Transcript Fog&Dew

Dew forms as the temperature drops to the dew point. Water droplets
love to collect on surfaces or tiny particles. It is very difficult for water
drop to join together without having a surface or tiny particle to cling
onto. These particles are called condensation nuclei
The air temperature starts out at 20 oC and less than 50% relative
humidity. During the evening the temperature drops until the
dew point is reached and then dew begins to form.
The air temperature starts out at 20 oC and less than 50% relative
humidity. During the evening the temperature drops until the dew point
is reached and then dew begins to form. The temperature continues to
drop until the dew becomes frozen.
The air temperature starts out at 20 oC and less than 50% relative
humidity. During the evening the temperature drops until the
dew point is reached and then frost begins to form. In this case
the dew point is less than 0 oC. Dew point temperature less than
0 oC are often referred to as the frost point.
Black Frost is really the absence of any frost or frozen dew.
Night time temperatures below freezing but with no visible
frost or frozen dew.
The data in the chart above represent the dew-point temperature and expected
minimum temperature near the ground for various clear mornings in a small town.
Assume that the dew point remains constant throughout the night.
a. on which morning would there be the greatest likelihood of observing visible frost?
b. on which morning would there be the greatest likelihood of observing frozen dew?
c. on which morning would there be the greatest likelihood of black frost with no
visible frost, dew, or frozen dew?
d. on which morning would there be the greatest likelihood of observing only dew?
e. on which morning would there be the greatest likelihood of no visible frost, dew, or
frozen dew and no black frost?
Answers on next slide. Try to figure it out first.
The data in the chart above represent the dew-point temperature and expected minimum temperature near the
ground for various clear mornings in a small town. Assume that the dew point remains constant throughout
the night.
a. on which morning would there be the greatest likelihood of observing visible frost? Morning 4
b. on which morning would there be the greatest likelihood of observing frozen dew? Morning 3
c. on which morning would there be the greatest likelihood of black frost with no visible frost, dew, or frozen
dew? Morning 2
d. on which morning would there be the greatest likelihood of observing only dew? Morning 5
e. on which morning would there be the greatest likelihood of no visible frost, dew, or frozen dew and no
black frost? Morning 1
Hygroscopic (attrack water very much)aerosols can result in wet
haze formation with as little as 75 % RH
Haze
Hygroscopic aerosols Wet Haze with
less than 75 % RH
Condensation nuclei needed to
form fog or cloud. These are essential
for cloud or fog formation.
Hydrophobic aerosols (repel water)
Provided there are condensation nuclei available in the air fog
will for when temperatures drop to the dew point.
The west coast is well known for the large number of foggy days each
year. West coast fog is primarily advection fog and forms when the
warm ocean air moves horizontally toward the coast within and cools
by contact with the very cold waters right along the coast.
New England also experience advection fog as the warm air
above the Gulf stream encounters the cool Labrador ocean
currents right along the northern New England coast.
Fog: A suspension of small visible water
droplets (or ice crystals) in the air that
reduce visibility at the earth's to less than
1,000 meters (5/8 miles).
Cloud or fog form when air temperature drop to the dew point AND
there are condensation nuclei present.
Radiation Fog or Valley Fog
Radiation fog form by radiative cooling. Usually on clear calm nights.
Radiation Fog or Valley Fog
Central Vsalley in California. Socked in
Advection Fog
Common fog on west coast of US
Warm moist air confined to a shallow marine layer passes over cold
water next to the coastline. This cools the warm moist air and
results in fog formation. Sea salt are the likely condensation nuclei.
Another form of advection fog as warm moist air travels over snow.
The warm moist air also cools as it move to higher elevations and
expands.
Rising air naturally cools as it expands adiabatically.
Up Slope Fog
The warm moist air cools as it move to higher elevations and expands.
Other Types of fogs:
* acid fog -- radiation fog that forms in polluted air with
oxides of sulfur and nitrogen.
* evaporation (mixing) fog -- mixing of two unsaturated air
masses (steam fog, winter breath, arctic sea smoke,
steamy pavement).
* frontal fog -- a type of evaporation fog; moist air mixes
with cooler air; can occur where warm air rides up
over cold air (warm front).
Two unsaturated air parcels far right at 30 oC and far left at -10 oC mix.
After mixing the relative humidity reaches 100% and fog forms. This
is the type of fog that comes out of your mouth on a cold day.
Two unsaturated air parcels A and B mix.After mixing the relative
humidity reaches 100% and fog forms. This is the type of fog that
comes out of your mouth on a cold day.
Air flow tends to be directed perpendicular to the coast
line. This gives rise to cloudy conditions near the
headlands and clear near inlet beaches.
Little Summer enjoying the view of Headland clouds and clear
inlet beach on the Oregon coast.
Mixing drier air with fog can help dissapate the fog.