Transcript Document
Atmospheric Forces and Winds Atmospheric pressure Measuring air pressure Surface and upper-air charts Why the wind blows Surface winds Atmospheric Pressure Atmospheric Pressure air pressure - definition air pressure and temperature pressure gradient force • Air pressure is, quite literally, the weight of the atmosphere above us. Stepped Art Fig. 6-2, p. 143 Measuring air Pressure Barometers mercury barometer aneroid barometer altimeter barograph Pressure Readings station pressure and sea-level pressure isobars Surface and Upper Air Charts Surface and Upper Air Charts isobaric maps contour lines ridges troughs • Color-filled contour maps are the same as ordinary contour maps, except that the area between adjacent lines is filled in with color. Filled Contour Maps Why the Wind Blows Newton’s Laws of Motion Newton’s first law: “An object at rest will remain at rest and an obect in motion will move in a straight line at constant speed unless acted on by an unbalanced force.” Newton’s second law: Newton’s third law: and opposite reaction.” “Every action has an equal Forces that Influence the Wind net force and fluid movement • Wind is the result of a balance of several forces. Pressure Gradient Force pressure gradient pressure gradient force strength and direction of the pressure gradient force • The horizontal (rather than the vertical) pressure gradient force is responsible for causing air to move horizontally. Fig. 6-11, p. 151 Coriolis Force real and apparent forces Coriolis force strength and direction of the Coriolis force factors that affect the Coriolis force • It is sometimes claimed that “water swirls down a bathtub drain in opposite directions in the northern and southern hemispheres”. This is not true. Fig. 6-14, p. 153 Straight-line Flow Aloft combination of the pressure gradient and Coriolis forces geostrophic wind • Geostrophic winds can be observed by watching the movement of clouds. Curved Winds Around Lows and Highs Aloft cyclonic and anticyclonic flow centripetal force gradient wind Winds on Upper-level Charts gradients in contour lines meridional and zonal winds • Height contours on upper-level charts are interpreted in the same way as isobars on surface charts. Stepped Art Fig. 6-19, p. 158 Surface Winds Surface Winds planetary boundary layer friction frictional effects on the wind • Most people rarely venture out of the planetary boundary layer. Winds and Vertical Motions Winds and Vertical Motions divergence and convergence hydrostatic equilibrium Summary of Atmospheric Forces “True” forces: •Gravity •Pressure Gradient •Friction “Ficticious” forces: •Coriolis force •Centrifugal force Summary of Atmospheric Force Balances Vertical: •Hydrostatic Balance Horizontal: •Geostrophic Balanice •Gradient Balance •Ekman Balance (see Table 6-1 in Ackerman and Knox) Atmospheric Circulations Scales of atmospheric motions Eddies - big and small Local wind systems Global winds Global wind patterns and the oceans Scales of Atmospheric Motions Scales of Atmospheric Motions scales of motion microscale synoptic scale planetary scale • Lots of important weather events occur on microscales, like evaporation of liquid water molecules from the earth’s surface. Eddies - Big and Small Eddies - Big and Small eddy rotor wind shear turbulence • Wind shear can sometimes be observed by watching the movement of clouds at different altitudes. Local Wind Systems Thermal Circulations isobars and density differences thermal circulations Stepped Art Fig. 7-4, p. 172 Sea and Land Breezes sea breeze land breeze • Sea and land breezes also occur near the shores of large lakes, such as the Great Lakes. Stepped Art Fig. 7-5, p. 174 Seasonally Changing Winds the Monsoon Monsoon wind system Asian monsoon other monsoons Mountain and Valley Breezes valley breeze mountain breeze • The nighttime mountain breeze is sometimes called gravity winds or drainage winds, because gravity causes the cold air to ‘drain’ downhill. Katabatic Winds drainage winds bora • Katabatic winds are quite fierce in parts of Antarctica, with hurricane-force wind speeds. Chinook Winds Chinook winds compressional heating chinook wall cloud • In Boulder, Colorado, along the eastern flank of the Rocky Mountains, chinook winds are so common that many houses have sliding wooden shutters to protect their windows from windblown debris. Fig. 7-14, p. 180 Santa Ana Winds Santa Ana wind compressional heating wildfires • Many Southern California residents regularly hose down their roofs to prevent fires during Santa Ana wind season. Desert Winds dust storms dust devils Global Winds General Circulation of the Atmosphere cause: unequal heating of the earth’s surface effect: atmospheric heat transport • Ocean currents also transport heat from the equator to the poles and back. Single-cell Model basic assumptions Hadley cell why the single-cell model is wrong • One of the world’s premier atmospheric science research facilities,the Hadley Centre for Climate Research, is named after George Hadley. Three-cell Model model for a rotating earth Hadley cell doldrums subtropical highs trade winds intertropical convergence zone • Many global circulation terms, westerlies including ‘trade winds’ and polar front ‘doldrums’, were named by polar easterlies mariners who were well acquainted with wind patterns. Fig. 7-21, p. 185 Average Surface Winds and Pressure: The Real World semipermanent highs and lows Bermuda high & Pacific high Icelandic low & Aleutian low Siberian high • The Bermuda High frequently brings hot, muggy weather to the eastern US. Fig. 7-22a, p. 188 Fig. 7-22b, p. 189 The General Circulation and Precipitation Patterns major controls ITCZ, midlatitude storms, polar front • Most of the world’s thunderstorms are found along the ITCZ. Westerly Winds and the Jet Stream jet streams subtropical jet stream polar front jet stream Global Wind Patterns and the Oceans Winds and Upwelling upwelling wind flow parallel to the coastline • Upwelling frequently occurs along the coast of California. El Niño and the Southern Oscillation El Niño events Southern Oscillation La Niña teleconnections • ENSO is an example of a global-scale weather phenomenon. Fig. 7-32, p. 196 Other Atmosphere-Ocean Interactions North Atlantic Oscillation Arctic Oscillation Pacific Decadal Oscillation • Other atmosphere-ocean interactions may very well be discovered in the coming years.