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 o C 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 o C 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 o C 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 o C. Dew point temperature less than 0 o C 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 o C and far left at -10 o C 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.