Cloud consists of water droplets or ice crystals in suspension in the atmosphere, produced when excess water vapor is condensed out of saturated air and present in sufficient quantity to produce a visible mass.
2. Reporting of Cloud. For cloud amount reporting the entire sky above a place is considered to be divided into 8 equal sectors and each sector is called Octa. Reporting is done based on the number of Octas of sky which is covered by clouds. The table below is important from perspective of examination.
Nil 1-2 Octa 3-4 Octa 5-6 Octa 7-8 Octa
Fine Fair Partly cloudy Cloudy Sky Obscured
SKC FEW SCT BKN OVC
Sky Clear Few Scattered Broken Overcast
3. Cloud Ceiling. Height of the lowest layer of cloud above ground level covering more than 4 Octa and below 6 Km (20,000 feet)
4. Cloud Base. Height of lowest cloud above ground level.
Note: In weather reports for pilots, cloud height is always reported above ground level and in Met charts it is AMSL.
5. Types of Cloud: Clouds are classified into various category/types based on certain characteristics. Let us see them one by one. First let us categorize them by their height. There are 3 types of cloud as per their height.
(a) Low Clouds (Height below 2 km/6500 feet)
(b) Medium Clouds (Height between 2 to 8 Km/6,500 - 25000 feet)
(c) High Clouds (Height between 6 to18 km/20,000 - 60000 feet)
Low cloud types are Stratus, Nimbostratus and Stratocumulus. Medium cloud types are Altostratus and Altocumulus. High cloud types are Cirrus, Cirrostratus and Cirrocumulus. Alto means Medium and Cirro means high so a prefix of Alto means it is a medium cloud and a prefix of Cirro means it is a high cloud.
Most of the clouding & weather phenomenon in atmosphere is restricted to Troposphere since cloud formation require instability in atmosphere. Atmosphere above troposphere in Stratosphere is stable.
There are two types of clouding which occur at very high altitude above troposphere. First is nacreous clouds (also called mother of Pearl clouds) are seen in upper Stratosphere (11-20 Km) in higher latitudes in winters. Second is Noctilucent clouds which are seen in upper Mesosphere (20 -80 Km) in polar regions.
On the basis of their vertical/horizontal extent clouds are divided into 2 types.
(a) Strati form Clouds: Clouds with large horizontal extent
(b) Cumuli form Clouds: Clouds with large vertical extent.
Clouds are formed due to vertical motion of air, hence they are also classified based on the type of situation which forced the air to rise. The four different reasons for this are often used to categorize the resulting clouds:
(a) Orographic cloud - Formed due to the ascent of air over high ground.
(b) Convection cloud - Formed due to surface heating of the air producing thermal up-currents forcing the air to rise.
(c) Turbulence cloud - Formed due to the air rising due to turbulence.
(d) Frontal cloud - Formed when there is a general rise of air over a wide area due to movement of air fronts of large air masses.
6. Virga. Virga is precipitation descending from the base of a cloud but not reaching the surface.
7. Cloud Formation. The basic reason why clouds form is adiabatic cooling. The rising air is subjected to a steady decrease of atmospheric pressure as it ascends. The decreasing pressure causes the air to expand. The expanding air cools adiabatically. At a certain height unsaturated air will become saturated. The height at which this saturation occurs is called the 'condensation level'. Condensation level is not the same as surface air's dew point temperature. This is due to the fact that as the pressure on air is reduced, so its capacity to hold water vapour at a given temperature increases.
Instrument. Instrument used to measure cloud base is called Ceilometer or Alidade.
8. When the mass of the water/ice droplets can no longer be supported by the air, it falls down on ground in the form of precipitation. Precipitation can be of four types. Drizzle, Rain, Snow, Sleet and Hail.
(a) Drizzle. very small water drops, very close together, so small that their impact on an exposed water surface is imperceptible. Visibility in drizzle drops more than it would drop in Rain.
(b) Rain. is the precipitation of water drops greater than 0.5 mm in diameter. On striking a water surface, raindrops make a distinct splash
(c) Snow is solid precipitation in the form of ice (2-5 mm)
(d) Sleet. either snow that is melting as it falls to earth or a mixture of snow and rain or even drizzle falling together
(e) Hail . comprises precipitation in the form of small balls or chunks of ice which may vary in size from 2 to 5 mm in soft hail (or graupel) to golf ball or tennis ball size in the hail(5-50 mm) ( .) associated with violent thunderstorms,