GEOG 140 Relative Humidity Lab: Dr. Rodrigue

GEOG 140

Introduction to Physical Geography

Lab 4: Atmospheric Moisture

The purpose of this lab is to introduce you to tracking the temperature and the relative humidity of an air parcel that moves through various elevations, which you can do if you know its initial temperature and absolute humidity and the topography of the landscape it crosses. You start out building basic skills in reading relevant graphs and tables and then apply those skills to this problem. There are five sections to this lab.

The first section, a, gets you up to speed on reading relative humidity charts if you know the dry bulb temperatures (DBT) and the wet bulb temperatures (WBT) from reading a sling psychrometer. A series of DBT and WBT are given. For each pair, you need to calculate the wet bulb depression (WBD) to get into the X-Y graph. The wet bulb depression is just the wet bulb temperature substracted from the dry bulb temperature.
The second section, b, gives you familiarity with figuring out the saturation quantity for a cubic meter of air, if you know its current temperature. You locate the given temperature on the X axis, read straight up to the curve, and then read left to the saturation quantity on the Y axis from that point on the curve.
The third section, c, shows you how to use the same X-Y graph in reverse, to figure out the dew point temperature for a package of air, if you know its current absolute humidity in grams per cubic meter. You locate the absolute humidity on the Y axis, read right to the curve, and then straight down to the temperature on the X axis. That temperature is the dew point temperature for an air mass having that particular given absolute humidity.
The fourth section, d, builds on your skills in using that X-Y graph to figure out what will happen to an air mass as it climbs up and down a couple of mountain ranges, kind of like crossing over the Coast Ranges in Central Caliofrnia into the San Joaquin Valley and then up over the Sierra Nevada down into Death Valley. Here is an overview of that section, which should give you the information you need to complete it successfully. These bullet statements will make no sense to you right now (so don't freak out. Yet.). Once you are in the throes of section d, though, you will find them really helpful. So, don't forget they're up here!
- You will be given the initial conditions: Absolute humidity and temperature.
- Knowing these facts about the air mass, you figure out the maximum amount of water vapor (the saturation quantity) it could potentially hold at the start of the cross-section (from knowing its temperature and then consulting Figure 2).
- Once you know the maximum potential vapor content or saturation quantity, you can compare the actual absolute humidity with this maximum, in order to figure out its relative humidity at the beginning.
- Knowing its actual absolute humidity, you can figure out its dew point temperature, again by consulting Figure 2.
- Once you know its initial dew point temperature, you can then figure out its saturation condensation level (or dew point elevation). You do this by
  - subtracting the dew point temperature from the starting temperature and then
  - dividing your answer by the dry adiabatic lapse rate.
- Once you know the SCL, you know that the air will cool at the dry adiabatic lapse rate up until that elevation and then will switch to the wet adiabatic lapse rate above that elevation (as long as the air is rising: descending air always warms at the dry rate). You will then be able to determine the temperature at the tallest peak pretty straightforwardly:
  - Subtract the SCL from the tallest elevation (which is?)
  - Multiply that answer by the wet adiabatic lapse rate
  - Then subtract that answer from the dew point temperature.
- While you're up at the highest elevation, you can also figure out its absolute humidity there at the top. Just remember that the actual absolute humidity cannot be greater than the saturation quantity for the coldest temperature (the temperature that can hold the least water vapor).
- You can calculate the temperature at the end by:
  - multiplying the kilometers of descent times the dry rate and then
  - adding the answer to the cold temperature at the top of the tallest peak.
- Look up the saturation quantity for the hot temperature at the end.
- Since you know the absolute humidity left in the air after its ascent to the tallest point in the cross-section and since you also know the saturation quantity at the end of the cross section, you are now in a position to calculate the relative humidity at the end.
- Helpful reminder: The formula for relative humidity is R = A x 100/S, where:
  - R = Relative humidity (what you're trying to figure out)
  - A = Absolute humidity (what you're given at A or what's left over in the air mass at D)
  - S = Saturation quantity for air at a particular temperature (here, at E)
The last section of the lab, e, will have you summarize why there is such a difference in temperature and relative humidity from the beginning to the end. Think about what latent heat is and what it has to do with water's phase changes.

Lab 4a: Relative Humidity

Give the relative humidity for each of the following, referring to Figure 1:




                        (1)        (2)        (3)        (4)        (5)

     -----------------------------------------------------------------------
     Dry bulb temp.:   40.0°C     15.0°C     30.0°C    -20.0°C     -5.0°C
     -----------------------------------------------------------------------
     Wet bulb temp.:   20.0°C     14.0°C     26.0°C    -21.0°C     -8.0°C
     -----------------------------------------------------------------------
     Rel. humidity:    ____ %     ____ %     ____ %     ____ %     ____ %
     -----------------------------------------------------------------------

Lab 4b: Saturation Quantity

Using Figure 2, give the saturation quantity (grams per cubic meter) for a mass of air having a temperature of:

35°C __________g/m³
25°C __________g/m³
20°C __________g/m³
0°C __________g/m³
-10°C __________g/m³

Lab 4c: Dew Point Temperature

Referring again to Figure 2, give the dew point temperature for an air mass having an absolute humidity of:

25 g/m³ __________°C
35 g/m³ __________°C
4 g/m³ __________°C
18 g/m³ __________°C
60 g/m³ __________°C

Lab 4d: Bringing It All Together

Here is where you apply your knowledge of how to read those two X-Y graphs. A mass of air has an initial temperature of 35°C and an absolute humidity of 9 g/m³.
It blows over a terrain having the cross section shown in Figure 3.

What is its relative humidity at the beginning of its ascent? __________ %

How did you figure this out?

     __________________________________________________________________________
   
     __________________________________________________________________________

What is the dew point for this parcel of air? __________°C

How did you figure this out?


     __________________________________________________________________________   

     __________________________________________________________________________

At which elevation will the air reach dew point? In other words, what is the
saturation condensation level? __________ km

How did you determine this?

     __________________________________________________________________________

     __________________________________________________________________________

Will it rain on slope A-B? _____yes _____no

Explain:

     __________________________________________________________________________

     __________________________________________________________________________

Will it rain on slope B-C or slope D-E? _____yes _____no

Explain:


     __________________________________________________________________________

     __________________________________________________________________________

Will it rain on slope C-D? _____yes _____no

Explain:

     __________________________________________________________________________

     __________________________________________________________________________

What will the temperature be at the highest point in the cross-section? __________°C

How did you figure this out?


     __________________________________________________________________________
   
     __________________________________________________________________________

What is the new dew point and the new absolute humidity for this air?
__________°C and __________g/m³

Why does there have to be a new dew point and absolute humidity?


     __________________________________________________________________________

     __________________________________________________________________________

What will the temperature be at E? __________°C

Explain how you calculated this:


     __________________________________________________________________________

     __________________________________________________________________________

What will the relative humidity be at E? __________%

How did you figure this out?


     __________________________________________________________________________

     __________________________________________________________________________

Lab 4e: Brief Lab Report

Reviewing your explanations above, compose a brief lab report (one paragraph or so), which answers the following:

Why is the temperature so much greater at E than it was when it started out at A?
Why is the relative humidity there so much lower?
How tall would a third mountain range to the right of E have to be to experience any precipitation from this particular air mass?

Figures

Figure 1: Relative Humidity Chart



     Dry-bulb        Wet-bulb depression (WB temp. - DB temp.)°C 
     temp.                                
     (air temp.)     0    1    2    3    4    5   10   15   20   25 
     ______________________________________________________________

     -20           100   41                    
                                         
     -15           100   58   18               

     -10           100   69   39   10         Relative humidity (%)
                                         
     - 5           100   77   54   32   11     
                                         
       0           100   82   65   47   31   15
                                                              
       5           100   86   71   58   45   32     
                                                               
      10           100   88   76   65   54   44     
                                                               
      15           100   90   80   70   61   52   12                 
                                                              
      20           100   91   82   74   66   58   24                 
                                                                     
      25           100   92   84   77   70   63   32    7           
                                                                    
      30           100   93   86   79   73   67   39   16            
                                                                    
      35           100   93   87   81   75   69   44   23    6     
                                                                   
      40           100   94   88   82   77   72   48   29   13     
                                                                   
      45           100   94   89   83   78   73   51   33   18    6
                                                                   
      50           100   95   89   84   79   75   54   37   23   11

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