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: weight And since weight is defined by weight = mass x gravity that means the pressure really just measures the mass of air above a given 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

3 As we move upwards in the atmosphere, the weight of the air above us (and hence the pressure) decreases weight pressure

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

5 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 a given pressure level (say 500 mb) must always be the same weight 500 mb 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 a 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

7 Surface Pressure The surface pressure measures the weight of the whole air column, from the ground to the top of the atmosphere 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 from the column tends to decrease surface pressure

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

9 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

10 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 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

11 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 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

12 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 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

13 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 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

14 Let's look at this again. Suppose we're drawing the contour representing 1004 mb. Well, we've got two stations with pressure equal to 1004 mb. Clearly the contour has to pass through these points But what about in between?

15 Well, we also have stations with 1000 mb and 1006 mb. Clearly, the 1004 mb contour must go somewhere between the two. But where?

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

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

18 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?

19 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

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

21 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

22 example surface pressure map, showing isobars (red lines) along with cold and warm fronts

23 Pressure Surfaces

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

25 Pressure Surfaces In the atmosphere, lower pressure means higher heights. Some good reference points to remember are: However, the exact heights of these levels vary from place to place, and day to day

26 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

27 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.

28 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. 5.3 km 5.4 km 5.5 km 5.6 km N E (Remember, by definition, a pressure surface has the same value of pressure everywhere, which would make for a boring plot if we plotted pressure.) Schematic upper-level chart showing height of the 500 mb pressure surface

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

30 5.3 km A schematic upper-level chart, with contours showing the height of the 500 mb surface 5.4 km 5.5 km 5.6 km N E The equivalent vertical cross-section, running along the dotted line above

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

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

33 An example 500 mb height chart (black contours)

34 500 mb surface 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... Height contours on an upper-level chart play the same role as isobars on a surface plot.