GEOG 140

Introduction to Physical Geography

Lab 4: Atmospheric Moisture

Answers

Lab 3a: Relative Humidity

1. 13%
2. 90%
3. 73%
4. 41%
5. 32%

Lab 3b: Saturation Quantity

(~ means "about")

1. ~39g/m³
2. ~23g/m³
3. ~17g/m³
4. ~4g/m³
5. ~2g/m³

Lab 3c: Dew Point Temperature

1. ~26° C
2. ~33° C
3. ~0° C
4. ~21° C
5. ~44° C

Lab 3d: Bringing It All Together

(only selected answers, not rationale)

1. ~23 %

3. ~10° C

5. ~2.5 km

7. no

9. no

11. yes

13. ~2.5° C, assuming you used the 5° C/km WALR)

15. ~2.5° C, depending on the wet rate you used)

17. ~42.5° C, if you used the 5° wet adiabatic lapse rate)

19. ~10%

Lab 3e: Brief Lab Report

Basically, it's so much warmer at E because both of the descents (B-C and D-E) were at the higher dry adiabatic lapse rate, while part of one ascent (C-D) was at the lower wet adiabatic lapse rate. In other words, the change in temperature during descent was strictly the result of the adiabatic concentration of heat energy by compression. During ascent, part of the change in temperature was just due to the adiabatic dispersion of heat energy due to expansion, but, once rain began, there was a release of latent heat when the water vapor condensed. This added sensible heat to the air mass, which became apparent when the air returned to sea level: It warmed due to adiabatic compression, but it warmed more than expected because of this additional sensible heat released during the rain on slope C-D.

It's much drier at E also, partly because some of the humidity in the air was taken out as precipitation on slope C-D, so there's less absolute humidity in the air. Also, the temperature at E, being much warmer, has a higher saturation quantity, so the denominator in the relative humidity formula is larger, which makes for a smaller relative humidity percentage.

A third mountain range would have to be at least 4 km tall (the elevation of Peak D) for the air mass to cool (at the dry rate) to the new dew point of the now drier air (assuming no other moisture is somehow added to the air mass).

This document is maintained by Dr. Rodrigue
First placed on the web: 10/19/99
Last Updated: 06/15/07