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Explore the concept of air pressure and its role in creating wind patterns. Learn about measuring air pressure, factors affecting wind, and the Coriolis Effect.
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Chapter19 Air Pressure and Wind
Air Pressure Defined 19.1 Understanding Air Pressure Air pressure is the pressure exerted by the weight of air. 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.
Atmospheric Pressure - force exerted by atmospheric gas molecules on a given area When air masses move around the earth due to differential heating, this value can change. Elevation also changes the value of atmospheric pressure. Atmospheric pressure is often called barometric pressure, as it is measured with a barometer.
Atmospheric pressure units Atmospheric pressure is often measured in millibars (1000 mb = 1 bar). Standard pressure at sea level is 1.0132 bar or 1013.2 mb, often rounded to 1000 mb. American pilots and TV weatherman express atmospheric pressure in “inches mercury” where 29.9 inches = standard pressure at sea level. Atmospheric pressure is often called barometric pressure, and it is measured with a barometer.
Measuring Air Pressure 19.1 Understanding Air Pressure A barometer is a 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.
Measuring atmospheric pressure – the barometer If you evacuate a tube (i.e. remove all the air) and put it in a dish of liquid, the liquid will fill the tube as the air pressure pushes on the liquid in the dish. If you tried this with a dish of water, the water would rise up to about 33 feet in the tube!
Measuring atmospheric pressure Using mercury, a very heavy liquid, we find that at normal sea-level barometric pressure, the liquid in the tube rises to a height of 760 mm (or 29.92 inches). This apparatus is the original form of the barometer, a device used for measuring barometric pressure. American TV weather reports are usually given in terms of inches of mercury Overseas reports use millibars.
Measuring barometric pressure – the modern way Mercury barometers are dangerous and difficult to use. Modern aneroid barometers use changes within a partially evacuated chamber to move the pointer to the correct value.
Boiling water at altitude Why does water boil at a lower temperature at a higher elevation? Less air pressure allows the water to change state (from liquid to gas) without being so “energetic”. Since boiling water at altitude is not as hot, cooking times must be altered.
Factors Affecting Wind 19.1 Understanding Air Pressure Wind is the result of horizontal differences in air pressure. Air flows from areas of higher pressure to areas of lower pressure. The unequal heating of Earth’s surface generates pressure differences. Solar radiation is the ultimate energy source for most wind. Three factors combine to control wind: pressure differences, the Coriolis effect, and friction.
Pressure and wind • Wind – movement of air from high to low pressure areas. • Wind is caused by pressure differences due to unequal heating of Earth’s atmosphere
Cold air is more dense than warm and exerts more Pressure than warm air at a given altitude
Factors Affecting Wind 19.1 Understanding Air Pressure Pressure Differences • A pressure gradient is the amount of pressure change occurring over a given distance. • Closely spaced isobars—lines on a map that connect places of equal air pressure—indicate a steep pressure gradient and high winds. Widely spaced isobars indicate a weak pressure gradient and light winds.
Isobars-Lines of Equal Pressure Drawn at the earth’s surface (there are upper-level charts as well) Corrected to “sea level” so only effects of weather shown, not elevation Units are millibars (mb) where 1013 mb is standard sea-level pressure. Note that lows and highs appear the same; like a bullseye. Look at numbers (and big H or L) to determine which is which.
Isobars-Lines of Equal Pressure Wind flags: Winds blow away from the flags and towards the dots Winds blow from high pressure to low pressure Winds blow towards the L, away from the H Although temperature infuences pressure, no real correlation Note that lows and highs appear the same; like a bullseye. Look at numbers (and big H or L) to determine which is which.
Factors Affecting Wind 19.1 Understanding Air Pressure Coriolis Effect • The Coriolis effect describes how Earth’s rotation affects moving objects. In the Northern Hemisphere, all free-moving objects or fluids, including the wind, are deflected to the right of their path of motion. In the Southern Hemisphere, they are deflected to the left.
Global Wind Patterns and the Coriolis Effect • Wind blows from high to low pressure • Wind blows straight in one direction, but earth turns underneath • We experience the wind as curving, not us moving! • This apparent “turning force” is the Coriolis Effect.
Global Wind Patterns and the Coriolis Effect • If you stand behind the wind and watch it move away from you in the N. Hemisphere, it appears to curve to the right • If you stand behind the wind and watch it move away from you in the S. Hemisphere, it appears to curve to the left.
Factors Affecting Wind 19.1 Understanding Air Pressure Friction • Friction acts to slow air movement, which changes wind direction. • Jet streams are fast-moving rivers of air that travel between 120 and 240 kilometers per hour in a west-to-east direction.
Jet Stream • Jet streams are not just straight across, but have a wavy pattern. • The jets follow the contours of low and high pressure areas (troughs and ridges, respectively), which move like waves in the atmosphere across the earth.
Jet Stream • In winter, the polar jet dips into the US, the troughs and ridges affect the kind of weather. • If a trough is sitting over you, it is generally very cold and snowy or rainy. • If a ridge is sitting over you, it is generally warm and dry.
