BISL 04-Weather and Climate

Normal visibility

6 miles

(10 km)

ADVECTION FOG

Formed when a mass of humid

and cool air moves over a surface

that is colder than the air

RADIATION FOG

This fog appears only on the ground

and is caused by radiation cooling

of the Earth's surface.

FRONTAL FOG

Formed ahead of a

warm front

The air becomes

saturated as it

ascends.

The densest fog affects visibility

at this distance and has

repercussions on car, boat, and

airplane traffic. In many cases,

visibility can be zero.

160 feet

(50 m)

ASCENDING

AIR

FOG

BLOCKEDFOG

FOG

FOG

1. 
   

2. 
   

4. 
   

3. 
   

Mist

Mist consists of salt and other dry

particles imperceptible to the

naked eye. When the concentration

of these particles is very high, the

clarity, color, texture, and form of

objects we see are diminished.

660 feet

(200 m)

0.6 mile

(1 km)

1.2 miles

(2 km)

1.9 miles

(3 km)

160 feet

(50 m)

DENSE

FOG

Means of
transport
are affected
by visibility.

THICK

FOG

FOG MIST

W

hen atmospheric water vapor condenses near the ground, it forms fog and mist. The

fog consists of small droplets of water mixed with smoke and dust particles. Physically

the fog is a cloud, but the difference between the two lies in their formation. A cloud

develops when the air rises and cools, whereas fog forms when the air is in contact with the

ground, which cools it and condenses the water vapor. The atmospheric phenomenon of

fog decreases visibility to distances of less than 1 mile (1.6 km) and can affect

ground, maritime, and air traffic. When the fog is light, it is called mist.

In this case, visibility is reduced to 2 miles (3.2 km).

44 METEOROLOGICAL PHENOMENA

Orographic barrier

Fog develops on lee-side mountain

slopes at high altitudes and occurs

when the air becomes saturated

with moisture.

OROGRAPHIC

FOG

Dew

The condensation of water

vapor on objects that have

radiated enough heat to

decrease their temperature

below the dew point

Wind

Warm air

High

landmasses

Types of Fog

Radiation fog forms during cold nights

when the land loses the heat that was

absorbed during the day. Frontal fog forms when

water that is falling has a higher temperature

than the surrounding air; the drops of rain

evaporate, and the air tends to become saturated.

These fogs are thick and persistent. Advection

fog occurs when humid, warm air flows over a

surface so cold that it causes the water vapor

from the air to condense.

Fog and Visibility

Visibility is defined as a measure of an observer's

ability to recognize objects at a distance through the

atmosphere. It is expressed in miles and indicates the visual

limit imposed by the presence of fog, mist, dust, smoke, or

any type of artificial or natural precipitation in the

atmosphere. The different degrees of fog density have

various effects on maritime, land, and air traffic.

Lost in the Fog

INVERSION FOG

When a current of warm, humid air

flows over the cold water of an ocean or

lake, an inversion fog can form. The

warm air is cooled by the water, and its

moisture condenses into droplets. The

warm air traps the cooled air below it,

near the surface. High coastal

landmasses prevent this type of fog

from penetrating very far inland.

WEATHER AND CLIMATE 45