Weather substantially influences our day-by-day life. The most simple explanation of weather: "It is all that happens outside". Weather includes the changes of hot and cold or wet and dry. It may be calm or stormy, and clear or cloudy. Weather is a variety of events that occur in the atmosphere all over the world. Broadly speaking, weather is the place-by-place and the day-by-day and/or season-by-season feeling of the state of the atmosphere, when the events take place on relatively small scales both spatially and temporally. Meteorologists record the weather every day. Continuous recording of various weather indices helps to determine the climate of an area. In the every day life people grasp mainly the short-term weather fluctuations, while human perception of climatic changes is rather limited. Climate is a powerful tool for dealing with the weather and represents the average of weather on some spatial scale over a long period of time. It puts somewhat wild, unpredictable everyday weather into long-term perspective. If one knows the climate of some region, he possesses information about what the weather may occur today, a week later, or next year. Climate is the time integral of weather over a period of decades or longer. It accumulates the totality of weather events, thus, may be broadly defined as the long-term behavior of the environmental system.
The fact is that Earth's climate is perpetually changing. Widespread climatologic investigations have shown that climate varies on all timescales from decades to millions of years. The past changes have ranged from slow and gradual to fast and even abrupt. An impact of climate fluctuations on the mankind is extremely significant, sometimes dramatic. Even in relatively recent fifteenth to eighteenth centuries the decrease of the mean annual surface air temperature (SAT) by one or two degrees produced the so-called "Little Ice Age" that had strongly altered many social and economic sectors in Europe. That time the populations died from the crop failure and famine. According to Lamb (1969) "... climatic history must be central to our understanding of human history ..."
Climate can be usually described in terms of normals,1 means and extremes of a variety of weather elements. SAT is typically of interest in any discussions of the large-scale climate variability. It is also a major driving factor in various investigations of the climate impact on many natural and managed systems as well as determining variable for different climatic models. Climate may vary on a large range of temporal and/or spatial scales. Spatial scales may be local (<105km2), regional, continental (10-100 million km2), or global. Temporal scales may vary from relatively short duration (annual/decadal) to long scales comparable with the characteristic times of the geological processes (hundreds of millions of years).
On the longer timescale the Earth's climate roughly represents the alternating of ice ages and interglacials, when the former are characterized by major extension of the polar ice sheets and growth of the mountain glaciers. Figure 1 shows the existing temperature deviations from the long-term temperature average over approximately the past half million years. The temperature anomalies were deduced from the measurements of the isotopic fractionation of oxygen in the ice core from Vostok, the Russian base in Antarctica (Petit et al., 1999; see also the web site of the National Climatic Data Center
'Climate normals or averages are used to summarize or describe the average climate conditions of a particular location.
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