METR 104: Our Dynamic Weather (Lecture w/Lab) Lab Exploration #5: Winds and Pressure Patterns Dr. Dave Dempsey Dept. of Geosciences SFSU, Spring 2012

(5 points)
(Lab Section 1: Wed., April 18; Lab Section 2: Fri, April 20)

Objectives. By the end of this lab exploration, students should be able to:

• Develop generalized, qualitative relationships between winds (both speed and direction) and atmospheric pressure patterns, based on a set of observations
• near the earth's surface
• higher in the troposphere ("aloft")
• Estimate the direction and relative strength of the horizontal pressure-gradient force (PGF) at specific locations on a contour map of pressure
• Recognize limitations of using PGF patterns to deduce wind patterns
• Understand the logical terms "prove", "confirm" or "support", "disconfirm" or "not support", and "disprove", and apply them to evaluate statements about relations between winds and pressure patterns based on a set of observations.

Materials. Maps of pressure and winds based on observations recorded at 12Z April 4, 2011:

• Figure 1: Sea-level pressure isobars (from a computer model analysis) and surface wind observations
• Figure 2: Sea-level pressure isobars and surface wind vectors (from a computer model analysis)
• Figure 3: Pressure pattern and wind observations, aloft (in mid-troposphere, around ~18,000–19,000 ft. above sea level where the pressure is around 500 mb)

Background. Concepts to be covered in lab beforehand (see handout, "Some Important Points about Winds and Pressure Patterns"):

• Pressure and force
• Contour lines of constant pressure (isobars) on a weather map
• sea level pressure
• pressure aloft (e.g., in mid-troposphere)
• Features of pressure patterns
• high and low pressure areas
• horizontal pressure gradient
• relation between pressure gradient and spacing and orientation of isobars
• relation between pressure gradient and net force on air due to pressure (pressure gradient force)

Also:

• The definitions and distinctions among the following terms of logic:  prove disprove confirm (or support) disconfirm (or not support)

Instructions. Respond in writing to the questions posed below and turn in your responses at the end of the lab session (or before the next lab session the following week).

Questions. For the questions below, refer to the accompanying Figures 1 through 3.

1. Figure 1 shows contour lines of sea level pressure (isobars), together with observed surface winds represented using the common stem-and-barb convention. (See "The Station Model" if you need to review this. Recall that the station is located at the head of the stem, at the opposite end of the stem from the barb(s).)

Figure 2 shows the same isobars, together with surface winds represented as vectors (arrows). (Faster winds are represented as longer arrows; the arrows point in the direction toward which the wind is blowing. By convention, the arrow represents the wind at the spot located at the tail end of the arrow.)

The two figures are based on observations recorded at the same time (12Z on April 4, 2011).

Are the winds on Figures 1 and 2 represented at the same individual locations? If not, how might you characterize the differences?

2. We know that we can deduce the direction and relative strength of the pressure-gradient force at any particular place based on the orientation and relative spacing of the isobars at that place. We also know that the pressure gradient at a location isn't zero, then there will be a pressure-gradient force pushing on air there.

For each of the statements (a) through (c) listed below, decide whether the information on Figures 1 and 2:
• disproves,
• disconfirms (doesn't support) (with a small minority of exceptions, perhaps),
• confirms (supports) (with a small minority of exceptions, perhaps),
• proves, or
• lacks any obvious bearing on the statement.

(For this purpose, Figure 2 is probably easier to use.) For each statement, explain your conclusion.

1. The pressure-gradient force alone determines the wind direction. (Hint: if this is true, in what direction should the wind be blowing?)

2. Winds are usually faster where the pressure gradient (and hence the pressure-gradient force) is larger. (For this question, compare land areas with other land areas and ocean areas with other ocean areas, but not land areas with ocean areas. Things might get more complicated in mountainous areas.)

3. For a given pressure gradient, surface winds tend to be faster over ocean areas than over land areas.

3. If it's possible based on these observations, how might you generalize the relation between wind direction relative to the orientation of the isobars, especially over land areas (e.g., parallel, or perpendicular and toward lower or higher pressure, or across isobars toward their lower or higher pressure side but angled clockwise or counterclockwise or to the right or left of the perpendicular [from a perspective facing in the direction of the wind])?

4. Consider the 1000 mb isobar around the low pressure area centered over the Great Lakes area. What would you say about the net flow of air across this isobar (that is, into or out from the area enclosed by the isobar)? (Determine this by considering each wind vector that lies on or very near the isobar, and note in each case whether or not the wind crosses the isobar and in what sense [inward or outward]. Then add the effects of all of these winds to get the net flow of air inward or outward.)

To compensate for this net flow, would you expect air in the region enclosed by the isobar to rise out of the region or subside (sink) into it? Why?

5. Consider the 1028 mb isobar around the high pressure region centered over Utah, Colorado, and Wyoming. What would you say about the net flow of air across this isobar? What might happen in this region if this pattern persisted and nothing were to compensate for this net flow? What do you think must happen to compensate for it?

6. Figure 3 shows the pattern of pressure in mid-troposphere, at an altitude of 18,000-19,000 feet (about 5-6 kilometers) above sea level. At this altitude the pressure is around 500 mb, roughly half of typical sea level pressures. (As a side note, about half the mass of air in the atmosphere lies below this level and the other half lies above it.)

The contour lines on Figure 3 aren't actually isobars, and the labels on the contours and on the maxima ("H") and minima ("L") locations aren't pressures. However, for our purposes the pressure pattern is the essentially the same as the pattern shown on Figure 3, and if the contour lines were actual isobars then the contour interval would be about 10 mb. Figure 3 also shows winds observed at this level by radiosondes, represented using the stem-and-barb convention.

Is the pressure pattern at this level similar to the pattern at sea level? Note any features that might be similar or very different.

7. Are the winds at the level in Figure 3 similar to those at the surface? If not, how would you describe the differences?

8. How might you generalize the relation between wind direction and the orientation of the contours at this level, if there seems to be one? Does this differ from your answer to Question (3) above?

9. Does there seem to be any connection between wind speed and magnitude of the pressure gradient at this level? If so, what?

10. Conclusion: At either the surface or aloft, does the pattern of pressure-gradient force alone (which we can determine from the pressure pattern) seem adequate to explain the observed pattern of winds (both speeds and directions)? Explain.

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