| NATS 101 Lecture 10 Condensation: Fog and Cloud 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 Types | |
| Reading - Ahrens pp 96-108 | |
| Problems - 4.16 |