Dew Point Depression

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Introduction

The dew point depression is the difference between the air temperature and the dew point temperature. The magnitude of this difference can give us a good estimate of the relative humidity or how close the air is to saturation when the relative humidity is not given.

Objectives

References

Relative Humidity

Instruction

Dew Point Depression (DPD)

The DPD is just the number of degrees that the dew point is below the air temperature. As a formula we write the dew point depression as

        DPD = T -  T_d

Study the following examples.

    If T = 80°F and T_d = 70°F, then the DPD = 80°F - 70°F = 10°F
    If T = 53°F and T_d = 46°F, then the DPD = 53°F - 46°F = 7°F
    If T = 25°F and T_d = 25°F, then the DPD = 25°F - 25°F = 0°F
    If T = 22°C and T_d = 9°C, then the DPD = 22°C - 9°C = 13°C

Observe that since the dew point can never be greater than the temperature, the DPD can never be negative. It will be 0 whenever T = T_d , i.e., whenever the air is saturated.

If we know the temperature and the dew point depression, we can calculate the dew point by subtracting the DPD from T. In other words, we may rearrange the formula above and write it as

     T_d = T - DPD

Study the following examples.

    If T = 80°F and DPD = 6°F, then T_d = 74°F
    If T = 53°F and DPD = 23°F, then T_d = 30°F
    If T = 25°F and DPD = 0°F, then T_d = 25°F
    If T = 22°C and DPD = 31°C, then T_d = -9°C  

Relative Humidity Calculated from the Temperature and Dew Point Depression

If we know T and DPD we can calculate RH. An example follows.

If T = 60°F and DPD = 10, then T_d = 50°F, e = 12.3 mb, e_s = 17.7 mb and RH = (12.3/17.7)100% = 69%

Complete the Practice Questions below before looking at the Web Examples.

Web Examples

Recall on the Meteograms that the upper line on the upper graph is temperature and the lower line on the upper graph is dew point. So the distance between the lines represents the dew point depression. Where the lines meet, the DPD = 0 and the RH is 100%. The farther apart the two lines are, the greater the DPD is and the lower the RH is. Furthermore, you can estimate the DPD by using the temperature scale on the left-hand side of the graph. Notice that the relative humidity is not given on a Meteogram. Review Meteograms for a few locations to be sure you can visually assess where the RH is the largest and smallest over the 24-hour period.

Meteogram

Review some station model data and notice that you can mentally calculate the dew point depression, but that neither the DPD or RH is given.

Regional Surface Plots (select Current Weather)

Notice in the Weather Roundups that the temperature, dew point, and relative humidity are all given.

Current Weather Reports for Maine

In the text form of a sounding, T, T_d, DPD, and RH are all given. Review a few lines of the text form of a sounding and observe that in every case DD = TEMP - DEWP

Soundings (select Text: List)

In the graphical form of a sounding, the rightmost line is the temperature and the leftmost is the dewpoint. So you can estimate the DPD by the distance between the two lines at any level. Where the lines touch, the relative humidity is 100%. The farther apart the lines are at any level, the lower the relative humidity is. Inspect a sounding in text form and in graphical form and notice that at any level the DPD that you read in text form is the distance between the temperature dew point graphs.

Soundings (select GIF: Stuve)

Practice Questions

1. Inspect the table of temperatures and dew point depressions below. Notice that for each combination of T and DPD you can determine T_d and then calculate the RH. One cell has been filled in corresponding to T=60°F and DPD=10°F(the example above). 

Relative Humidity for Combinations of Temperature and Dew Point Depression

2. From the completed table above, we can see that for a given temperature, as the DPD increases, the RH
    a. increases
    b. stays the same
    c. decreases
    d. there is no obvious pattern

3. From the completed table above, we can see that if the DPD is close to 0, the RH is close to _______ %.

4. Based on your response to the above questions, calculate the DPD for each of the locations in the Weather Roundup below and place them in order of decreasing RH.

5.  Dallas/Ft. Worth is in the central time zone. Inspect the Dallas/Ft. Worth Meteogram below and indicate the local time when the RH reached a maximum and a minimum over the 24-hour period.

Answers

1.

2. c

3. 100%

4.Order of decreasing RH: e, a, c, g, i, d, b, h, f (note that the RH was given on the original Weather Roundup, you do not need to calculate it)

5. The DPD maximum/RH minimum was reached at 23Z or 5pm, CST on Jan 23.
The DPD minimum/RH maximum was reached at 13Z or 7am, CST on Jan 24.

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