Dew, frost and fogsDew, frost and fogs

Review of last lecture:Review of last lecture:Components of global water cycleComponents 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)

Water (HWater (H22O ) is unique on earth because it O ) is unique on earth because it

can exist in all 3 states (phases)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

Evaporation and CondensationEvaporation 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.

Sublimation and DepositionSublimation 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.

• Humidity: amount of water vapor in air

• Humidity expressed in a number of ways Indices

1. Vapor Pressure: the partial pressure exerted by water vapor. 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

2. Absolute Humidity: density of water vapor expressed in g/m3

3. 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

4. Mixing Ratio: amount of water vapor (g) relative only to mass of dry air (kg). Saturation mixing ratio: maximum mixing ratio

5. Relative Humidity: the amount of water vapor in the air relative to the possible maximum.

6. Dew point temperature: temperature at which saturation occurs in air (generally colder than Tair, equals to Tair when saturated)

Indices of Water Vapor Content

RH=q/qsx100

Saturation vapor pressureSaturation 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.

Video: Water cycleVideo: Water cycle

• http://www.youtube.com/watch?v=9e1ccP2P0Fk

• 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).

Methods to achieve saturation and condensationMethods to achieve saturation and condensation

Forms of Condensation:

• saturation droplets or ice crystals• condensation/deposition dew, frost, fog, clouds

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

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

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

Fog

• 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

• 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

Radiation 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

Advection Fog

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

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

Steam Fog

• 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

Weather doctor almanac 2002

Steam Fog – can see plumes rising

Different types of fog found throughout the U.S.

• 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.

Summary