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Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloud cover." Weather." Merriam-Webster Dictionary. Retrieved on 27 June 2008. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the troposphere, just below the stratosphere. Weather refers to day-to-day temperature, precipitation, and other atmospheric conditions, whereas climate is the term for the averaging of atmospheric conditions over longer periods of time. When used without qualification, "weather" is generally understood to mean the weather of Earth.
Weather is driven by air pressure, temperature, and moisture differences between one place and another. These differences can occur due to the Sun's angle at any particular spot, which varies with latitude. The strong temperature contrast between polar and tropical air gives rise to the largest scale atmospheric circulations: the Hadley cell, the Ferrel cell, the polar cells, and the jet stream. Weather systems in the middle latitudes, such as extratropical cyclones, are caused by instabilities of the jet streamflow. Because Earth's axial tilt relative to its orbital plane (called the ecliptic), sunlight is incident at different angles at different times of the year. On Earth's surface, temperatures usually range ±40 °C (−40 °F to 104 °F) annually. Over thousands of years, changes in Earth's orbit can affect the amount and distribution of solar energy received by Earth, thus influencing long-term climate and global climate change.
Surface temperature differences in turn cause pressure differences. Higher altitudes are cooler than lower altitudes, as most atmospheric heating is due to contact with the Earth's surface while radiative losses to space are mostly constant. Weather forecasting is the application of science and technology to predict the state of the atmosphere for a future time and a given location. Earth's weather system is a chaos theory; as a result, small changes to one part of the system can grow to have large effects on the system as a whole. Human attempts to weather control have occurred throughout history, and there is evidence that human activities such as agriculture and industry have modified weather patterns.
Studying how the weather works on other planets has been helpful in understanding how weather works on Earth. A famous landmark in the Solar System, Jupiter's Great Red Spot, is an anticyclonic storm known to have existed for at least 300 years. However, the weather is not limited to planetary bodies. A stellar corona is constantly being lost to space, creating what is essentially a very thin atmosphere throughout the Solar System. The movement of mass ejected from the Sun is known as the solar wind.
Weather occurs primarily due to air pressure, temperature and moisture differences from one place to another. These differences can occur due to the sun angle at any particular spot, which varies by latitude in the tropics. In other words, the farther from the tropics one lies, the lower the sun angle is, which causes those locations to be cooler due to the spread of the sunlight over a greater surface.NASA. World Book at NASA: Weather. Archived copy at WebCite (10 March 2013). Retrieved on 27 June 2008. The strong temperature contrast between Polar region and tropical air gives rise to the large scale atmospheric circulation cells and the jet stream.John P. Stimac. [4] Air pressure and wind. Retrieved on 8 May 2008. Weather systems in the mid-latitudes, such as extratropical cyclones, are caused by instabilities of the jet stream flow (see baroclinity).Carlyle H. Wash, Stacey H. Heikkinen, Chi-Sann Liou, and Wendell A. Nuss. A Rapid Cyclogenesis Event during GALE IOP 9. Retrieved on 28 June 2008. Weather systems in the tropics, such as 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).Windows to the Universe. Earth's Tilt Is the Reason for the Seasons! Retrieved on 28 June 2008. 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).Milankovitch, Milutin. Canon of Insolation and the Ice Age Problem. Zavod za Udz̆benike i Nastavna Sredstva: Belgrade, 1941. .
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.Ron W. Przybylinski. The Concept of Frontogenesis and its Application to Winter Weather Forecasting. Retrieved on 28 June 2008. Higher altitudes are typically cooler than lower altitudes, which is the result of higher surface temperature and radiational heating, which produces the adiabatic lapse rate. In some situations, the temperature actually increases with height. This phenomenon is known as an inversion and can cause mountaintops to be warmer than the valleys below. Inversions can lead to the formation of fog and often act as a cap that suppresses thunderstorm development. On local scales, temperature differences can occur because different surfaces (such as oceans, forests, ice sheets, or human-made objects) have differing physical characteristics such as albedo, roughness, or moisture content.
Surface temperature differences in turn cause pressure differences. A hot surface warms the air above it causing it to expand and lower the density and the resulting surface air pressure.Michel Moncuquet. Relation between density and temperature. Retrieved on 28 June 2008. The resulting horizontal pressure gradient moves the air from higher to lower pressure regions, creating a wind, and the Earth's rotation then causes deflection of this airflow due to the Coriolis effect.Encyclopedia of Earth. Wind. Retrieved on 28 June 2008. The simple systems thus formed can then display Emergence to produce more Pressure system and thus other weather phenomena. Large scale examples include the Hadley cell while a smaller scale example would be sea breeze.
The atmosphere is a chaos theory. As a result, small changes to one part of the system can accumulate and magnify to cause large effects on the system as a whole.Spencer Weart. The Discovery of Global Warming. Retrieved on 28 June 2008. This atmospheric instability makes weather forecasting less predictable than tidal waves or eclipses. Although it is difficult to accurately predict weather more than a few days in advance, weather forecasters are continually working to extend this limit through meteorology research and refining current methodologies in weather prediction. However, 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.
In the United States, the National Weather Service has an annual report for fatalities, injury, and total damage costs which include crop and property. They gather this data via National Weather Service offices located throughout the 50 states in the United States as well as Puerto Rico, Guam, and the Virgin Islands. As of 2019, tornadoes have had the greatest impact on humans with 42 fatalities while costing crop and property damage over 3 billion dollars.United States. National Weather Service. Office of Climate, Water, Weather Services, & National Climatic Data Center. (2000). Weather Related Fatality and Injury Statistics.
The Little Ice Age caused crop failures and in Europe. During the period known as the Grindelwald Fluctuation (1560–1630), volcanic forcing eventsJason Wolfe, Volcanoes and Climate Change , NASA, 28 July 2020). Date retrieved: 28 May 2021. seem to have led to more extreme weather events. These included droughts, storms and unseasonal blizzards, as well as causing the Swiss Grindelwald Glacier to expand. The 1690s saw the worst famine in France since the Middle Ages. Finland suffered a severe famine in 1696–1697, during which about one-third of the Finnish population died." Famine in Scotland: The 'Ill Years' of the 1690s". Karen J. Cullen (2010). Edinburgh University Press. p. 21.
Once an all-human endeavor based mainly upon changes in barometric pressure, current weather conditions, and sky condition,Weather Doctor. Applying The Barometer To Weather Watching. Retrieved on 25 May 2008.Mark Moore. Field Forecasting: A Short Summary. Retrieved on 25 May 2008. forecast models are now used to determine future conditions. On the other hand, human input is still required to pick the best possible forecast model to base the forecast upon, which involves many disciplines such as pattern recognition skills, , knowledge of model performance, and knowledge of model biases.
The chaos theory nature of the atmosphere, the massive computational power required to solve the equations that describe the atmosphere, the error involved in measuring the initial conditions, and an incomplete understanding of atmospheric processes mean that forecasts become less accurate as of 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.Klaus Weickmann, Jeff Whitaker, Andres Roubicek and Catherine Smith. The Use of Ensemble Forecasts to Produce Improved Medium Range (3–15 days) Weather Forecasts. Retrieved on 16 February 2007.Todd Kimberlain. Tropical cyclone motion and intensity talk (June 2007). Retrieved on 21 July 2007.Richard J. Pasch, Mike Fiorino, and Chris Landsea. TPC/NHC’S Review of the NCEP Production Suite For 2006. Retrieved on 5 May 2008.
There are a variety of end users to weather forecasts. Weather warnings are important forecasts because they are used to protect life and property.National Weather Service. National Weather Service Mission Statement. Retrieved on 25 May 2008. Forecasts based on temperature and precipitation are important to agriculture,Blair Fannin. Dry weather conditions continue for Texas. Retrieved on 26 May 2008.Dr. Terry Mader. Drought Corn Silage. Retrieved on 26 May 2008.Kathryn C. Taylor. Peach Orchard Establishment and Young Tree Care. Retrieved on 26 May 2008.Associated Press. After Freeze, Counting Losses to Orange Crop. Retrieved on 26 May 2008. and therefore to commodity traders within stock markets. Temperature forecasts are used by utility companies to estimate demand over coming days.The New York Times. Futures/Options; Cold Weather Brings Surge In Prices of Heating Fuels. Retrieved on 25 May 2008.BBC. Heatwave causes electricity surge. Retrieved on 25 May 2008.Toronto Catholic Schools. The Seven Key Messages of the Energy Drill Program. Retrieved on 25 May 2008.
In some areas, 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 to survive through them.
Tropical weather forecasting is different from that at higher latitudes. The sun shines more directly on the tropics than on higher latitudes (at least on average over a year), which makes the tropics warm (Stevens 2011). And, the vertical direction (up, as one stands on the Earth's surface) is perpendicular to the Earth's axis of rotation at the equator, while the axis of rotation and the vertical are the same at the pole; this causes the Earth's rotation to influence the atmospheric circulation more strongly at high latitudes than low latitudes. Because of these two factors, clouds and rainstorms in the tropics can occur more spontaneously compared to those at higher latitudes, where they are more tightly controlled by larger-scale forces in the atmosphere. Because of these differences, clouds and rain are more difficult to forecast in the tropics than at higher latitudes. On the other hand, the temperature is easily forecast in the tropics, because it does not change much.
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:
The effects of inadvertent weather modification may pose serious threats to many aspects of civilization, including , , food and fiber production, economic development, and human health.
The coldest average annual temperature in a permanently inhabited location is at Eureka, Nunavut, in Canada, where the annual average temperature is .
The windiest place ever recorded is in Antarctica, Commonwealth Bay (George V Coast). Here the gales reach . Furthermore, the greatest in a period of twelve occurred in Mount Rainier, Washington, US. It was recorded as of snow.
One of the most famous landmarks in the Solar System, Jupiter's Great Red Spot, is an Anticyclone storm known to have existed for at least 300 years. On other , the lack of a surface allows the wind to reach enormous speeds: gusts of up to 600 metres per second (about ) have been measured on the planet Neptune. This has created a puzzle for planetary scientists. The weather is ultimately created by solar energy and the amount of energy received by Neptune is only about of that received by Earth, yet the intensity of weather phenomena on Neptune is far greater than on Earth. , the strongest planetary winds discovered are on the extrasolar planet HD 189733 b, which is thought to have easterly winds moving at more than .
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