Factors Affecting Wind

Transcription

1 Understanding Air Pressure Average air pressure at sea level is about 1 kg per cm 2 (14.7 lbs/in 2 ) Roughly the same pressure that is produced by a column of water 10 m (33 ft) high The pressurized suits used by astronauts on space walks are designed to duplicate the atmospheric pressure experienced at Earth s surface o Without these protective suits to keep body fluids from boiling away, astronauts would perish in minutes Factors Affecting Wind Wind air flowing horizontally with respect to Earth s surface Results from differences in air pressure o Air flows from areas of higher pressure to areas of lower pressure Wind is nature s attempt to balance inequalities in air pressure Unequal heating of Earth s surface generates these differences o Solar radiation is the ultimate energy source for most wind If Earth did not rotate, and there was no friction, air would flow in a straight line Wind is controlled by 1) Pressure-gradient force 2) Coriolis Effect 3) Friction Pressure-Gradient Force Shultz revision of Schott 04 1

2 Pressure differences create wind, and the greater these differences, the greater the wind speed Variations in air pressure are determined from readings taken at hundreds of weather stations o Pressure data are shown on a weather map using isobars lines that connect places of equal air pressure iso = equal; bar = barometer Pressure gradient- the amount of pressure change occurring over a given distance o Closely spaced isobars indicate a steep pressure gradient and high winds and vice versa o Driving force of wind, and it has both magnitude and direction Once air starts to move the Coriolis effect and friction come into play, but only to modify the movement, not to produce it Shultz revision of Schott 04 2

3 Coriolis Effect Wind does not cross the isobars at right angles as the pressure-gradient force directs it o This deviation is the result of Earth s rotation o Coriolis Effect (force) the deflective force of Earth s rotation on all free-moving objects, including the atmosphere and oceans Named after the French scientist who first thoroughly described it Free-moving objects are deflected to the right in the Northern Hemisphere This deflection: Friction o 1) is always directed at right angles to the direction of air flow o 2) affects only wind direction, not speed o 3) is affected by wind speed (stronger speed = greater deflection) o 4) is strongest at the poles and weakens equatorward, becoming nonexistent at the equator Only important near the surface Acts to slow the axis movement Shultz revision of Schott 04 3

4 Surface Winds vs. Winds Aloft Eventually, the Coriolis Effect will balance the pressure-gradient force, and the wind will blow parallel to the isobars o Upper-air winds generally take this path and are called geostrophic winds Usually above 600 m (2000 ft) Travel at higher speeds than surface winds Most prominent feature of upper-level flow are jet streams o First encountered by high-flying bombers during WWII o Fast-moving rivers of air that travel between 120 and 240 km ( mph) per hour in a west-to-east direction o Can have Zonal or Meridional Flow Below 600 m, friction complicates the airflow o Friction lowers the windspeed and reduces the Coriolis effect Upper air flow is nearly parallel to the isobars, whereas the effect of friction causes the surface winds to move more slowly and cross the isobars at an angle Highs & Lows One of the most common features on any weather maps are areas designated as pressure centers. o Low, or cyclones, are centers of low pressure Shultz revision of Schott 04 4

5 Kyklon = moving in a circle o Highs, or anticyclones, are centers of high pressure Cyclones o Centers of low pressure o Pressure decreases toward the center o Wind blow inward and counter-clockwise due to friction Anticyclones o Centers of high pressure o Pressure increases toward the center o Winds blow outward & clockwise Divergence Weather Generalizations Rising air is associated with cloud formation and precipitation Subsidence produces clear skies In a cyclone, the net inward transport of air causes a shrinking of the area occupied by the air mass o Horizontal convergence o Whenever air converges horizontally, it must pile up or increase in height to allow for the decreased area it now occupies Generates a taller, and therefore heavier air column Divergence aloft must occur at a rate equal to the inflow below to maintain a low pressure center Convergence aloft accompanies divergence at the surface and general subsidence of the air column Shultz revision of Schott 04 5

6 o Descending air is compressed and warmed Fair Weather! General Circulation of the Atmosphere Underlying cause of wind is unequal surface heating o The atmosphere acts as a giant heat-transfer system, moving warm air poleward and cool air equatorward Circulation on a Nonrotating Earth On a hypothetical nonrotating planet with a smooth surface of either all land or all water, two large thermally produced cells would form o Heated equatorial air would rise until it reached the tropopause, which would deflect the air poleward o Reaches the poles and sinks, spreads out in all directions at the surface and moves back toward the equator This hypothetical circulation system has upper-level air flowing poleward and surface air flowing equatorward If we add the effect of rotation, this simple convection system will break down into small cells Idealized Global Circulation Near the equator, the rising air is associated with the pressure zone known as the equatorial low a region marked by abundant precipitation As air reaches 20 or 30 N or S, it sinks back to the surface Shultz revision of Schott 04 6

7 o This subsidence and associated adiabatic heating produce hot, arid conditions Subtropical High center of this zone of subsiding dry air which encircles the globe near 30 Location of Great deserts At the surface, airflow is outward from the center of the subtropical high o Some of the air travels equatorward and is deflected by the coriolis effect. Creates the Trade Winds Steady easterly winds are called TRADE winds because sailors relied on them to carry cargoes from Europe to the West Indies and South America. o Some travels poleward and is deflected by the coriolis effect Generates the Prevailing Westerlies of the mid-latitudes Because they blow from the West to the East, they are called Prevailing Westerlies. As the Westerlies move poleward, they encounter the cool Polar Easterlies in the region of the subpolar low o The interaction of these warm and cool winds produces the stormy belt known as the polar front Mixing of warm & cold air along polar front has a major effect on weather changes in the U.S. The source region for the variable polar easterlies is the Polar High and here, Doldrums cold air is subsiding and spreading equatorward. Surface at equator is strongly heated warm air rises steadily = LOW Pressure Shultz revision of Schott 04 7

8 o Cool air moves in and is warmed rapidly and rises. Little motion = weak winds Regions near the equator with little to no winds are called the doldrums Horse Latitudes Warm air that rises at the equator divides and flows both north and south. o 30 N & S, air stops moving toward the poles and sinks Forms a belt of calm air Name is from 100 s of years ago, sailors stuck in these waters ran out of food and water for their horses and had to throw them overboard. Influence of Continents The only truly continuous pressure belt is the subpolar low in the Southern Hemisphere o Ocean is uninterrupted by landmasses Where the landmasses break-up the ocean surface, large seasonal temperature differences disrupt the: 1. Global pressure patterns 2. Global wind patterns The circulation over the oceans is dominated by semi-permanent cells of high pressure in the subtropics & cells of low pressure over the subpolar regions Large landmasses become cold in the winter and develop a seasonal high pressure system from which surface flow is directed off-land o Seasonal changes in wind direction are known as the monsoons During warm months air flows onto land Shultz revision of Schott 04 8

9 Warm, moist air from the ocean = rainy, summer monsoon Dry continental air blows toward the ocean = winter monsoon The Westerlies Does not fit the convection system proposed for the tropics Between 30 & 60, the general west to-east flow is interrupted by migrating cyclones and anticyclones A close correlation exists between the paths taken by these surface pressure systems and the position of the upper-level air flow, upper air strongly influences the movement of these systems o Steep temp gradient across the middle latitudes in the winter months corresponds to a stronger flow aloft. o Polar jet stream fluctuates seasonally such that its average position migrates southward in winter & northward in summer The # of cyclones generated is also seasonal o Most in cooler months when temp. gradients are greatest Local Winds Winds than influence much smaller areas Produced from pressure differences that arise from temperature differences caused by unequal heating of earth s surface Local winds are simply small scale winds produced by a locally generated pressure gradient Land & Sea Breezes Land heats more quickly than water o Air above the land surface heats up, expands & rises Shultz revision of Schott 04 9

10 Area of Low Pressure Creates a sea breeze because cooler air over water (higher pressure) moves toward the warmer land (lower pressure) o Sea breezes can be a significant moderation influence At night, the reverse may take place o Land Breeze Land cools more rapidly than the sea Mountain & Valley Breezes During daylight hours, the air along the slopes of mountains is heated more intensely than air at the same elevation over the valley floor o Warmer, less dense air glides up along the slope Valley Breeze Indentified by cumulus clouds the develop on adjacent mountain peaks Common during the warm air season Reverses at night mountain slope cools more rapidly and drains downslope into the valley o Mountain Breeze Dominant in cold season Shultz revision of Schott 04 10

11 Chinook & Santa Ana Winds Chinook winds are warm, dry winds common on the eastern slopes of the Rockies o Snow-Eaters o Created when air descends the leeward side of a mountain and warms by compression o Occur mostly in the winter and spring A Chinook like wind that occurs in southern California is the Santa Ana o Increase the threat of fire How Wind Is Measured Direction and speed are two important basic measurements o Winds are always labeled by the direction from which they blow Wind Vane instrument used to determine wind direction o Always points into the wind Cup Anemometer used to determine wind speed o Anemo=wind, metron=measuring instrument Because changes in wind direction often bring changes in temp. and moisture conditions, the ability to predict winds can be very useful El Niño & La Niña The cold Peruvian current flows equatorward along the coast of Ecuador and Peru o Encourages upwelling of cold, nutrient filled waters Near the end of each year a warm current that flows southward along the coasts of Ecuador and Peru replaces the cold Peruvian current Shultz revision of Schott 04 11

12 o Called El Niño = The child (Christmas season) Every 3-7 years these counter currents become unusually strong and replace the cold off shore waters with warm equatorial water El Niño devastates the fishing industry Some inland areas that are normally arid receive an abnormal amount of rain Two of the strongest El Niño Events on record occurred between & o El Niño brought ferocious storms to California and heavy rains and floods to the southern U.S. and destroyed hurricanes in the Atlantic Each time an El Niño occurs, the barometer pressure drops over large portions of the SE Pacific, whereas in the Western Pacific, near Indonesia and North Australia, the pressure rises o When El Niño comes to an end, the pressure difference swings back This see-saw pattern of atmospheric pressure between the East and West Pacific is called the Southern Oscillation o ENSO El Niño/ Southern Oscillation The steady westward flow of the trade winds creates a warm surface current that moves east to west along the equator and results in a piling up of a thick layer of warm surface water that produces higher sea level (by 30 cm) in the W. Pacific o Eastern Pacific is characterized by a strong Peruvian current, upwelling of cold water and lower sea levels When the Southern Oscillation occurs, this normal situation changes o Pressure rises in Indonesia causing a weakened or reversed pressure gradient along the equator Trade Winds diminish or change direction Shultz revision of Schott 04 12

13 During most El Niños, warmer than normal winters occur in the Northern U.S. and Canada Drought conditions are generally observed in Indonesia, Australia and the Philippines La Niña Opposite of El Niño o Surface temps in the eastern Pacific are colder than average A typical La Niña winter blows colder than normal air over the Pacific NW and the northern Great plains while warming the rest of the U.S. Greater precip. is expected in the NW Usually results in greater hurricane activity o Cost of hurricane damage is 20x greater in La Niña years Shultz revision of Schott 04 13

14 Global Distribution of Precipitation In general regions influenced by high pressure, with its associated subsidence and diverging winds, experience relatively dry conditions Regions under the influence of low pressure and its converging winds and ascending air receive ample precipitation. o Rainest region equator (low pressure) Latitudinal variation in precip due to air temp and capacity for water o Cold air and low latitudes =less precip Distribution of Land & Water affects precip as well o Large landmasses in the mid-latitudes commonly experience decreased precipitation toward their interiors o Mountain barriers also affect precipitation! Shultz revision of Schott 04 14