Chapter 19: Air Pressure and Wind

1. Chapter 19: Air Pressure and Wind Overview

2. What is air pressure? • Air pressure is simply the pressure exerted by the weight of air above • Air pressure is exerted in all directions—down, up, and sideways • The air pressure pushing down on an object exactly balances the air pressure pushing up on the object

3. Measuring Air Pressure • Barometer – device used for measuring air pressure • When air pressure increases, the mercury in the tube rises; when air pressure decreases, so does the height of the mercury column • Scientists will often use the more portable aneroid barometer to record changes over time of air pressure

4. Factors Affecting Wind • Wind is the result of horizontal differences in air pressure • Air flows from areas of higher pressure to areas of lower pressure • Wind is nature’s way of balancing out pressure inequalities • The unequal heating of Earth’s surface generates pressure differences • Solar radiation is the ultimate energy source for most wind • If Earth did not rotate, and if there were no friction between moving air and Earth’s surface, air would flow in a straight line from high to low pressure areas • Three factors combine to control wind: pressure differences, the Coriolis effect, and friction

5. Pressure Differences • Wind is created from differences in pressure—the greater these differences are, the greater the wind speed is • Over Earth’s surface, variations in air pressure are determined from barometric readings taken at hundreds of weather stations • Pressure Gradient – the amount of pressure change occurring over a given distance • Equal pressures are connected on a map using isobars • Closely spaced isobars indicate a steep pressure gradient and high winds • Widely spaced isobars indicate a weak pressure gradient and light winds • The pressure gradient is the driving force of wind

6. Coriolis Effect • Coriolis Effect – the apparent deflective force of earth’s rotation on all free-moving objects • All free-moving objects or fluids, including the wind, are deflected to the right of their path in the Northern Hemisphere; in the Southern Hemisphere, they are deflected to the left • This deflection: 1) is always directed at right angles to the direction of airflow; 2) affects only wind direction and not wind speed; 3) is affected by wind speed—the stronger the wind, the greater the deflection; and 4) is strongest at the poles and weakens toward the equator, becoming nonexistent at the equator

7. Friction • The effect of friction on wind is important only within a few kilometers of Earth’s surface • Friction acts to slow air movement, which changes wind direction • When air is above the friction layer, the pressure gradient causes air to move across isobars • The most prominent features of airflow above the friction layer are jet streams • Jet Streams – fast-moving rivers of air that travel between 120 to 240 kilometers per hour in a west-to-east direction • The roughness of the terrain determines the angle of airflow across the isobars; the smoother the terrain, the smaller the angle of airflow • Slower wind speeds caused by friction decrease the Coriolis effect

8. Pressure Centers and Winds: Highs and Lows • Cyclones – centers of low pressure • Anticyclones – centers of high pressure • In cyclones, pressure decreases from the outer isobars toward the center • In anticyclones, the values of the isobars increase from the outside toward the center • When the pressure gradient and the Coriolis effect are applied to pressure centers in the Northern Hemisphere, wind blows counterclockwise around a low and clockwise around a high • In either hemisphere, friction causes a net flow of air inward around a cyclone and a net flow outward around an anticyclone • The usual “villain” in weather reports is the low-pressure center

9. Global Winds on a Non-Rotating Earth • The underlying cause of wind is the unequal heating of Earth’s surface • The atmosphere balances these differences by acting as a giant heat-transfer system • The system (atmosphere) moves warm air toward high latitudes and cool air toward the equator

10. Global Winds on a Rotating Earth • Trade Winds – two belts of winds that blow almost constantly from easterly directions and are located on the north and south sides of subtropical highs • Westerlies – dominant west-to-east motion of the atmosphere that characterizes the regions on the poleward side of the subtropical highs • Polar Easterlies – winds that blow from the polar high toward the subpolar low • Polar Front – stormy frontal zone separating cold air masses of polar origin from warm air masses of tropical origin

11. Global Winds – Influence of Continents • Where landmasses break up the ocean surface, large seasonal temperature differences disrupt the global pattern of pressure zones in the atmosphere • Large landmasses can become cold in the winter when a seasonal high-pressure system develops, and the surface airflow will be directed off the land • Monsoons – seasonal reversals of wind direction associated with large continents, especially Asia; in the winter, the wind blows from land to sea, and in the summer, the wind blows from sea to land

12. Local Winds • The local winds are caused either by topographic effects or by variations in surface composition—land and water—in the immediate area • During warm summer months, coastal land is heated more intensely than the water, producing an area of low pressure which the cooler ocean air moves into fill, creating a breeze in the afternoon (sea breeze); at night the reverse may take place (land breeze) • The same happens in the mountains, with the air from the valley coming up to replace the air from the mountain slopes (valley breeze); at night the reverse takes place (mountain breeze)

13. Sea and Land

14. How Wind is Measured • Two basic wind measurements—direction and speed—are especially important to the meteorologist • Winds are always labeled by the direction from which they blow • The wind vane is the instrument used to determine wind direction • Prevailing Wind – when wind consistently blows more often from one direction than from any other • In the United States, the westerlies consistently move weather from west to east across the continent • Anemometer – device to measure wind speed

15. El Nino • At irregular intervals of three to seven years, the warm countercurrents, along the coasts of Peru and Ecuador, become unusually strong and replace normally cold offshore waters with warm equatorial waters – El Niño • The warm waters block the nutrients from reaching the surface waters, causing many fish to die off, and greatly affects the fishing industries of Peru and Ecuador • Some inland areas that are normally arid get an abnormal amount of rain, increasing their crop production • These episodes mostly effect the eastern tropical Pacific, but is a part of the global circulation and affects the weather all over the world

17. La Nina • The opposite of El Niño is an atmospheric phenomenon known as La Niña • Researchers have come to recognize that when surface temperatures in the eastern Pacific are colder than average, a La Niña event is triggered that has a distinctive set of weather patterns • A typical La Niña winter blows colder than normal air over the Pacific Northwest (with more precipitation) and the Northern Great Plains • It warms much of the rest of the United States

19. Global Distribution of Precipitation • Areas dominated by the convergent Trade winds (equatorial low) have mainly rain forests and abundant precipitation • Areas dominated by the subtropical high-pressure cells are regions of extensive deserts • The interiors of large land masses commonly experience decreased precipitation • You will be able to explain much about global precipitation through your knowledge of global winds and pressure systems

20. Global Distribution of Preciptiation

21. Assignment • Read Chapter 19 (pg. 532-549) • Do Chapter 19 Assessment • #1-29 (pg. 553-554) • # 1-6 (pg. 555)