Although the poetic view of snow is one of a uniform white blanket softly covering the ground, snow is, in fact, far from homogeneous. Initially snow crystals precipitate in a wide variety of shapes, depending on the atmospheric conditions where they form and on the temperature and wind speed near the ground where they are deposited. Patterns of snow deposition and topography also contribute to variability in snow accumulation. Once deposited, snow crystals bond together to form a new material - the snow cover. With each snowfall the snow cover is refreshed with a new layer whose properties may be quite different from the older snow beneath it. As the snow cover ages, its physical properties continue to evolve in response to weather conditions and to thermodynamic stresses within the ice-water-vapor system. These changes alter physical and chemical processes within the pack, which then affect the climate through complicated feedback mechanisms. For example, modifications in air temperature and radiation at the surface change temperature gradients within the snow that drive grain growth and metamorphism. Larger grain sizes, in turn, decrease the snow albedo and cause more heat to be retained by the earth's surface, which, on a larger scale, can increase atmospheric moisture and produce heavier snowfalls. Ablation of the polar snow cover in summer exposes bare ice and leads to more absorption of incident sunlight and further melting. Post-depositional metamorphism also changes the nature of the interstitial air space, thereby altering the permeability of snow to flows of air and water. Increased ventilation of the snow cover can lead to increased sublimation and crystal change. Increased water flow accelerates crystal growth, which in turn increases the permeability and accelerates snow-cover runoff. Snow is an extremely effective insulator, trapping heat in the ground and slowing sea-ice growth in winter. But as

Snow and Climate: Physical Processes, Surface Energy Exchange and Modeling, ed. Richard L. Armstrong and Eric Brun. Published by Cambridge University Press. © Cambridge University Press 2008.

snow compacts, its thermal conductivity increases, which leads to cooling of the ground and to warming of the atmosphere.

Snow is, therefore, a dynamic, complicated medium, and its microstructure plays a key role in the behavior of snow on many scales. The large-scale behavior or appearance of snow is often due to its small-scale properties. Avalanches, for example, can be launched because of thin weak hoar layers, and remotely sensed signals from satellites orbiting the planet are sensitive to snow crystal type and size.

This chapter focuses on snow physics. Our purpose is to provide a unified introduction to the subject, which addresses both recent issues and the results of past investigations. It is fundamental to the physics of snow that all three phases of water may coexist in relationships that are strictly governed by laws of thermal and mechanical equilibrium. While much of our understanding of snow-cover processes derives from observation, it is also important to examine the microscale and theoretical background for these behaviors. We begin with a description of the origin and characteristics of deposited snow, followed by a discussion of snow classification and metamorphism, which engages the theory of phase equilibria. The next sections on thermal and fluid flow discuss heat and mass transfer within the snowpack and its interaction with the evolving snow characteristics. They include a relatively new presentation of the role of air flow in thermal and vapor transport and of capillary forces and unstable wetting in water flow. We conclude with a section on radiative characteristics, which includes a detailed discussion of snow albedo -a key parameter controlling surface heat exchange.

Snow investigators now have the benefit of detailed computer models, which serve as tools to synthesize and examine current parameterizations of snow properties and processes. Comparisons of computer simulations with the measurement of bulk properties (snow depth and mass), in-snow profiles (snow density, temperature, grain size, and liquid water content), and boundary fluxes (surface exchange and basal outflow) validate the models and highlight areas of snow physics that require further study. A shortcoming of these point models is that they do not treat two- and three-dimensional processes. While we discuss important multi-dimensional processes, such as windpumping, large-scale phenomena are, in general, not addressed by this chapter.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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