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.

Figure 6.2: (a) Two air columns, each with identical mass, have the same surface air pressure. (b) Because it takes a shorter column of cold air to exert the same surface pressure as a taller column of warm air, as column 1 cools, it must shrink, and as column 2 warms, it must expand. (c) Because at the same level in the atmosphere there is more air above the H in the warm column than above the L in the cold column, warm air aloft is associated with high pressure and cold air aloft with low pressure. The pressure differences aloft create a force that causes the air to move from a region of higher pressure toward a region of lower pressure. The removal of air from column 2 causes its surface pressure to drop, whereas the addition of air into column 1 causes its surface pressure to rise. (The difference in height between the two columns is greatly exaggerated.) Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. 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: “Every action has an equal and opposite reaction.”

Forces that Influence the Wind net force and fluid movement Wind is the result of a balance of several forces.

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.

Figure 6.11: The pressure gradient between point 1 and point 2 is 4 mb per 100 km. The net force directed from higher toward lower pressure is the pressure gradient force. 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.

Figure 6.14: On nonrotating platform A, the thrown ball moves in a straight line. On platform B, which rotates counterclockwise, the ball continues to move in a straight line. However, platform B is rotating while the ball is in flight; thus, to anyone on platform B, the ball appears to deflect to the right of its intended path. 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.

Figure 6.19: An upper-level 500-mb map showing wind direction, as indicated by lines that parallel the wind. Wind speeds are indicated by barbs and flags. (See the blue insert.) Solid gray lines are contours in meters above sea level. Dashed red lines are isotherms in °C. 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 Fig 7.1 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

Figure 7.4: A thermal circulation produced by the heating and cooling of the atmosphere near the ground. The H’s and L’s refer to atmospheric pressure. The lines represent surfaces of constant pressure (isobaric surfaces). 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.

Figure 7. 5: Development of a sea breeze and a land breeze Figure 7.5: Development of a sea breeze and a land breeze. (a) At the surface, a sea breeze blows from the water onto the land, whereas (b) the land breeze blows from the land out over the water. Notice that the pressure at the surface changes more rapidly with the sea breeze. This situation indicates a stronger pressure gradient force and higher winds with a sea breeze. Stepped Art Fig. 7-5, p. 174

Seasonally Changing Winds - the Monsoon Monsoon wind system Asian monsoon other monsoons

Mountain and Valley Breezes 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.

Figure 7.14: A chinook wind can be enhanced when clouds form on the mountain’s windward side. Heat added and moisture lost on the upwind side produce warmer and drier air on the downwind side. 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 westerlies polar front polar easterlies Many global circulation terms, including ‘trade winds’ and ‘doldrums’, were named by mariners who were well acquainted with wind patterns.

Figure 7.21: The idealized wind and surface-pressure distribution over a uniformly water-covered rotating earth. Watch this Active Figure on ThomsonNow website at www.thomsonedu.com/login. 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.

Figure 7.22: Average sea-level pressure distribution and surface wind-flow patterns for January (a) and for July (b). The heavy dashed line represents the position of the ITCZ. 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.

Figure 7.32: In diagram (a), under ordinary conditions higher pressure over the southeastern Pacific and lower pressure near Indonesia produce easterly trade winds along the equator. These winds promote upwelling and cooler ocean water in the eastern Pacific, while warmer water prevails in the western Pacific. The trades are part of a circulation (called the Walker circulation) that typically finds rising air and heavy rain over the western Pacific and sinking air and generally dry weather over the eastern Pacific. When the trades are exceptionally strong, water along the equator in the eastern Pacific becomes quite cool. This cool event is called La Niña. During El Niño conditions—diagram (b)—atmospheric pressure decreases over the eastern Pacific and rises over the western Pacific. This change in pressure causes the trades to weaken or reverse direction. This situation enhances the countercurrent that carries warm water from the west over a vast region of the eastern tropical Pacific. The thermocline, which separates the warm water of the upper ocean from the cold water below, changes as the ocean conditions change from non-El Niño to El Niño. 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.