To investigate the concept of pressure we will simulate the use of tall, red blocks and short, blue blocks that weigh the same. One tall block and one short block are shown in Figure 5A-1. Red blocks are cube-shaped while the shorter blue blocks have the same size base but are half as high. T Figure 5A-1. One tall red block and one short blue block on table T. Whether red or blue, the blocks have the following common characteristics: Each block has the same weight regardless of the volume it occupies. Each block has the same size square base. Each block exerts the same downward pressure on the surface beneath them (because the equivalent weight is acting on the same size base). 1. Figure 5A-1 shows one tall red block and one short blue block side-by-side on their square bases on the flat horizontal surface of a table T. Because both blocks weigh the same and their bases are the same size, the blocks exert pressure on table T. a. equal b. unequal 2. The short block occupies half of the volume as the tall block while containing an equal mass. Because density is a measurement of mass per unit volume, the blue block is a. half b. twice as dense as the red block. In Figure 5A-2 two blocks identical to the originals are placed on top of each block, creating a red stack and a blue stack. c. greater T amount of pressure to the pressure exerted on the table by the L Figure 5A-3. Three red blocks in a stack and three blue blocks in a stack, with imaginary surface 1 inserted beneath two red blocks and one blue block. Figure 5A-4 shows two more blocks added for a total of five blocks in each stack. Imaginary surface 2 is added beneath the top three red blocks and top blue block, so there are two red blocks and four blue blocks beneath it. Figure 5A-2. A stack of two red blocks and a stack of two blue blocks on table T. 3. In Figure 5A-2, the amount of pressure exerted on table T by each stack is on the table exerted by each single block. a. twice the b. the same 4. The pressure exerted on the table by the red stack is blue stack. a. equal b. not equal 5. Another identical block is added to each stack, for a total of 3 identical blocks in each stack in Figure 5A-3, and imaginary surface 1 is inserted horizontally through the two stacks. One red block and two blue blocks are beneath imaginary surface 1. Compare the pressure exerted on imaginary surface 1 by the overlying blocks. The red stack exerts pressure on imaginary surface 1 than does the blue stack. a. less b. equal b. three-fourths c. the same as 8. On imaginary surface 2, the downward pressure exerted by the blue block is exerted by the red blocks. a. one-third the pressure b. one-half c. equal to 9. Starting at the table T and moving upward in Figure 5A-4, the difference in downward pressure on imaginary surfaces 1 and 2 exerted by the overlying portions of the two stacks a. increases b. decreases 10. In the stack, the pressure decreases more rapidly with height. Figure 5A-4. Five red blocks in a stack and five blue blocks in a stack, with imaginary surface 2 inserted beneath two red blocks and one blue block and imaginary surface 1 beneath four red blocks and three blue blocks. a. short, more dense blue b. tall, less dense red Click on the image of the next two figures to print or draw on them digitally. 11. Figure 5A-5 is a side view of the two stacks of pressure blocks, the same blocks seen in the previous figure. Following the example line for the first level of blocks, draw straight lines connecting the mid- points of the bases of each level that exerts the same pressure. These lines connecting equal pressure become steeply inclined as the pressure increases. 6. In Figure 5A-4, the pressure exerted on the table by the red stack is the table by the blue stack. to the pressure exerted on a. less b. more a. equal b. unequal 7. On imaginary surface 1 in Figure 5A-4, the pressure exerted by the overlying blue blocks is pressure exerted by the overlaying red blocks. a. one half the e i g H & 60 Surface Tall Blocks Short Blocks Figure 5A-5. Side view of pressure block stacks. To this point we have been examining the change in pressure with height through stacks of blocks of different density (short blocks versus tall blocks). Now we apply it to the rate air pressure drops with altitude in the atmosphere. Figure 5A-6 shows a cross section of the atmosphere based on upper-air soundings from radiosondes simultaneously on 7 December at Tallahassee, FL and at Long Island, NY, approximately 1520 km (945 mi.) apart. Air pressure values in millibars (mb) are plotted at their observed altitudes, starting with nearly equal values (1013 mb) at Earth's surface. 16,000+ Altitude (meters) 14,000 150 mb > < 150 mb 200 mb > 12,000 < 200 mb. 250 mb > < 250 mb 10,000 300 mb > < 300 mb 8,000 400 mb > < 400 mb 6,000 500 mb > < 700 mb 2,000 850 mb > < 850 mb 925 mb > < 925 mb 0 1000 mb > Florida south 1000 mb New York north Figure 5A-6. Vertical cross-section of air pressure for Florida and New York. 12. Using Figure 5A-6, over Florida, the atmosphere exerted a pressure of 200 mb at an altitude of approximately m above sea level. a. 11,500 b. 11,800 c. 12,200 13. The atmosphere above Long Island, NY was colder and denser than the air above the more southern, warmer Tallahassee, FL. Following the example shown at the surface, draw straight lines connecting equal air-pressure arrowheads on the graph. For most of the atmosphere above Earth's surface these lines, representing equal air pressures, are a. horizontal b. slightly inclined 14. Compare the lines of equal pressure you drew on Figure 5A-5 and Figure 5A-6, one of rigid blocks and the other of compressible air, yet both reveal the effect of density on pressure. The lines of equal pressure slope from the lower-density red blocks or warm air column above Florida to the higher-density blue blocks or cold air column above New York. 16. Imagine you are piloting a plane flying from Florida to New York. Over Florida, the onboard pressure altimeter indicates that the aircraft is at 5800 m above sea level. From Figure 5A-6, the air pressure at that altitude over Florida is about mb. a. 400 b. 500 c. 600 17. Relying on the pressure altimeter, you fly toward New York along a constant pressure level with an indicated altitude of 5800 m. En route, the air temperature outside the aircraft gradually falls but you forget to alter the calibration between air pressure and altitude. Over New York, the pressure altimeter still reads 5800 m, the indicated altitude of the aircraft, however, from Figure 5A-6 the true altitude of the aircraft over New York is the altitude indicated by the altimeter. a. lower than b. the same as c. higher than a. upward b. downward 15. Because of the slope of the equal-pressure lines in Figure 5A-6, it is evident that at 12,200 m above sea level, the air pressure in the warmer air over Florida is the air pressure in the colder New York air at the same 12,200 m altitude. a. lower than b. the same as c. higher than The influence of air temperature on the rate of pressure drop with altitude has important implications for pilots with airplanes equipped with air pressure altimeters. An air pressure altimeter is a barometer in which altitude is calibrated against air pressure. 18. The true altitude of the plane over New York is about m. a. 4500 b. 5500 c. 6000 19. You flew along a constant pressure surface (500 mb), which is at a cold air. altitude in warm air than in a. lower b. higher A pilot must adjust a plane's pressure altimeter to correct for changes in the altitude of pressure surfaces due to changes in air temperature en route. This correction ensures a more accurate calibration between air pressure and altitude. Air Pressure Change in the Horizontal As air masses travel over Earth's surface, they bring changing air pressure and weather with them. Those changes are particularly dramatic at or near their fronts. Highs and lows on surface weather maps identify the highest or lowest pressure centers of broad-scale pressure systems and several may be on a weather map of the coterminous United States, moving from west to east across the Northern Hemisphere middle latitudes. Air pressure at locations in the path of these migrating systems fall as a low approaches or when a high departs, even as air pressure rises with approaching highs or departing lows. Fronts can mark the boundaries of a high, and frequently anchor a low. Where fronts reach Earth's surface, they create a transition zone within the atmosphere, separating two air masses of different density. Figure 5A-7 is a cross-section schematic of a cold front moving from west to east. The vertical scale of the cross section is greatly exaggerated for clarity. The warm (red) air mass is being replaced by the cold (blue) air mass. Each air mass would have a high-pressure center as shown. a. fall then rise b. rise then fall Horizontal changes in pressure help to identify frontal boundaries and consequently can describe what is happening within the air column. Summary Variations in air temperature cause differences in air pressure. Air pressure and air density drop rapidly with altitude in the lower troposphere and then more gradually aloft. Within the atmosphere, at constant pressure, air density is inversely proportional to air temperature. Hence, all other factors being equal, cold air masses are denser and exert higher pressure at Earth's surface than do warm air masses. Air pressure drops more rapidly with altitude in a column of cold air than warm air. A constant pressure surface usually slopes downward toward colder air. H Cold Air Warm Air H East West Figure 5A-7. Cross-section of idealized cold front with scale greatly exaggerated for clarity. Imagine being located where the air pressure is highest in the warm air mass. You would find that pressure, temperature, and other weather parameters change as the front approaches and passes your location. (For a map view, see Figure 2A-1 of Investigation 2A.) 20. As the cold front passes your location, the temperature would a. fall b. rise c. remain steady 21. As the cold front moves toward and passes your location, the air pressure would