Quick Review: Pressure

Transcription

1 Quick Review: Pressure The pressure force is the force exerted by an air mass on its surroundings (and vice-versa) In the atmosphere, the surroundings are usually other air masses three air masses, each exerting pressure forces on the others

2 In the atmosphere, the pressure is essentially a measure of weight. Specifically: For any given height, the pressure equals the weight of the air above that height weight pressure the pressure force from below holds the weight of the air above

3 In the atmosphere, the pressure is essentially a measure of weight. Specifically: For any given height, the pressure equals the weight of the air above that height weight And since weight is defined by weight = mass x gravity that means the pressure is really just a measure of the mass of air above that height - more mass above you means higher pressure...less mass, lower pressure pressure the pressure force from below holds the weight of the air above

4 As we move upwards in the atmosphere, the weight of the air above us (and hence the pressure) decreases - pressure decreases with height weight pressure

5 As we move upwards in the atmosphere, the weight of the air above us (and hence the pressure) decreases - pressure decreases with height weight And of course, as we go downwards, the weight of the air above us increases - so pressure increases as we move downwards pressure

6 Some things we can infer (corollaries, if you will): (1) Since pressure is determined solely by the mass of air above, the mass of air above any given pressure level (say 500 mb) must always be the same weight 500 mb pressure

7 Some things we can infer (corollaries, if you will): (1) Since pressure is determined solely by the mass of air above, the mass of air above any given pressure level (say 500 mb) must always be the same (2) By the same reasoning, given two specific pressure levels (say 500 mb and 200 mb) the mass of air between the two levels is always the same mass A mass B 200 mb 500 mb

8 Some things we can infer (corollaries, if you will): (1) Since pressure is determined solely by the mass of air above, the mass of air above any given pressure level (say 500 mb) must always be the same (2) By the same reasoning, given two specific pressure levels (say 500 mb and 200 mb) the mass of air between the two levels is always the same mass A mass B 200 mb 500 mb the weight of mass B is 300 mb regardless of temperature, time of day, season, etc.

9 Surface Pressure The surface pressure measures the weight of the whole air column, from the ground to the top of the atmosphere So...the only way to change the surface pressure is to increase or decrease the mass of the column. In particular...

10 Surface Pressure The surface pressure measures the weight of the whole air column, from the ground to the top of the atmosphere So...the only way to change the surface pressure is to increase or decrease the mass of the column. In particular... i. Air converging into the column tends to increase the surface pressure, and... convergence in the column tends to increase surface pressure

11 Surface Pressure The surface pressure measures the weight of the whole air column, from the ground to the top of the atmosphere So...the only way to change the surface pressure is to increase or decrease the mass of the column. In particular... i. Air converging into the column tends to increase the surface pressure, and... ii. Air diverging from the column tends to decrease the surface pressure divergence in the column tends to decrease surface pressure

12 But we have to be careful (Danger, Will Robinson!) because divergence at one level usually means convergence at another (and vice-versa) - (Why?)

13 But we have to be careful (Danger, Will Robinson!) because divergence at one level usually means convergence at another (and vice-versa) - (Why?) So when we talk about surface pressure changes, we really have to talk about net divergence or net convergence - Most of the time, the upper-level process wins net divergence means decreasing surface pressure

14 Surface Pressure Maps To represent surface pressure, we plot out the data on a map and draw lines (called isobars) connecting points of constant value 1006 Note that most of the time the lines go between two points Where we draw the line depends on which of the two values is closest

15 Surface Pressure Maps To represent surface pressure, we plot out the data on a map and draw lines (called isobars) connecting points of constant value 1006 Note that most of the time the lines go between two points Where we draw the line depends on which of the two values is closest

16 Surface Pressure Maps To represent surface pressure, we plot out the data on a map and draw lines (called isobars) connecting points of constant value 1006 Note that most of the time the lines go between two points Where we draw the line depends on which of the two values is closest

17 Surface Pressure Maps To represent surface pressure, we plot out the data on a map and draw lines (called isobars) connecting points of constant value 1006 Note that most of the time the lines go between two points Where we draw the line depends on which of the two values is closest

18 Let's look at this again. Suppose we're drawing the contour representing mb

19 Let's look at this again. Suppose we're drawing the contour representing mb. Well, we've got two stations with pressure equal to mb. Clearly the contour has to pass through these points

20 Let's look at this again. Suppose we're drawing the contour representing mb. Well, we've got two stations with pressure equal to mb. Clearly the contour has to pass through these points But what about in between?

21 Well, we also have stations with 1000 mb and 1006 mb. Clearly, the mb contour must go somewhere between the two. But where?

22 Well, we also have stations with 1000 mb and 1006 mb. Clearly, the mb contour must go somewhere between the two. But where? We'll assume the pressure between between the two stations changes uniformly, in the sense that halfway between the two stations in distance, the pressure is also halfway between (i.e., 1003 mb)

23 Now the full pressure difference between the stations is 6 mb, but the difference between our mb contour and the 1000 mb station (say) is only 4 mb mb 6 mb 1000

24 Now the full pressure difference between the stations is 6 mb, but the difference between our mb contour and the 1000 mb station (say) is only 4 mb. So...if the pressure changes uniformly, this tells us that the mb contour must lie a fraction of (4 mb)/(6 mb) = 2/3 of the distance between the two stations mb 6 mb 1000

25 Now the full pressure difference between the stations is 6 mb, but the difference between our mb contour and the mb station (say) is only 4 mb. So...if the pressure changes 1006 uniformly, this tells us that the mb contour must lie a fraction of (4 mb)/(6 mb) = 2/ of the distance between the two stations.

26 And of course the same reasoning applies for the other contours as well

27 And of course the same reasoning applies for the other contours as well

28 And of course the same reasoning applies for the other contours as well

29 One caveat: It's most useful to compare pressures at the same height - For surface pressure maps, this chosen height is sea level

30 One caveat: It's most useful to compare pressures at the same height - For surface pressure maps, this chosen height is sea level But dude, what if we're not at sea level? What if we live in Tibet?

31 One caveat: It's most useful to compare pressures at the same height - For surface pressure maps, this chosen height is sea level But dude, what if we're not at sea level? What if we live in Tibet? - In mountainous regions, the pressure has to be adjusted down to sea level by applying an altitude correction The correction tries to account for the weight of the air that would have been between the height of the ground (up on the mountain) and sea level

32 in mountainous regions, pressure at the ground is adjusted to give an equivalent pressure at sea level

33 In practice, surface pressure contours are usually smoothed out to highlight the big features (rather than the wiggles) surface pressure map before smoothing...and after

34 Pressure Surfaces

35 Pressure Surfaces (Not to be confused with surface pressure!)

36 Pressure Surfaces In the atmosphere, lower pressure means higher heights. Some good reference points to remember are: 200 mb: upper troposphere ( km) 500 mb: mid-troposphere (5 6 km) 850 mb: lower-troposphere (~ 1.5 km) 1000 mb: ground level (-0.25 to 0.25 km) However, the exact heights of these levels vary from place to place, and day to day

37 Suppose we're interested in where the pressure equals 500 mb in particular. On a given day, the heights where the pressure drops to 500 mb might be different at different locations (as illustrated) heights where pressure equals 500 mb above three observing stations

38 Suppose we're interested in where the pressure equals 500 mb in particular. On a given day, the heights where the pressure drops to 500 mb might be different at different locations (as illustrated) Definition: A surface in space that connects all points with a given value of pressure (say 500 mb) is called a pressure surface heights where pressure equals 500 mb above three observing stations

39 Suppose we're interested in where the pressure equals 500 mb in particular. 500 mb pressure surface On a given day, the heights where the pressure drops to 500 mb might be different at different locations (as illustrated) Definition: A surface in space that connects all points with a given value of pressure (say 500 mb) is called a pressure surface heights where pressure equals 500 mb above three observing stations

40 A pressure surface is a surface in space that connects all points with a given value of pressure (in this case 500 mb). As we'll see, the height of a pressure surface depends on the temperature.

41 Upper-Level Charts Upper-level data is usually plotted on pressure surfaces (e.g., 500 mb, 200 mb, etc) rather than at constant height levels. But instead of pressure, the 5.3 km 5.4 km 5.5 km 5.6 km contours on the plot show the height of the pressure surface above the ground (Remember, by definition, a pressure surface has the same value of pressure everywhere, which would make for a boring plot if we plotted pressure.) N E Schematic upper-level chart showing height of the 500 mb pressure surface

42 Upper-Level Charts Upper-level charts read like topographic maps, with the contours showing the height of the pressure surface...

43 A schematic upper-level chart, with contours showing the height of the 500 mb surface A' C B 5.3 km 5.4 km 5.5 km 5.6 km A N E The equivalent vertical cross-section, running along the dotted line above

44 A schematic upper-level chart, with contours showing the height of the 500 mb surface A' C B 5.3 km 5.4 km 5.5 km 5.6 km A N E height N A B C A' The equivalent vertical cross-section, running along the dotted line above

45 A schematic upper-level chart, with contours showing the height of the 500 mb surface A' C B 5.3 km 5.4 km 5.5 km 5.6 km A N E height N 5.6 km 5.3 km The equivalent vertical cross-section, running along the dotted line above A B C A'

46 A schematic upper-level chart, with contours showing the height of the 500 mb surface A' C B 5.3 km 5.4 km 5.5 km 5.6 km A N 500 mb surface E height N 5.6 km 5.3 km The equivalent vertical cross-section, running along the dotted line above A B C A'

47 South North a schematic pressure surface in 3D, with red colors showing higher heights and blue colors lower heights

48 North South the same thing as an upper-level chart, with contours showing lines of constant height

49 today's (Mon) 500 mb height chart (black contours)

50 500 mb surface A B

51 500 mb surface pressure less than 500 mb A B pressure greater than 500 mb

52 500 mb surface pressure less than 500 mb A B pressure greater than 500 mb pressure at point A is greater than pressure at point B

53 500 mb surface pressure less than 500 mb At a given altitude (such as A to B in the figure) regions of higher heights will have higher pressures, while regions of lower heights will have lower pressures And as a result... A pressure greater than 500 mb pressure at point A is greater than pressure at point B B Height contours on an upper-level chart play the same role as isobars on a surface plot. - You can think of higher height contours as higher pressure, and lower height contours as lower pressure