Chapter 9 Flashcards
Fog is generally considered as
an atmospheric hazard
Fog is a form of
condensation such as dew, frost and clouds but it is formed above the ground at low levels.
The essential deference between fog and cloud is
the method and place of formation.
Clouds need an
upward motion to form
. Clouds need an upward motion to form. The lifting process causes
the parcel of air to lift to a higher levels and cools adiabatically to condense.
Fog results from
cooling or by the addition of enough water vapor to cause saturation.
Fog results from cooling or by the addition of enough water vapor to cause saturation. Fog needs a
light to moderate wind to form
Fog results from cooling or by the addition of enough water vapor to cause saturation. Fog needs a light to moderate wind to form. Strong wind
disperses the air before it reaches saturation
1- Fog formed by cooling:
- Radiation fog
- Advection fog
- Upslope fog
Fog formed by cooling: (how it forms)
When temperature of a layer of air in contact with the ground falls below its dew point condensation produces fog
Radiation fog (conditions)
radiation cooling of the ground and adjacent air.
In clear nights and fairy high relative humidity the continuous cooling may produce fog in early morning.
A necessary condition to form a fog is a light wind. A light wind of 3-4km/h creates enough turbulence to carry the fog upward 10 to 30 meters above the ground without dispersing it
Advection fog (conditions)
When warm and moist air is blown over a cold surface, it becomes chilled by contact and to a certain extent, by mixing with the cold air created by cold surface below. If cooling is sufficient the result is advection fog.
Advection fog needs a wind of 10-30 km/h, because that advection fog is often thick may have a thickness of 300-600 m.
Radiation fog may form at anytime of a day and persist for many hours.
Upslope fog:
Upslope fog is created when relatively humid air moves up a gradual sloping plain or in some cases, up the steep slopes of a mountain because of the upward movement, air expands and cools adiabatically. In a stable air if the dew point is reached an extensive layer of fog may form.
Fog formed by evaporation:
- Steam fog
- Frontal fog
Fog formed by evaporation (how it forms)
Fog may form when saturation occurs primarily because of the addition of water vapor. The resulting fog is called evaporation fog
Steam fog
When cool air moves over warm water enough moisture may evaporate from the water surface to saturate the air immediately above
Frontal fog:
When precipitations fall passing through the frontal slope, part of these precipitations evaporates in cold air below the frontal slope. If the cold air near saturation, the evaporation may reach saturation and fog may form. This type of fog is called frontal or precipitation fog.
Frost and dew:
These phenomena are forms of
condensation at the ground
Frost and dew:
These phenomena are forms of condensation at the ground. They are generally results from
radiation cooling on clear, cold nights
Dew is
the condensation of water vapor on objects that have radiated sufficient heat to lower their temperature below the dew point of the surrounding air.
Because different objects radiate heat at different rates dew may
form on some surfaces but not on others.
Dew is more frequent on
grass because the transpiration of water vapor by the blades rises the relative humidity to higher levels directly above the grass.
Dew is an important source of
of moisture in dry regions. During summer time it is very necessary.
The conditions of dew formation are:
- clear, cold nights; 2. calm wind
Frost has similar conditions of formation as
dew except that it forms when the temperature falls below freezing point
Frost has similar conditions of formation as dew except that it forms when the temperature falls below freezing point, thus frost forms when
water vapor changes directly from gas into solid without entering the liquid state. This process of changing state is called deposition
Frost has similar conditions of formation as dew except that it forms when the temperature falls below freezing point, thus frost forms when water vapor changes directly from gas into solid without entering the liquid state. This process of changing state is called deposition. It produces
delicate patterns of ice crystals that frequently decorate windows in winter
Types of frost:
- Radiation frost
- Advection frost
Radiation frost forms under these conditions:
- sufficient cooling at clear nights;
- the temperature falls below 0oC; and
- calm to light wind.
Radiation frost forms in
valleys early morning, when the air is stable (in clear, cold nights).
Advection frost:
It forms when a cold air mass moves toward a dry surface. It forms anytime on mountain slope. A mixing of the two types of frost may occur.
