Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Air Pressure and Wind Definition of Air Pressure Air pressure is simply the pressure exerted by the WEIGHT of the AIR ABOVE. Average air pressure at sea level is about 1 kg/cm2 or the weight of a column of water 10 meters high! This is called one atmosphere. So the air pressure exerted on the top of your desk is more than 5000 kg!!! So why doesn’t your desk collapse? Your desk doesn’t collapse because AIR PRESSURE IS EXERTED IN ALL DIRECTIONS The air pressure pushing DOWN on an object EXACTLY BALANCES the air pressure pushing UP on the object. 10 m high aquarium filled with water Measuring Air Pressure • Barometer – a device for measuring air pressure (bar = pressure, metron = measuring instrument) • As air pressure goes up, the mercury in the tube rises. Torricelli, a student of Galileo, invented the mercury barometer in 1643 Aneroid Barometer Mercury barometers are neither small nor portable, so the aneroid barometer was developed. It uses a partially empty metal chamber that is very sensitive to changes in air pressure (expanding as pressure decreases, and compressing as pressure increases). This type of barometer can be connected to a recording device, but nowadays there are digital barometers with built-in memory. What Causes Wind? Wind is the result of HORIZONTAL differences in air pressure. Air flows from areas of HIGHER pressure to areas of LOWER pressure. Why? The kinetic theory of matter! Wind is nature’s way of balancing inequalities in air pressure. Ultimately, the sun is responsible for wind: unequal heating of the Earth’s surface generates pressure differences. So why doesn’t wind flow straight from high to low pressure? Two reasons: • The Earth rotates • There is friction between moving air and Earth’s surface So there are THREE factors that control wind: pressure differences, the Coriolis effect, and friction. Pressure differences The greater the difference in pressure, the greater the wind speed will be. Isobars are lines on a weather map that connect places of equal pressure. The spacing of isobars indicates the amount of pressure change happening over a given distance. You are familiar with a similar type of map, a CONTOUR map of elevation. Recall that closely spaced lines mean steep changes in elevation, while lines that are far apart indicate a gentle slope or flat land. Pressure Gradients Closely spaced isobars indicate a steep pressure gradient and high winds. Widely spaced isobars indicate a weak pressure gradient and light winds. •The MAGNITUDE of the pressure gradient is shown by the spacing of the isobars. The closer the spacing, the bigger the pressure gradient. •Its DIRECTION goes from higher pressure towards lower pressure, and PERPENDICULAR to the isobars. H L However, the other two factors (Coriolis effect and friction) modify the wind direction, and friction also modifies the wind speed. Coriolis Effect The Earth’s rotation affects freely-moving objects, by deflecting them to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Let’s watch a brief movie about this effect. • The horizontal path of an object is really a straight line (as it would appear to someone looking down from space), but to someone on Earth’s surface, it appears that the object veers away from its intended path. Of course, what is really happening is that the Earth is rotating ‘out from under’ the object’s path, at a rate of 15 degrees per hour eastward. But what about winds traveling east or west? The Coriolis effect causes the same deflection to the right or left! This is more complicated to explain as it involves centripetal acceleration (what keeps something moving in a circular orbit). Air moving east is going faster than the Earth’s rotation, so it wants to move outward toward space. Gravity holds it down though, so instead the air moves towards the equator. Air moving west is going slower, so it wants to dive down, can’t, and moves towards the poles instead. So how does the Coriolis effect act on wind? • At right angles (perpendicular) to the direction of air flow • It is strongest at the poles and nonexistent at the equator • It affects only wind direction, not wind speed • However, the faster wind is moving, the MORE it is deflected Inertial circles • Another result of the Coriolis effect is that a moving air (or water) mass travels in a circular trajectory called an 'inertial circle'. The circles are bigger at the equator than at the poles. • Let’s watch a bit more of that movie that demonstrates this on the merry-go-round So how important is the Coriolis effect? • For hurricanes, the deflection is about 40 km a day. • Over the large scale of most air masses, and the days it takes them to travel, this adds up. • In New Mexico a softball hit 100m down the right field line will be deflected 1.5cm to the right. • For the inertial circles, a wind speed of 10 m/s at Hobbs’ latitude means a circle would be about 200 km (124 miles) across, with one rotation in 14 hours Friction Friction acts opposite to the direction of wind flow Friction (cont.) • Over the oceans, there is little friction and winds tend to flow parallel to the isobars. • Over rougher terrain, winds can be deflected as much as 45 degrees from the isobars, toward lower pressure areas. PGF – pressure gradient force CF – Coriolis ‘force’ • For higher altitudes, friction is unimportant. For wind speeds high enough, the Coriolis effect exactly balances the pressure-gradient force, and the winds flow parallel to the isobars. This flow is called a geostrophic wind. Jet Streams The most important winds at higher altitudes are the jet streams, rivers of air traveling 120 to 240 km/h. The one you are most familiar with travels west to east across the U.S. at the polar front, the boundary between cold, polar air and moist subtropical air. Pressure Centers and Winds When you look at a weather map, you see highs and lows. The are pressure centers known as ANTICYCLONES and CYCLONES (from the Greek kyklon meaning ‘moving in a circle.’ CYCLONES are centers of LOW pressure (pressure decreases toward the center). ANTICYCLONES are centers of HIGH pressure (pressure increases toward the center). Yes, hurricanes are called cyclones in the Indian Ocean, but that’s not what we mean here! Cyclonic and Anticyclonic Winds We know that winds are most affected by the pressure gradient and the Coriolis effect. These two factors cause winds in the Northern Hemisphere to blow COUNTERCLOCKWISE around a LOW and CLOCKWISE around a HIGH . Southern Hemisphere The opposite is true in the Southern Hemisphere: winds in lows circulate clockwise, while winds in highs circulate counterclockwise. Cyclonic and Anticyclonic Winds Friction was the other factor, and it causes air to flow INWARD around a LOW OUTWARD around a HIGH Weather and Air Pressure Remember convergence as a way to lift air? This happens as winds flow into a low pressure system. To balance the inflow (CONVERGENCE), there must be outflow (DIVERGENCE) aloft at the same rate. In an anticyclone (high), surface air diverges (outflow), which means there must be convergence (inflow) and SUBSIDING air aloft. Weather of Cyclones Convergence at the surface, divergence aloft. Because of this upward movement of air, cyclones (lows) are often associated with stormy weather and unstable conditions. Weather of Anticyclones Anticyclones (highs) have the opposite pattern of flow, with winds converging aloft and subsiding air at the surface. Highs are usually associated with clear skies and stable air. Weather Forecasting So now you can see why weather reports emphasize the locations and possible paths of lows and highs, especially the lows. Lows move roughly west-to-east across the contiguous US, taking days to do so. Their paths are somewhat unpredictable because surface conditions are linked to the air above them, so meteorologists need to understand the total atmospheric circulation. Exploration Lab • Break here for Exploration Lab activity Global winds Remember, it’s the sun that ultimately causes winds. More solar radiation is received at the equator than is lost back to space. Less solar radiation is received at the poles than is lost back to space. The atmosphere acts as a giant heattransfer system by balancing these differences, moving warm air toward the poles, and cold air toward the equator. If the Earth didn’t rotate Notice that there are TWO CELLS, one in the Northern Hemisphere and one in the Southern Hemisphere. Global winds would be simple, with heated air at the equator rising to the tropopause, flowing toward the poles, sinking, and flowing along the surface back to the equator. There would be permanent lows along the equator and highs at the poles. Rotating Earth Model Notice that each cell has now become THREE CELLS. •Hadley cells: At the equator, rising air produces a pressure zone called the EQUATORIAL LOW, an area with abundant precipitation and little wind. Air then flows aloft northward and southward to about 30° latitude where the air subsides and heats due to compression. These are the SUBTROPICAL HIGHS. The stable, dry conditions associated with the subtropical highs are responsible for the great deserts of Africa (the Sahara), Australia, and Arabia. At these subtropical highs, air flows along the surface, some toward the equator and some toward the poles, deflected due to the Coriolis effect. The TRADE WINDS are two belts of wind between 30° latitude and the equator that blow almost constantly from easterly directions. You will recall from your social studies classes how important these winds were for global exploration and trade. •Ferrel Cells between 30° and 60° latitude. At the poles, cold air sinks and spreads towards the equator. This is the POLAR HIGH. The prevailing WESTERLIES are two belts of wind that dominate the surface weather patterns in this cell. At 60° latitude is the SUBPOLAR LOW where surface winds converge and rise to the tropopause. •Polar Cells, from 60° latitude to the poles. The POLAR EASTERLIES are winds that blow from the polar high to the subpolar low, but they’re not constant like the trade winds. The interaction between the warm air masses from the Ferrell Cell and the cool air masses from the Polar Cell produce a storm belt called the POLAR FRONT. Here is another view of the global circulation that shows what kinds of biomes are associated with each cell and the location of the primary jet streams. Influence of Continents The only truly continuous pressure belt is the subpolar low in the Southern Hemisphere. Here the ocean is uninterrupted by landmasses. At other latitudes, particularly in the Northern Hemisphere where landmasses break up the ocean surface, large seasonal temperature differences disrupt the pressure pattern. July average global air circulation Large landmasses, particularly Asia, become cold in the winter when a seasonal high-pressure system develops. From this system, surface airflow is directed off the land. In the summer, landmasses are heated and develop lowpressure cells, which permit air to flow onto the land. Look at the map above to see this general pattern. These seasonal changes in wind direction are known as the MONSOONS. During warm months, areas such as India experience a flow of warm, water-laden air from the Indian Ocean, which produces the rainy summer monsoon. The winter monsoon is dominated by dry continental air. A similar situation exists to a lesser extent over North America. Regional Wind Systems • Middle latitude circulation doesn’t fit the model for the tropics, being complex. • A PREVAILING WIND is a wind that consistently blows more often from one direction than any other • The general west-to-east flow in the contiguous United States (the ‘westerlies’) is interrupted by migrating cyclones and anticyclones. Local winds • Caused by either topographic effects or variations in surface composition—land and water—in the immediate (LOCAL) area • Land and sea breezes affect areas on coasts or near large lakes, causing small areas of high or low pressure that drive short-range winds. VIDEO Valley and mountain breezes In mountain regions, there is a similar effect. During the day, air along the slopes of mountains is heated more intensely than air in the valleys. This is reversed at night, with cold air flowing down the slopes into the valleys. Mountain breezes are most common during winter, while valley breezes are most common during summer. Wind Measurement Direction • Labeled by the direction from which they blow • Using a wind or weather vane (N, E, S, W, or degrees with 0° at north, 90° at east, etc.) Speed • Anemometer (‘anemo’=wind, ‘metron’=measuring instrument) • Beaufort wind scale Global Distribution of Precipitation • The tropical region in the area of the Equatorial Low is the rainiest region on Earth, including the rain forests of South America and Africa. • The regions in the area of the subtropical highs are deserts