focuses on observations for accurate numerical weather analysis and prediction, timely watches and warnings in advance of hazardous weather, and the special requirements of climate monitoring.

The charge to this committee was to (1) focus on time scales less than 48 hours, but keep longer time scales in mind; (2) focus on U.S. and adjacent coastal regions, but keep global observing system requirements in mind; (3) focus on ground-based in situ and remote sensing observations, but keep the utility of satellite observations in mind; (4) focus on the atmospheric boundary layer, but keep the deep troposphere in mind. Within this context, the hazardous weather events most important to detect, monitor, and predict are

  • flooding from a large-scale storm

  • Nor’easters

  • snowstorms and ice storms

    (For the above three items, precipitation type, intensity, and amount [in the case of snow and ice, liquid equivalent and accumulation on the ground] are all important.)

  • hurricanes and tropical storms

  • air pollution1

  • thunderstorms, including mesoscale convective systems

    • lightning

    • flash floods

    • hail

    • straight-line damaging winds (resulting from squall lines or bow echoes)

    • tornadoes

  • windstorms without precipitation

    • downslope windstorms

    • pressure-gradient windstorms

  • fire weather

  • aviation hazards

    • in-cloud icing

    • downbursts

    • aircraft turbulence

The order in the above list is roughly by size and longevity. The time and space scales associated with these phenomena are depicted in Figure 2.1. All


This report covers the release of toxic substances, accidentally or deliberately. This topic is closest to “air pollution,” but, since it is not a natural phenomenon, it is not treated in Appendix A, nor is it mentioned in Table 2.1. The spatial and temporal scales for toxic releases (0.2 to 2.0 km and 15 min to hrs, respectively) are generally smaller than those for air pollution.

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