1. Introduction to Atmospheric Sciences Plymouth State University
DEW, FROST & FOG
Dr. Sam Miller
Introduction to Atmospheric Sciences
Plymouth State University 1

2. Formation Processes

3. 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

4. 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

5. 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

6. Outcomes

7. Dew and Frozen Dew Dew Frozen 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 AFTER condensation occurs

8. Dew

9. Frozen Dew

10. Frozen Dew

11. Frost and Rime Frost Rime Ice
Deposition of ice crystals occurs when air temperature and dew 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

12. Frost
FIGURE 4.13 These are the delicate ice-crystal patterns that frost exhibits on a window during a cold winter morning.
Fig. 4-13, p.90

13. Rime Ice

14. Rime Ice

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

16. 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

17. Fog formed by cooling Radiation fog Advection fog Upslope 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

18. Radiation fog
FIGURE 4.15 Radiation fog nestled in a valley.
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.

19. Advection fog
FIGURE 4.16 Advection fog rolling in past the Golden Gate Bridge in San Francisco. As fog moves inland, the air warms and the fog lifts above the surface. Eventually, the air becomes warm enough to totally evaporate the 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.

20. Upslope fog Air from the valley rides up the side of the mountain.
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.

21. 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

22. 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

23. 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

24. 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

25. Foggy Weather

26. Foggy Weather More prevalent in Reduced visibility Driving issues
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

27. Average annual number of days with dense fog
FIGURE 4.18 Average annual number of days with dense fog throughout the United States.
Average annual number of days with dense fog

28. Driving in foggy weather in California!

29. Driving in foggy weather in California!

30. Review

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

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

33. Additional Graphics Sources