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Wind • Horizontally moving air • Air/wind naturally flows from areas of high pressure to low pressure. • Wind is natures attempt to balance out inequalities in atmosphere Direction • Winds are always labeled by the direction from which they come from – A north wind comes from the north and blows towards the south • Wind vane – Always points into the wind – Prevailing winds are winds blowing from more than one direction – Changes in wind direction tend to mean changes in temperature Speed • Measure speed using a cup anemometer • Read from a computer similar to a speedometer What effects wind? • Three factors: – Pressure-Gradient Force – Coriolis Effect – Friction 1. Pressure-Gradient Force • Differences in pressure cause wind; greater the pressure, the greater the wind speed • Isobars show pressure differences on maps • Spacing of isobars indicates the amount of pressure change occurring over a given distance. Closer the isobars, the greater the pressure gradient. Widely spaced isobars indicate a weak pressure gradient. 2. Coriolis Effect • Air moves out of regions with high pressure into regions of low pressure. • Coriolis effect: winds are deflected due to rotation of the Earth; winds turn right in the N. Hemisphere and to the left in the S. Hemisphere. • Effects direction, not speed • Deflection was made clearly evident in WWII by the US Navy • http://www.classzone.com/books/earth_scien ce/terc/content/visualizations/es1904/es1904 page01.cfm 3. Friction • Friction on wind is only important within a few km of the surface. • Acts to slow air movement by about 20% and thus alter wind directions. • Two types of friction: – Molecular friction: friction between individual air molecules – Friction between two surfaces: air and land • Friction between two surfaces: Air rubbing against Earth’s surface causes friction. It is surface friction that causes the phenomenon of "gusty winds" at the surface. • Upper level winds (jet stream) tend to be stronger than surface winds. Jet Stream • Jet streams are a narrow, variable band of very strong, predominantly westerly air currents encircling the globe several miles above the earth • River-like currents of fast moving air traveling 75-150 mph in a west to east direction • Located in lower stratosphere • Typically cover long distances and can be thousands of miles long • Can be discontinuous and meander across the atmosphere but they all flow east at a rapid speed • Separated into: – Polar jet stream – (Sub) tropical jet stream Jet Stream and El Niño/La Niña • Jet stream helps carry El Niño and La Niña El Niño • An irregularly occurring climatic changes affecting the equatorial Pacific region every few years. • Characterized by the appearance of unusually warm, nutrient-poor water off northern Peru and Ecuador, typically in late December. • Two strongest events on record occurred between 1982-83 and 1997-98 • 1997-98: Particularly bad for United States – Huge storms struck west coast and gulf causing large-scale beach erosion and floods • When El Niño happens: air pressure drops in southeastern pacific and rises in western pacific. • When event ends (hurricane/storms), the pressure “swings” in the opposite direction; rises in southeast and decreases in west. • Called Southern Oscillation. La Niña • Characterized by the appearance of unusually cooler surface temperatures in eastern Pacific • Typical winter blows colder than normal over the pacific northwest and the great plains. • Winter 1998-1999: – Record snowfall in Washington state – Greater hurricane activity than El Nino Global Atmospheric Circulation • Atmospheric Circulation: large scale movement of air • Underlying cause of wind is unequal heating (difference in pressure). • Scientist made models to help understand circulation Single Cell – Non-Rotating Earth • First and most basic model of atmospheric circulation • Circulation is known as a Hadley Cell Rotating Earth • • • • • • • Equatorial Low/Doldrums Trade Winds Subtropical High Westerlies Polar Front Polar Easterlies Polar Highs Equatorial Low/Doldrums • Place of inactivity/low pressure • Abundant precipitation Trade Winds • Wind blowing steadily towards the equator from the northeast (N. hemisphere) or the southeast (S. hemisphere) • Prevailing and most persistent winds of the tropics Subtropical High • Belt of high pressure located at latitudes 30° N and S • Semi-permanent Westerlies • Move in western direction, towards poles • Govern most of air flow/weather in North America • Move from high to low pressure • Prevailing surface winds • Mid-latitudes, between 30° and 60° Polar Front • Fronts are boundary between two different air masses • Stormy front • Difference in temperature and density • Do not readily mix – Results in the formation of the polar front – The polar front is a zone of strong convergence and rapidly rising air Polar Easterlies • Cool, dry, prevailing winds • Move east, towards areas of low pressure Polar Highs • Cold air is moving south, but does not go beyond polar front • Sinks • Circulation results in high pressure Local Air Circulation • Land and Sea breezes • Mountain and Valley breezes • Chinook and Santa Ana winds Land and Sea Breezes • Water, unlike air and land, does not change temperature quickly. • Water absorbs or loses a lot of heat energy before the temperature changes. • During the day the sun heats the land quickly. As land heats, the air above is heated. The hot air rises, become less dense creating low pressure. The air over the sea is cooler because it takes much longer to heat water. The air over the sea keeps its high pressure • The differences in pressure creates wind or a breeze. • Winds blow from the sea to the land to try to equalize the pressure. This is called a sea breeze. • At night land cools rapidly. Because the air of the sea is warmer, the wind's direction changes. • The cooler heavier air over the land forms a high pressure causing the air to move towards the sea where the air pressure is lower. This is called a land breeze. • http://www.classzone.com/books/earth_scien ce/terc/content/visualizations/es1903/es1903 page01.cfm Mountain and Valley Breezes • Localized wind that occurs one right after the other in a daily cycle • Characterized by upslope flow during the day, and down slope flow during the night • During the day, the air along the valley wall becomes warmer • The cooler air sinks into the valley as the rest of the air (warm air) blows up slope creating a valley breeze. • No as it gets darker, the air over the valley is forced to ascend or rise into the atmosphere while the cooler air blows down slope. • This creates a mountain breeze. Santa Ana Winds • Strong, dry winds that blow westward • Can bring hot to cold temperatures • Sweep through Southern California in fall and winter • Sometimes travels 80-90 mph • Contribute to forest and brush fires Chinook Winds • Name comes from the Chinook Indians who lived in the region • Warm, dry wind • Strong and gusty (reach 100 mph or higher) • Causes temperature to rise rapidly – January 1966, in 4 minutes time, temperature rose 38°F