Weather Bug

Friday, December 24, 2010

UPDATED 9 AM EST, December 24, 2010

Posted by ILIAS at 7:39 AM

Christmas Eve Outlook By WeatherBug Sr. Meteorologist, James West
	Click to Enlarge Related Content Current Radar Current Temperatures Highs Today United States Alert Map Your Photos Last-minute shoppers and travelers beginning the Christmas holiday weekend will not have to deal with many weather issues across much of the U.S. A few minor delays are possible in the U.S.`s midsection. WeatherBug Meteorologist Gretchen Mishek has the latest in her exclusive WeatherBug National Outlook. A winter storm taking shape over the Midwest and Mississippi Valley will add a bit of trouble today. Light to moderate snow will expand out of the Upper Midwest and northern Plains into the central Mississippi Valley, making roads a bit slick from Arkansas to the Dakotas. A nice frosting of 1 to 2 inches of snow should put places like Kansas City, St. Louis, and Chicago in the Christmas spirit. Across the southern Plains from south of Dallas into eastern Oklahoma and central Arkansas, roads will be wet and umbrellas will be the accessory of choice for last-minute shoppers as a steady rain falls later in the day. Northern Arkansas and southern Missouri will be the transition zone between the snow and rain, so a wintry mix of sleet and freezing rain could make things slick. The next Pacific storm arriving in the Pacific Northwest will produce rain from the northern California coast all of the way to Seattle. The highest elevations of the Cascades will see fresh snow, just in time for Santa`s arrival late tonight. The rest of the U.S. will be quiet and seasonably cold. Plenty of sunshine will lighten the mood from southern California to the Plains and up and down the East Coast from Florida into New England. Temperatures will be coldest in the northern Plains, with teens and 20s for highs common. Southern Texas and Florida will be the warmest spots as the thermometer peaks in the 70s. Everywhere else will see seasonable temperatures with the Northeast, Great Lakes and Central Plains seeing 20s and 30s. The South, Southeast, Gulf Coast, Southwest and California will be in the 50s and 60s. Be sure to keep WeatherBug active to receive the latest weather in your neighborhood and get the latest updates anywhere on Twitter at WeatherBug WeatherBuzz. What do you think of this story? Click here for comments or suggestions.

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Cause

On Earth, common weather phenomena include wind, cloud, rain, snow, fog and dust storms. Less common events include natural disasters such as tornadoes, hurricanes, typhoons and ice storms. Almost all familiar weather phenomena occur in the troposphere (the lower part of the atmosphere).[3] Weather does occur in the stratosphere and can affect weather lower down in the troposphere, but the exact mechanisms are poorly understood.[5]

Weather occurs primarily due to density (temperature and moisture) differences between one place to another. These differences can occur due to the sun angle at any particular spot, which varies by latitude from the tropics. In other words, the farther from the tropics you lie, the lower the sun angle is, which causes those locations to be cooler due to the indirect sunlight.[6] The strong temperature contrast between polar and tropical air gives rise to the jet stream.[7] Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow (see baroclinity).[8] Weather systems in the tropics, such as monsoons or organized thunderstorm systems, are caused by different processes.

Because the Earth's axis is tilted relative to its orbital plane, sunlight is incident at different angles at different times of the year. In June the Northern Hemisphere is tilted towards the sun, so at any given Northern Hemisphere latitude sunlight falls more directly on that spot than in December (see Effect of sun angle on climate).[9] This effect causes seasons. Over thousands to hundreds of thousands of years, changes in Earth's orbital parameters affect the amount and distribution of solar energy received by the Earth and influence long-term climate. (see Milankovitch cycles).[10]

The uneven solar heating (the formation of zones of temperature and moisture gradients, or frontogenesis) can also be due to the weather itself in the form of cloudiness and precipitation.[11] Higher altitudes are cooler than lower altitudes, which is explained by the lapse rate.[12][13] On local scales, temperature differences can occur because different surfaces (such as oceans, forests, ice sheets, or man-made objects) have differing physical characteristics such as reflectivity, roughness, or moisture content.

Surface temperature differences in turn cause pressure differences. A hot surface heats the air above it and the air expands,lowering the air pressure and its density.[14] The resulting horizontal pressure gradient accelerates the air from high to low pressure, creating wind, and Earth's rotation then causes curvature of the flow via the Coriolis effect.[15] The simple systems thus formed can then display emergent behaviour to produce more complex systems and thus other weather phenomena. Large scale examples include the Hadley cell while a smaller scale example would be coastal breezes.

The atmosphere is a chaotic system, so small changes to one part of the system can grow to have large effects on the system as a whole.[16] This makes it difficult to accurately predict weather more than a few days in advance, though weather forecasters are continually working to extend this limit through the scientific study of weather, meteorology. It is theoretically impossible to make useful day-to-day predictions more than about two weeks ahead, imposing an upper limit to potential for improved prediction skill.[17]

Modification

The aspiration to control the weather is evident throughout human history: from ancient rituals intended to bring rain for crops to the U. S. Military Operation Popeye, an attempt to disrupt supply lines by lengthening the North Vietnamese monsoon. The most successful attempts at influencing weather involve cloud seeding; they include the fog- and low stratus dispersion techniques employed by major airports, techniques used to increase winter precipitation over mountains, and techniques to suppress hail.[41] A recent example of weather control was China's preparation for the 2008 Summer Olympic Games. China shot 1,104 rain dispersal rockets from 21 sites in the city of Beijing in an effort to keep rain away from the opening ceremony of the games on August 8, 2008. Guo Hu, head of the Beijing Municipal Meteorological Bureau (BMB), confirmed the success of the operation with 100 millimeters falling in Baoding City of Hebei Province, to the southwest and Beijing's Fangshan District recording a rainfall of 25 millimeters.[42]

Whereas there is inconclusive evidence for these techniques' efficacy, there is extensive evidence that human activity such as agriculture and industry results in inadvertent weather modification:[41]

* Acid rain, caused by industrial emission of sulfur dioxide and nitrogen oxides into the atmosphere, adversely effects freshwater lakes, vegetation, and structures.
* Anthropogenic pollutants reduce air quality and visibility.
* Climate change caused by human activities that emit greenhouse gases into the air is expected to affect the frequency of extreme weather events such as drought, extreme temperatures, flooding, high winds, and severe storms.[43]

The effects of inadvertent weather modification may pose serious threats to many aspects of civilization, including ecosystems, natural resources, food and fiber production, economic development, and human health.[44

Forecasting

Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Human beings have attempted to predict the weather informally for millennial, and formally since at least the nineteenth century.[25][26] Weather forecasts are made by collecting quantitative data about the current state of the atmosphere and using scientific understanding of atmospheric processes to project how the atmosphere will evolve.[27]

Once an all-human endeavor based mainly upon changes in barometric pressure, current weather conditions, and sky condition,[28][29] forecast models are now used to determine future conditions. Human input is still required to pick the best possible forecast model to base the forecast upon, which involves pattern recognition skills, teleconnections, knowledge of model performance, and knowledge of model biases. The chaotic nature of the atmosphere, the massive computational power required to solve the equations that describe the atmosphere, error involved in measuring the initial conditions, and an incomplete understanding of atmospheric processes mean that forecasts become less accurate as the difference in current time and the time for which the forecast is being made (the range of the forecast) increases. The use of ensembles and model consensus helps to narrow the error and pick the most likely outcome.[30][31][32]

There are a variety of end users to weather forecasts. Weather warnings are important forecasts because they are used to protect life and property.[33] Forecasts based on temperature and precipitation are important to agriculture,[34][35][36][37] and therefore to commodity traders within stock markets. Temperature forecasts are used by utility companies to estimate demand over coming days.[38][39][40] On an everyday basis, people use weather forecasts to determine what to wear on a given day. Since outdoor activities are severely curtailed by heavy rain, snow and the wind chill, forecasts can be used to plan activities around these events, and to plan ahead and survive them.

References

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Weather Bug

Friday, December 24, 2010

UPDATED 9 AM EST, December 24, 2010

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