atmospheric modeling has made Environment Canada's Canadian Meteorological Centre in Montréal a key player in the verification of the Comprehensive Nuclear Test Ban Treaty--a landmark agreement that commits countries around the world to refrain from conducting nuclear weapon explosion tests. The Treaty is due to come into effect in the early 2000s, and although it has already been signed by all declared nuclear-weapon states, these verification procedures are being put in place to deter any clandestine testing. Natural Resources Canada and Health Canada also play an important role in the verification process through the operation of seismic, hydroacoustic and radionuclide monitoring stations.

Computer modeling of the atmosphere forms the basis of Environment Canada's weather forecasts, and is also used to predict the transport and dispersion of airborne materials. Developed in the wake of the 1986 nuclear accident at Chernobyl, this advanced technology has many and varied applications including the support of safe aircraft operations, such as forecasts of volcanic ash clouds travelling around the earth following an eruption.

In the case of a nuclear explosion, computer models can estimate the time it takes for radioactive material to move from an explosion site through the atmosphere to different regions of the globe. The path of the nuclear cloud can then be mapped based on the speed of atmospheric motion. Depending on the location of the explosion and the particular atmospheric patterns, it can take up to 10 days before radioactive material is detected by ground-based monitoring systems--although the search is made easier by the unique atmospheric "signature" of the cloud.

Although the fact that radioactivity is not always vented into the atmosphere can make nuclear test verification more difficult, the seismic activity caused by a nuclear explosion is very different from that of an earthquake or other natural rumble. Environment Canada's Atmospheric Transport Model is valuable because it can be used—in conjunction with monitoring of seismic activity and the presence of radionuclides in the air—in three verification-related scenarios, including one where an event is only suspected and no cloud has been emitted.

This application, known as backward modeling, uses data on non-conventional radioactive materials collected by radionuclide samplers to track the origin of a nuclear cloud based on possible explosion sites. In forward modeling, it predicts the direction and flow of radionuclides from a suspected event and the type and location of sampling equipment that could be used to detect them. Where both the time and location of an event are known, the model predicts where the nuclear cloud will travel and helps to identify the explosive yield of the nuclear device.

The use of atmospheric modeling to detect and track radioactive material is just one example of how meteorological technology is being expanded for use in a variety of fields. This application not only assists humankind in its pursuit of peace, but also increases preparedness in the case of nuclear-reactor accidents and other large-scale toxic releases, and promotes the free and open exchange of meteorological data and products within the international community.

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