Dew, frost and fogs
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)
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
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.
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.
Indices of Water Vapor Content
•
•
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.
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
mv
q=
mv + md
4.
Mixing Ratio: amount of water vapor (g) relative only to mass of
dry air (kg).
Saturation mixing ratio: maximum mixing ratio
r=
5.
Relative Humidity: the amount of water vapor in the air relative
to the possible maximum.
RH=q/qsx100
6.
Dew point temperature: temperature at which saturation occurs in
air (generally colder than Tair, equals to Tair when saturated)
mv
md
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.
Video: Water cycle
• http://www.youtube.com/watch?v=9e1ccP
2P0Fk
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).
- pDa = cv DT
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
Radiation Fog
• 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
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
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
Steam Fog – can see plumes rising
Weather doctor almanac 2002
Different types of fog found throughout the U.S.
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.