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Chapter 6: Air Pressure and Winds Air pressure – pressure exerted by the weight of the air above Pressure is measured in force (weight) per unit area Standard air pressure is commonly expressed in several different units: 14.7 lbs / in2 US units 101 325 N / m2 SI units 1013.25 millibar US meteorology units 750 torr from Galileo’s student, Torricelli in 1643, sometimes in mm Hg 29.92 in Hg US weather stations Methods used to measure air pressure Mercury barometer Liquid mercury trapped in a tube with the only open end placed in an open dish of mercury If the tube is about 1 m tall, a vacuum will form at the closed end and air pressure will force mercury up into the tube about 760 mm Aneroid barometer – aneroid meaning without liquid A metal canister with a thin metal top that can change shape as it is compressed by high air pressure or expand outward in low air pressure is connected to a pointer system with a series of levers to magnify the changes Barograph An aneroid barometer connected to a recording mechanism (which can be as simple as a rotating drum) for a continuous record of air pressure For purposes of weather prediction, meteorologists are concerned with horizontal differences in air pressure and, therefore, must compensate for differences in altitude In other words, air pressure reported from weather stations around the world is always corrected to sea level pressure As altitude increases, the weight of a column of air above the observer will decrease because the column of air is shorter Correction to air pressure is made by simply calculating the weight of a theoretical column of air from the height of the observation to sea level and adding the pressure such a column would produce to the measured air pressure Change in air pressure with altitude is not a constant – it is an exponential change The change is approximately ½ for every 5 km of altitude Example: Approximate the air pressure at a height of 15 km 15 km = 3 (5 km) Sea level pressure is about 1013 mbar The new pressure is 1013 mbar x (½)3 = 1013 mbar x (1/8) = 127 mbar The standard atmosphere In meteorology, the standard atmosphere depicts the idealized vertical distribution of atmospheric pressure corrected for both pressure and temperature (see Table 6-1 on page 167 of Lutgens Tarbuck. The Atmosphere, 9th edition. 2004.) Horizontal Variations in Air pressure To compare air pressure from station to station, corrections must be made for differences in elevation (see the standard atmosphere chart above) Extreme difference in pressure 30 mbar above average sea level pressure (about 1 inch of mercury) 60 mbar below average sea level pressure (about 2 inches of mercury) Influence of temperature and water vapor on air pressure Polar regions in the winter receive very little incoming solar radiation and are snow covered so they reflect what little light they receive Temperatures of –34°C (and extremes of –40°C to –50°C) are common Air molecules at these temperatures move more slowly and therefore pack more closely and so their density is much higher (a Canadian high pressure is formed) The opposite occurs with air moving from near the equator over the Gulf of Mexico Warm air has fast moving molecules which reduce the density of the air and create a low pressure area Effect of water vapor Most people perceive humid air as being ‘heavy’ but, in fact, water molecules (H2O = 18 amu) are significantly lighter than the nitrogen (N2 = 28 amu) and oxygen (O2 = 32 amu) molecules in the air This results in humid air being much less dense than dry air at the same temperature Influence of air flow on air pressure Convergence – when air flows into a region from many sides, the air piles up into a taller column of air and the pressure increases Divergence – when air flows out of a region in all directions, the air pressure will decrease Summary: Cold, dry air is more dense and is associated with high pressure areas Warm, humid air is less dense and is associated with low pressure areas Winds moving in to an area result in higher pressures Winds moving out of an area result in lower pressures Factors affecting winds 1. the pressure-gradient force 2. the Coriolis force 3. friction Pressure-gradient force – horizontal air pressure differences result in an unbalanced force causing air to move (wind) from a higher pressure region to a lower pressure region Data from weather stations all over the United States pool data and make maps showing air pressure Isobars – lines connecting areas of equal air pressure Spacing of isobars indicate the amount of pressure change between areas Widely spaced isobars represent small pressure differences Closely spaced isobars represent large pressure differences Temperature, horizontal pressure gradients, and wind Sea breeze is a microcosmic example of how temperature causes horizontal pressure gradients 1. heating the Earth’s surface causes air above the surface to expand and rise 2. the rising air causes a low pressure area on the shore 3. the rising air hits cooler air aloft and creates a high pressure aloft 4. cool water creates a high pressure area just above the water 5. density difference creates low pressure area high above the water 6. results in offshore breeze aloft 7. also results in an onshore surface breeze Figure 1: Sea breeze formation From http://oceanservice.noaa.gov/education/yos/resource/JetStream/ocean/sb_circ.htm Pressure gradient forces alone would result in weak winds that quickly equalize regions of unequal pressure by quickly filling in the low pressure areas Vertical pressure gradient Fact: Air pressure decreases with increasing altitude Fact: Air will flow from high to low pressure areas Question: Why isn’t there a vertical wind from high pressure near the surface to the low pressure regions at high altitudes? Answer: The force of gravity exactly balances the force of the vertical pressure gradient – this balance is called hydrostatic equilibrium Other forces like the Coriolis force, friction, and gravity (the only one of these forces that can generate wind) generally act to modify and sometimes even enhance the development of Earth’s pressure systems Coriolis force – because of Earth’s rotation, all freely moving objects are deflected to the right in the Northern hemisphere and to the left in the Southern hemisphere The Coriolis force 1. is always directed at right angles to the direction of airflow 2. affects only wind direction, not wind speed 3. is affected by wind speed; the stronger the wind, the greater the deflecting force 4. is strongest at the poles and nonexistent at the equator Friction – a force that acts to slow a moving object Unopposed, the pressure gradient force would cause winds to constantly accelerate Friction is effective at slowing this acceleration only near the surface Winds aloft – above a few kilometers, friction is negligible and so Coriolis force is responsible for balancing the pressure gradient force Begin with a stationary parcel of air Coriolis force only affects moving objects, so the initial movement of a parcel of air under the influence of a pressure gradient force will be directly toward the center of a nearby low pressure area (see bottom of graphic to the right) As the wind pick up, the Coriolis force increases (note in graphic to the right that as wind speed increases the Coriolis force also increases) The Coriolis force always acts perpendicular to the wind direction, to the right in the Northern hemisphere and to the left in the Southern hemisphere The wind will theoretically move in a circular motion between isobars at a constant speed – this balanced, constant wind speed is called a geostrophic wind Buys Ballot’s law: in the Northern hemisphere, if you stand with your back to the wind, low pressure will be found to your left hand and high pressure to your right Therefore, cyclonic airflow (counterclockwise) will occur around a low pressure center and anticyclonic airflow (clockwise) will occur around a high pressure center Surface winds – near the surface, friction is no longer a negligible force Since friction always acts in the direction opposite the wind, the force of the wind (and therefore the Coriolis force) will never reach the magnitude of the pressure gradient force The result is that near the surface winds will always spiral into the low pressure area instead of moving in a circle at constant speed How winds generate vertical air motion Surface low pressure (cyclone in the Northern hemisphere) Will cause a cyclonic convergence of air at the surface The air must go someplace and therefore will rise The rate is very slow – about 1 km per day Result is unstable air (cloud formation) usually leading to precipitation The air column tends to build air pressure which slows the rising column The rising column must diverge aloft (anticyclonic in the Northern hemisphere) Surface high pressure (anticyclone in the Northern hemisphere) Will cause an anticyclonic divergence of air at the surface The air must come from someplace and therefore will generate a falling column of air The falling air warms adiabatically Result is stable air usually leading to fair weather The falling air column tends to build air pressure at the surface due to adiabatic warming which slows the falling column The falling column causes convergence aloft (cyclonic in the Northern hemisphere) Note again that low pressure systems tend to generate unstable air and cloud formation resulting in precipitation This is called pressure tendency or barometric tendency Falling barometer (a low pressure system) tends to generate precipitation Rising barometer (a high pressure system) tends to generate fair weather conditions Factors promoting vertical air flow Air moving from over water (less friction) to land (higher friction) results in surface convergence generating a rising air column and cloud formation Conversely, air moving from land to water results in surface divergence generating a falling air column Air passing over mountains compresses vertically causing horizontal divergence aloft On the lee side, vertical air expands causing horizontal convergence Air flow toward the equator where the Coriolis force is weakened causes divergence and subsidence Air flow toward the pole favors convergence and slow uplift Wind measurement Wind vanes – always point into the wind Often connected to a remote dial that shows wind direction N, NE, E, etc. 0° (N also 360°), 90° (E), 180° (S), and 270° (W) Prevailing winds – when the wind blows more often from one direction than from any other Wind rose – lines in each of the eight directions (N, NE, E, SE, etc.) are drawn such the length of the line in each cardinal direction indicates the percentage of time winds blow from that direction Cup anemometer – three hemispherical cups that catch wind and by the speed of the rotation of the device can measure wind speed Aerovane – resembles a wind vane with a propeller on the windward side that measures wind speed Wind socks – often seen a airports, a conical, open-ended, material sock catches wind showing both speed by virtue of how far the sock extends before the end drops and wind direction