| NATS 101 Lecture 9 Atmospheric Moisture |
| Hydrological Cycle |
| Humid Air |
| Humid air is a mixture of molecules that make up dry air (mostly N2 and O2) and lighter water vapor (H2O) molecules. | |
| Each type of molecule contributes a fraction of total air pressure, or a partial pressure, proportional to the number molecules per unit volume. The partial pressure of water vapor is termed the vapor pressure. |
| Saturation Vapor Pressure |
| The partial vapor pressure at which the rate of evaporation equals the rate of condensation in a closed system is called the saturation vapor pressure or SVP. The SVP effectively denotes the maximum water vapor that air the can ŇholdÓ. | |
| SVP depends strongly on temperature. | |
| Vapor pressure and SVP provide a measure of the actual water vapor content and the airŐs potential capacity, respectively. |
| Slide 5 |
| SVP and Temperature |
| SVP nearly doubles with a 10oC warming | |
| SVP and T Graph | |
| Supercooled water droplets can exist to temps of -40oC | |
| For temps below 0oC, SVP runs 10%-30% lower over ice |
| Relative Humidity |
| Air with a RH=100% is said to be saturated. | |
| RH depends on air temperature (SVP). | |
| RH changes by either changing airŐs water vapor content or the airŐs temperature. |
| Relative Humidity |
| Relative Humidity (Ahrens, Appendix B) |
| Consider air that is saturated at 0oC | |
| Temp VP SVP RH | |
| 0oC 6 mb 6 mb 100% | |
| 10oC 6 mb 12 mb 50% | |
| 20oC 6 mb 23 mb 28% | |
| 30oC 6 mb 42 mb 14% | |
| 40oC 6 mb 68 mb 9% |
| Other Measures of Humidity |
| Dew Point |
| DP-temperature to which air must be cooled at constant pressure to become saturated. Higher DP Þ Higher water vapor content. | |
| DP is a good indicator of the actual water vapor content since air pressures vary very little along the earthŐs surface. | |
| DP is plotted on surface weather maps. | |
| DP depression (Temp-DP) is plotted aloft. |
| Arizona Dew Point Ranges |
| Vapor Pressure Dew Point | |
| 24 mb 20oC | |
| 12 mb 10oC | |
| 6 mb 0oC | |
| 3 mb -10oC |
| Wet Bulb Temperature |
| Wet Bulb Temp -Lowest temp to which air can be cooled by evaporation of water into it. | |
| Warmer than dew point since moisture is being added to air which raises dew point. | |
| Measured with sling psychrometer. |
| Wet Bulb Temperature (Ahrens, Appendix D) |
| Wet bulb temperature lies about 30% of the way from the dew point to the temperature | |
| Application-Wet bulb temp gives maximum possible efficiency for a swamp cooler | |
| Month MAX Dew P Wet Bulb June 100oF 37oF 65oF July 100oF 63oF 75oF |
| Heat Index |
| Humid Air is Less Dense |
| Slide 17 |
| Summary: Moisture |
| Water vapor comes from the evaporation of sea water and resides in atmo. for ~1 week. | |
| Air has a saturation level for water vapor | |
| Saturation level depends on air temperature | |
| Humid air is less dense than dry air | |
| Water vapor content can be quantified by RH, dew point temp, wet bulb temp |
| Assignment for Next Lecture |
| Topic - Cloud and Fog Formation | |
| Reading - Ahrens pages 89-96 | |
| Problems - 4.3, 4.13, 4.14, 4.15 |