this is for just pilots who are doing CPL & ATPL.

1. FOG & CLOUDS
CAPT FURQAN HUSSAI

2. INTRODUCTION:
• A cloud is made up of water droplets or ice crystals suspended in the air. Cloud is defined as a
visible aggregation of minute water droplets and/or ice particles in the air, usually above the
ground level. These particles have a diameter ranging from 20 to 50µ.
• Clouds are the most important form of suspended water droplets caused by condensation. If these
are brought down to the ground, these would exactly look like fog. Conversely, a fog raised above
the ground would appear to be a cloud. The clouds are produced when the air above the ground is
cooled below its dew point.

3. • The cooling may come through many processes, but the rising air is usually involved in their formation. If
the movement of the air is generally horizontal, the clouds will be formed in layers, and are called strati-
form clouds.
• If the movement is vertical, then these are designated as cumuliform. As convectional currents are limited
to the troposphere only, so this part of the atmosphere contains all clouds.
• Weather of any area in the world is directly associated with clouds. All types of precipitation are caused by
the clouds. Although all the clouds may not produce precipitation, but they play a significant role in
modifying the weather of a given area.
• The type of precipitation depends upon the type of cloud. The type and height of clouds vary from the
tropical areas to polar areas.

4. • Heat Energy and Clouds:
• Heat energy of any area is greatly influenced by the clouds. Solar radiation is absorbed by the
clouds. Some part of the solar radiation is reflected back to the space by the clouds, whereas some
part of the radiation is diffused.
• Some part of terrestrial radiation is also absorbed by the clouds and simultaneously terrestrial
radiation is re-radiated back to the earth’s surface. Clouds behave like a black body. Heat energy
radiated by the clouds depends upon the temperature of the clouds.

5. CAUSE OF CLOUD FORMATION
• Clouds form when air rises and cools adiabatically. If rising air cools to its dewpoint, the water vapour
will condense out as water droplets.
• The height at which this occurs is the condensation level. This is also the level the cloud base occurs.
• There are several different lifting processes that can lead to cloud formation.
They are as follows:
• 1. Turbulence
• 2. Convection
• 3. Orographic uplift
• 4. Frontal uplift

6. • Turbulence: Strong winds blowing over uneven ground strike against the various obstructions and the air
gets deflected upwards. This causes a thorough mixing of the air and as the air rises, it cools adiabatically and if
cooled below its dew point, clouds will form. These clouds will be of an even layer type (stratus) and their bases
will generally be not more than 600m high. Turbulence clouds will be formed in great quantities if the air is
maritime (moisture laden) and blows Over cold uneven ground. Over sea, wind speeds of more than 13 knots
only can produce sea waves of sufficient height to create
turbulence clouds.

7. • Orographic lifting: When a warm, moist wind blows against a mountain range, it begins to
climb up the mountainside. During this ascent, it cools adiabatically and after cooling below its dew
point orographic clouds are formed. These are of the stratus type. If the mountain is quite high,
further ascent results in nimbostratus and continuous precipitation. On the windward side of the
mountain peak, clouds are forming steadily whereas on the leeward side, the clouds are dissipating
at the same rate (while descending,

9. • Convection: When a parcel of air gets heated, due
to any local cause, it expands, becomes less dense than
surrounding air, and rises. This is called convection
current. This local heating can be caused by contact
with warm sea or ground. So long as the parcel is
warmer than the surrounding air at each level, it will
continue to rise (unstable condition of air).

10. • Frontal lifting: Where a warm air mass and a cold air mass are in contact, their line of separation,
at sea level, is called a front. The boundary between them is not vertical. It is inclined towards the
colder air mass because the cold air, being denser, acts like a wedge and lifts up the warm air.
• In the case of a warm front, the slope is gradual and the up-sliding warm air forms stratiform clouds –
nimbostratus, altostratus, cirrostratus and finally cirrus.
• In the case of a cold front, the slope is very steep and the up-sliding warm air forms cumulonimbus and
cumulus clouds.

11. CLASSIFICATION:
• Clouds are classified on the basis of their height, shape, colour and transmission or reflection of light.
There are three basic cloud forms: cirrus (feathery or fibrous), stratus (stratified or in layers) and
cumulus (in heaps). Different forms of clouds are either pure forms or modifications and combinations
of them at different elevations.
• High level clouds = cirrus
• Medium level clouds = stratus
• Low level clouds = cumulus

13. HIGH LEVEL CLOUD:
• Cirrus:-
• Cirrus clouds are delicate, feathery clouds that are made
mostly of ice crystals. Their wispy shape comes from wind
currents which twist and spread the ice crystals into
strands.
Weather prediction: A change is on its way!

14. HIGH LEVEL CLOUDS:
• Cirrostratus:
• Cirrostratus clouds are thin, white clouds that cover the
whole sky like a veil. These clouds are most commonly
seen in the winter, and can cause the appearance of a halo
around the sun or the moon.
Weather prediction: Rain or snow will arrive within 24
hours!

15. HIGH LEVEL CLOUD:
• Cirrocumulus clouds : are thin,
sometimes patchy, sheet-like clouds. They
sometimes look like they’re full of ripples or are
made of small grains.
Weather prediction: Fair, but cold. However, if
you live in a tropical region, these clouds could
be a sign of an approaching hurricane!

16. MEDIUM LEVEL CLOUDS:
• Altocumulus clouds:
• Altocumulus clouds have several patchy white or gray
layers, and seem to be made up of many small rows of
fluffy ripples. They are lower than cirrus clouds, but still
quite high. They are made of liquid water, but they
don’t often produce rain.
Weather prediction: Fair

17. MEDIUM LEVEL CLOUDS:
• Altostratus clouds:
• Altostratus clouds are gray or blue-gray mid-level
clouds composed of ice crystals and water
droplets. The clouds usually cover the entire sky.
Weather prediction: Be prepared for continuous
rain or snow!

18. MEDIUM LEVEL CLOUDS:
• Nimbostratus clouds:
Nimbostratus clouds are dark, gray clouds that seem to
fade into falling rain or snow. They are so thick that they
often blot out the sunlight.
Weather prediction: Gloomy with continuous rain or
snow

19. LOW LEVEL CLOUDS:
• Cumulus clouds:
• Cumulus clouds look like fluffy, white cotton
balls in the sky. They are beautiful in sunsets,
and their varying sizes and shapes can make
them fun to observe!
Weather prediction: Fair

20. LOW LEVEL CLOUDS:
• Stratus cloud:
• Stratus cloud often look like thin, white sheets covering the
whole sky. Since they are so thin, they seldom produce much
rain or snow. Sometimes, in the mountains or hills, these
clouds appear to be fog.
Weather prediction: Fair, but gloomy

21. LOW LEVEL CLOUDS:
• Stratocumulus clouds:
• Stratocumulus clouds are patchy gray or white clouds
that often have a dark honeycomb-like appearance.
Weather prediction: Fair weather for now, but a storm
might be on its way.

22. CLOUD WITH VERTICAL DEVELOPMENT:
• Cumulonimbus clouds.
• Mass of heavy, grey cloud with its base at low
cloud level, having great vertical extent and its top
well in high cloud level. It has a threatening
appearance and is called a thundercloud. grow on
hot days when warm, wet air rises very high into
the sky. From far away, they look like huge
mountains or towers.
Weather prediction: Look out for rain, hail, and
tornadoes!

23. WHAT IS FOG:
• Fog is often described as a stratus cloud resting near the ground. Fog forms when the temperature
and dew point of the air approach the same value (i.e., dew-point spread is less than 5°F) either
through cooling of the air (producing advection, radiation, or upslope fog) or by adding enough
moisture to raise the dew point (producing steam or frontal fog). When composed of ice crystals, it is
called ice fog.
• We have few types of fog

24. • Fog Characteristics
• A general summary of characteristics important to fog formation and dissipation are given here. This
is followed by general fog forecasting guidance and guidance specific to advection, radiation, and
frontal fogs. (1) Formation. Fog forms by increasing moisture and/or cooling the air. Moisture is
increased by the following: • Precipitation. • Evaporation from wet surfaces. • Moisture advection.
Cooling of the air results from the following: • Radiational cooling. • Advection over a cold surface. •
Upslope flow. • Evaporation.

25. TYPES OF FOG:
• Radiation Fog: This fog forms when all solar energy exits the earth and allows the
temperature to meet up with the dew point. The best condition to have radiation fog is when
it had rained the previous night. This help to moisten up the soil and create higher dew
points. This makes it easier for the air to become saturated and form fog. However, the winds
must be light less than 15 mph to prevent moist and dry from mixing.

26. • Advection fog: Advection fog forms due to moist air moving over a colder surface, and the
resulting cooling of the near-surface air to below its dew-point temperature. Advection fog occurs over
both water (e.g., steam fog) and land.
• Upslope Fog: This fog forms adiabatically. Adiabatically is the process that causes sinking air to
warm and rising air to cool. As moist winds blow toward a mountain, it up glides and this causes the air
to rise and cool. The cooling of the air from rising causes to meet up with the dew point temperature.
Fog forms on top of the mountains.

27. • Frontal fog. Associated with frontal zones and frontal passages, this type of fog can be divided into
three types: warm-front pre-frontal fog; cold front post-frontal fog; and frontal-passage fog. Pre and post-
frontal fog are caused by rain falling into cold stable air thus raising the dew point.
• Frontal passage fog can occur in a number of situations: when warm and cold air masses, each near
saturation, are mixed by very light winds in the frontal zone; when relatively warm air is suddenly cooled
over moist ground with the passage of a well marked precipitation cold front.
• Valley Fog: Valley fog forms in the valley when the soil is moist from previous rainfall. As the skies
clear solar energy exits earth and allow the temperature to cool near or at the dew point. This form deep
fog, so dense it's sometimes called tule fog

28. LAND & SEA BREEZE:
• Land breeze: blowing breeze from land towards the sea is called a land breeze.
• They are formed during the night when sea water and land both lose heat, specific heat
capacity of land being very low as compared to that of sea water, land loses heat energy fast
and cools more rapidly as compared to the sea. Sea water being at higher temperature, the
air becomes lighter and rises up. Air from land being at higher pressure. So air from land
starts blowing towards the sea and gives rise to a land breeze.

29. LAND & SEA BREEZE:
• Sea breeze: blowing breeze from sea towards land during the day is called sea breeze.
• They are formed during the day time when land and sea both are heated equally by the sun,
but land has very low specific heat capacity as compared to the sea, so it is heated up more
quickly, thus air above land due to heat becomes lighter and rises up. Thus pressure
decreases and cold and humid air above the sea blowing towards land.