DEW, FROST & FOG: Formation, Processes, and Impacts

1. DEW, FROST & FOG Dr. Sam Miller Weather & Climate – MTDI 1200OL Plymouth State University 1

2. Ahrens, Ch. 4

3. Formation Processes

4. Condensation • Process by which water vapor becomes a liquid • Most efficient when air reaches saturation (T=Td, RH = 100%) and • Is cooled further or • More water vapor is added • Clouds, fog and dew • Form when water vapor condenses into tiny droplets of liquid water • More about clouds in next installment

5. Condensation Nuclei • Particles are needed to begin condensation of water vapor in the atmosphere • If they are present, condensation starts when RH = 100%. • Clouds, fog, or dew form • If they did not exist, condensation would not occur until RH ≈ 400%. (This is only possible at a microscopic level.) • Hygroscopic nuclei – very active (“water seeking”) condensation nuclei • Can start tiny droplets of condensation at RH=75% • Causing haze

6. Deposition • Process by which water vapor becomes a solid (ice) • Most efficient when air reaches saturation (T=Td, RH = 100%) and • Temperature is very low, and is • Is cooled further or • More water vapor is added • Frost • Form when water vapor forms ice crystals, either in clear air or on cold surfaces

7. Outcomes

8. Dew and Frozen Dew • Dew • Water condensed onto objects at ground level when the temperature falls to the dew point • Wind speeds are more than about 7 mph • If wind speeds are low, fog usually results instead • Frozen Dew • Liquid dew freezes forming beads of ice • Air temperature falls below freezing AFTERcondensation occurs

9. Dew

10. Frozen Dew

11. Frozen Dew

12. Frost and Rime • Frost • Deposition of ice crystals occurs when air temperatureanddew point fall below freezing • Water goes directly from vapor to solid state, without passing through liquid state • Rime Ice • Supercooled water droplets in clouds or fog freeze onto surfaces • Air temperature is already below freezing

13. Frost Fig. 4-13, p.90

14. Rime Ice

15. Rime Ice

16. Fog • Cloud formed near the ground under stable conditions • Wind speed less than about 7 mph • Air near the surface reaches saturation

17. Fog • Saturation can be achieved in two ways: • By cooling air down to the dew point • radiation fog (beneath a radiation inversion) • advection fog • upslope fog • By adding water vapor to the air • steam fog (a.k.a “sea smoke”) • seeing your breath in the winter

18. Fog formed by cooling • Radiation fog • Surface temperature cools by radiational cooling • Common in valleys (valley fog) • Forms beneath radiation inversions when there is enough moisture • Advection fog • Temperature cools by moving warm (moist) air over a cold surface • E.g.: San Francisco • Upslope fog • Temperature cools by being forced up a mountain • Cooling occurs adiabatically

19. Radiation fog Cool air collects in valley – warmer air remains on hillsides. Cool air in valley becomes saturated, possibly by absorbing water vapor from an open water source. Saturation may also occur by dropping temperature down to dew point.

20. Advection fog Warm air from another area (in this case land surface) moves over a cooler area (in this case the San Francisco Bay). Temperature in cooling air drops to dew point – air becomes saturated – fog forms.

21. http://www.nepalfriends.org/pages/fogproject.htm Upslope fog Air from the valley rides up the side of the mountain. As the air moves upward to lower pressure, it expands and cools. The temperature drops to the dew point, and the air becomes saturated.

22. Fog formed by adding vapor • Steam fog (“sea smoke”) • Need cold, dry air on top of warm water • Evaporation occurs from the warm water • Air in direct contact w/water gets warm and moist • Modified air rises and mixes with colder air above • Mixture is saturated • Fog looks like steam rising from water • Also called evaporation-mixing fog

23. Steam fog in Yellowstone Park Water vapor is added to cool air and condenses into a fine mist of water droplets (fog). Fig. 4-17, p.94

24. Steam fog over the Atlantic Ocean Cool (or cold) Arctic air moves over a warm ocean (or lake) surface. The dry air absorbs water vapor from the warm open water. Water vapor condenses into a fine mist of water droplets (fog). Fig. 4-17, p.94

25. Fog formed by adding vapor • Seeing your breath • Warm moist air from mouth • Mixes with cold air outside • Outside air must be close to saturation (cold) • Mixture is saturated • Looks like steam coming from mouth

26. Foggy Weather

27. Foggy Weather • More prevalent in • coastal regions adjacent to cold ocean currents • Reduced visibility • Airport delays and cancellations • Traffic accidents • Driving issues • High-beams worse than low-beams • Most pile-ups caused by vehicles slowing down after reaching low visibility • Chain reaction

28. Average annual number of days with dense fog

29. Driving in foggy weather in California!

30. Driving in foggy weather in California!

31. Review

32. Important formation processes for dew, frost and fog • Condensation • Condensation & hygroscopic nuclei • Deposition • Outcomes • Dew • Frozen dew • Frost • Rime ice • Fog

33. Types of fog • Radiation, advection and upslope • Steam fog and “breath” fog • Mechanisms responsible for two classes

34. Additional Graphics Sources http://www.dpcprints.com/print.php?IMAGE_ID=17421 http://epod.usra.edu/archive/images/firstfrost.jpg http://members.shaw.ca/warren.long/frost.jpg http://www.touchingthelight.co.uk/rhinog/1996_104020.jpg http://www.nepalfriends.org/pages/fogproject.htm http://drivecare.com/images/Fog.jpg http://www.angelfire.com/zine/mr2/2002_03_01_weblogarchive.html http://en.wikipedia.org/wiki/File:SeaSmoke.jpg http://ftourini.deviantart.com/art/silver-dew-drops-108726172 http://islandjoe.deviantart.com/art/Spiderweb-25770900