| NATS 101-05 Lecture 7 Atmospheric Moisture: |
| Water vapor | |
| Condensation & Cloud Formation |
| 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 (at which point dew forms). | |
| 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 the atmosphere for ~1 week. | |
| Air has a saturation level for water vapor | |
| Saturation level depends on air temperature | |
| Humid air is slightly less dense than dry air | |
| Water vapor content can be quantified by RH, dew point temp, wet bulb temp |
| NowÉCondensation: Fog Formation |
| Cloud Condensation Nuclei |
| Small, airborne particles are necessary on which water vapor can condense to produce cloud droplets | |
| Without such particles, RH>100% would be needed to produce clouds | |
| Such surfaces are called Cloud Condensation Nuclei (CCN) | |
| CCN are light and stay suspended for days |
| Cloud Condensation Nuclei |
| Sources | |
| Dust, volcanic ash, smoke, soot, salt, sulfate particles | |
| Concentrations | |
| 1,000-10,000 per cc Highest over cities Highest at surface |
| Cloud Condensation Nuclei |
| Haze over Melting Snow |
| Condensation |
| When air becomes supersaturated from either the temperature cooling or the addition of water vapor, water condenses onto CCN as small cloud droplets | |
| Diameters of droplets are 2-20 microns, (10-100 times smaller than human hair) | |
| Concentrations are 50-1,000 droplets per cc |
| Size of Cloud Droplets |
| What is Fog? |
| Fog - a cloud at the ground | |
| There are four basic types of fog | |
| Radiation Fog | |
| Advection Fog | |
| Upslope Fog | |
| Steam Fog | |
| Combinations exist (Radiation-Advection) |
| Radiation or Ground Fog |
| Valley Fog |
| Radiation Fog in Valley |
| Fog Dissipation in the Morning |
| Advection Fog |
| Advection Fog at Golden Gate |
| Upslope Fog |
| Steam Fog or Sea Smoke |
| Why You See Your Breath on a Cold Winter Night |
| Condensation can occur if moist, warm air mixes with cold, dry air. | |
| Temp RH SVP VP | |
| Air 0oC 20% 6 mb 1 mb | |
| (clear) | |
| Breath 36oC 80% 63 mb 55 mb | |
| (clear) | |
| 50-50 18oC 140% 20 mb 28 mb (cloud) | |
| 90-10 4oC 90% 8 mb 6 mb | |
| (clear) | |
| The 50-50 mix condenses into a cloud that quickly evaporates as your exhalation is diluted further |
| Steam Fog over Thermal Pool |
| Precipitation or Frontal Fog |
| Number of Days with Fog |
| Summary: Condensation |
| Condensation | |
| Can occur by cooling or moistening of air | |
| CCN permit condensation at RH near 100% | |
| Small (<0.2 to 1 microns) airborne particles | |
| Responsible for Haze formation at RH < 100% |
| Summary: Fog |
| Fog - a cloud at the ground | |
| Composed of small (20 micron) water drops | |
| Four primary types of Fog | |
| Radiation-Advection-Upslope-Steam | |
| Occur under distinct weather conditions |
| Next Lecture Assignments |
| Topic - Cloud Formation and Types | |
| Reading - Ahrens pages 89-105 | |
| Problems - 4.3, 4.13 - 4.16 |