1. Dew, frost and fogs

2. Review of last lecture: Components of global water cycle
Ocean water
Land soil moisture, rivers, snow cover, ice sheet and glaciers
Sea ice
Atmosphere water vapor, clouds, precipitation
Water in biosphere (including human beings)

3. Water (H2O ) is unique on earth because it can exist in all 3 states (phases)
An H2O molecule
3 states (gas, liquid, solid) depending on how the molecules are connected together
Can change from any state to any other state. Latent heat is consumed or released in a phase change
e.g. Evaporation -> liberation of water molecules, requires energy

4. Evaporation and Condensation
Molecules escape into the overlying volume as water vapor during evaporation. Energy must be available at the water surface. Water vapor increases in air as surface water evaporates.
Water vapor molecules randomly collide with the water surface and bond with adjacent molecules during condensation.
There is an equilibrium between evaporation and condensation during saturation. Upon saturation, evaporation rate equals condensation rate.

5. Sublimation and Deposition
Water vapor (gas) can change directly into ice or snow (solid) during deposition.
Ice or snow (solid) can turn directly into water vapor (gas) during sublimation.
There is an equilibrium between deposition and sublimation during saturation. Upon saturation, deposition rate equals sublimation rate.

6. Indices of Water Vapor Content
Humidity: amount of water vapor in air
Humidity expressed in a number of ways  Indices
Vapor Pressure: the partial pressure exerted by water vapor. Saturation vapor pressure (SVP) – maximum amount of vapor that can exist at a given temperature, increase w/ Tair
Absolute Humidity: density of water vapor expressed in g/m3
Specific Humidity: mass of water vapor (g) per mass of air (kg) (in g/kg) Saturation specific humidity (qs): highest specific humidity for a given temperature and pressure
Mixing Ratio: amount of water vapor (g) relative only to mass of dry air (kg) Saturation mixing ratio: maximum mixing ratio
Relative Humidity: the amount of water vapor in the air relative to the possible maximum.
Dew point temperature: temperature at which saturation occurs in air (generally colder than Tair, equals to Tair when saturated)
RH=q/qsx100

7. Saturation vapor pressure
Saturation vapor pressure is temperature dependent.
Saturation vapor pressure increases with temperature. Warmer air can hold more water vapor.
It’s a non-linear increase. At low temperatures the saturation vapor pressure increases slowly but it increases rapidly at higher temperatures.

8. Video: Water cycle

9. Methods to achieve saturation and condensation
Diabatic processes – add/remove heat
Conduction (e.g. movement of air mass over a cold surface)
Radiation (e.g. cooling of boundary layer air by longwave radiation)
Adiabatic processes - no addition/removal of heat
Add water vapor to air
Mix warm air with cold air
Cooling of air parcel when it rises (because air parcel expands when it rises, like a balloon)
1st Law of Thermodynamics  expanding air cools, compressed warms (like a manual hand air pump).

10. Forms of Condensation:
saturation  droplets or ice crystals
condensation/deposition  dew, frost, fog, clouds

11. Forms of Condensation:
Dew
liquid condensation on surface
occurs early morning on windless cloudless days
air immediately above ground cools, reaches dew point
diabatic process

12. Frost deposits white ice crystals  known as hoar frost
~ similar to dew BUT saturation occurs below 0oC
deposits white ice crystals  known as hoar frost
e.g. car windshield
phase change from vapor directly to solid (deposition)
diabatic process

13. Frozen Dew
results when saturation occurs slightly above 0oC  liquid dew
formed, when Temp drops liquid dew freezes
forms thin sheet of ice, tightly bound to surface
dangerous – black ice

14. Fog air has either been: cooled to dew point had moisture added
can be considered a cloud with base at ground level
air has either been:
cooled to dew point
had moisture added
mixed with warm moist air
5 different types
radiation
advection
upslope
precipitation
steam

15. Radiation Fog longwave radiation  reaches Dew Pt
occurs when near surface air chills diabatically through loss of
longwave radiation  reaches Dew Pt
requires cloudless nights and light wind to create mixed layer
‘burns’ off with sunrise – evaporates from below due to surface
heating

16. Advection Fog occurs when warm moist air moves across a cooler surface
air is chilled diabatically to saturation
common on the U.S. west coast  warm, moist air from
Pacific advects over the cold California current
Frequently develop near boundaries of opposing ocean temperatures
e.g: northeast coast of the U.S., Gulf Stream
and Labrador current

17. Upslope Fog
develops due to adiabatic cooling
occurs when air is lifted over topographic barriers, mountains
air expands and cools as it rises
common in region between Great Plains and Rocky Mountain foothills

18. Precipitation Fog
Rain occurs and some evaporates as it falls toward Earth
Sometimes this will lead to saturation near surface and cause fog
Adiabatic process
Weather doctor almanac 2002

19. Steam Fog Adiabatic process (no net change of energy)
Mixing of warm, moist air with cold air
Adiabatic process (no net change of energy)
e.g., common when cold air move over warm lakes/streams in autumn
Steam Fog – can see plumes rising
Weather doctor almanac 2002

20. Different types of fog found throughout the U.S.

22. Summary
Water Vapor Basics (names of different phase changes, latent heat)
Humidity indices (there are 6 total). Saturation vapor pressure increases non-linearly with temperature
Two methods of achieving saturation and condensation (diabatic vs. adiabatic processes). Different types of condensation - dew, frost, fog (radiation, advection, upslope, precipitation, steam), clouds.