In this article, I want to walk you through the climatology topic: Cloud Formation & Types Of Clouds for UPSC IAS Examination.
Meteorologists use the term ‘parcel’ to define a small body of air. Temperature and humidity conditions define a parcel’s ability to take off from the ground. Stability refers to the tendency of a parcel of air with its water vapor to either remain stationary or to ascend or descend. A stationary parcel is called Stable while an Ascending or descending parcel is called unstable.
An air parcel is considered unstable when it continues to rise until it reaches an altitude where the surrounding air has density similar to its own.
As a parcel ascends its pressure decreases with height. Due to a decrease in pressure, there is a drop in the temperature of the air parcel. As there is no external heat exchange, the process is called the Adiabatic process. As the temperature is reduced, it is called adiabatic cooling.
The point to be remembered is that it is different from the environmental lapse rate. In the case of the environmental lapse rate, the temperature decrease with an increase in altitude but the air is not moving from its place. In the case of Adiabatic cooling, the air parcel itself is moving and there is a drop of temperature in an air parcel. The rate of change of temperature is called the Adiabatic rate of cooling.
Similarly, as the parcel descends, the pressure in parcel increases hence there is an increase in temperature. This is called Adiabatic heating.
Condition for instability: When the Adiabatic lapse rate of cooling is lower than the local lapse rate, there is a condition for unstable air.
Cloud formation occurs when there is unstable air condition. As the air rises it starts cooling adiabatically. As the humidity present in it is in vapour form, it is called dry adiabatic rate. As the temperature reaches the dew point, the vapour starts condensing into small droplets forming clouds. The height which the condensation starts is called condensation limit. If the parcel is still unstable it will rise more, now because liquid water is present, it is wet adiabatic cooling. The wet adiabatic rate is higher than dry adiabatic rate.
- Clouds form when the invisible water vapor in the air condenses into visible water droplets or ice crystals.
- There is water around us all the time in the form of tiny gas particles, also known as water vapour.
- There are also tiny particles floating around in the air – such as salt and dust – these are called aerosols.
- The water vapour and the aerosols are constantly bumping into each other.
- When the air is cooled, some of the water vapour sticks to the aerosols when they collide – this is condensation.
- Eventually, bigger water droplets form around the aerosol particles, and these water droplets start sticking together with other droplets, forming clouds.
- Clouds form when the air is saturated and cannot hold any more water vapour, this can happen in two ways:
- The amount of water in the air has increased – for example through evaporation – to the point that the air cannot hold any more water.
- The air is cooled to its dew point – the point where condensation occurs – and the air is unable to hold any more water.
- The warmer the air is, the more water vapour it can hold.
- Clouds are usually produced through condensation – as the air rises, it will cool and reducing the temperature of the air decreases its ability to hold water vapour so that condensation occurs.
- The height at which dew point is reached and clouds form is called the condensation level.
What causes clouds to form?
There are five factors that can lead to air rising and cooling and clouds forming.
- Surface heating
- Orographic barrier
1. Surface heating – This happens when the ground is heated by the sun which heats the air in contact with it causing it to rise. The rising columns are often called thermals. Surface heating tends to produce cumulus clouds.
2. Topography or orographic forcing – The topography – or shape and features of the area – can cause clouds to be formed. When air is forced to rise over a barrier of mountains or hills it cools as it rises. Layered clouds are often produced this way.
3. Frontal – Clouds are formed when a mass of warm air rises up over a mass of cold, dense air over large areas along fronts. A ‘front’ is the boundary between warm, moist air and cooler, drier air.
4. Convergence – Streams of air flowing from different directions are forced to rise where they flow together, or converge. This can cause cumulus cloud and showery conditions.
5. Turbulence – A sudden change in wind speed with height creating turbulent eddies in the air.
The range of ways in which clouds can be formed and the variable nature of the atmosphere results in an enormous variety of shapes, sizes and textures of clouds.
Types Of Clouds
There are four basic cloud categories observed in our atmosphere –
The names for clouds are usually are combinations of the following prefixes or suffixes:
- Stratus/strato = flat/layered and smooth
- Cumulus/cumulo = heaped up/puffy, like cauliflower
- Cirrus/cirro = High up/wispy
- Alto = Medium level
- Nimbus/Nimbo = Rain-bearing cloud
A combination of these four basic types can give rise to the following types of clouds:
|Classification of clouds||Types of clouds|
|High clouds||Cirrus, Cirrostratus, Cirrocumulus|
|Middle clouds||Altostratus, Altocumulus|
|Low clouds||Stratocumulus, Stratus, Nimbostratus|
|Clouds with extensive vertical development||Cumulus, Cumulonimbus|
High Altitude clouds: These are found 20,000ft or higher above the land surface. Cirrus, Cirrostratus, and Cirrocumulus are the cloud types found here.
Middle Altitude Clouds: These are found between 6,500ft to 20,000ft above the land surface. Altostratus and Altocumulus are the cloud types found here.
Low Altitude Clouds: These cloud types can be found from ground level to about 6,500ft above it. They include Stratus, Stratocumulus, and Nimbostratus clouds.
Vertical Clouds: These are clouds that extend from the lower to the higher altitude s of the atmosphere. They form by thermal convection or frontal lifting, sustained by the powerful convectional current that holds and pushes the moisture in the clouds further upward. An example of a vertical cloud is the Cumulonimbus cloud.
Foggy Clouds: These form close to the ground. Sometimes they make visibility very poor such that you can hardly see more than 60 away.
- Detached clouds in the form of white, delicate filaments, mostly white patches or narrow bands.
- They may have a fibrous (hair-like) and/or silky sheen appearance.
- Cirrus clouds are always composed of ice crystals, and their transparent character depends upon the degree of separation of the crystals.
- Transparent, whitish veil clouds with a fibrous (hair-like) or smooth appearance.
- A sheet of cirrostratus which is very extensive, nearly always ends by covering the whole sky.
- A milky veil of fog (or thin Stratus) is distinguished from a veil of Cirrostratus of a similar appearance by the halo phenomena which the sun or the moon nearly always produces in a layer of cirrostratus.
- Thin, white patch, sheet, or layered of clouds without shading.
- They are composed of very small elements in the form of more or less regularly arranged grains or ripples.
- Grey or bluish cloud sheets or layers of striated or fibrous clouds that totally or partially covers the sky.
- They are thin enough to regularly reveal the sun as if seen through ground glass.
- Altostratus clouds do not produce a halo phenomenon nor are the shadows of objects on the ground visible.
- White and/or grey patch, sheet or layered clouds, generally composed of laminae (plates), rounded masses or rolls.
- They may be partly fibrous or diffuse.
- When the edge or a thin semi-transparent patch of altocumulus passes in front of the sun or moon a corona appears.
- This colored ring has red on the outside and blue inside and occurs within a few degrees of the sun or moon.
- The continuous rain cloud. Resulting from thickening Altostratus, This is a dark grey cloud layer diffused by falling rain or snow.
- It is thick enough throughout to blot out the sun.
- The cloud base lowers into the low level of clouds as precipitation continue.
- Grey or whitish patch, sheet, or layered clouds which almost always have dark tessellations (honeycomb appearance), rounded masses or rolls.
- Except for virga they are non-fibrous and may or may not be merged.
- A generally grey cloud layer with a uniform base which may, if thick enough, produce drizzle, ice prisms, or snow grains.
- When the sun is visible through this cloud, its outline is clearly discernible.
- Often when a layer of Stratus breaks up and dissipates blue sky is seen.
- Detached, generally dense clouds and with sharp outlines that develop vertically in the form of rising mounds, domes, or towers with bulging upper parts often resembling a cauliflower.
- The sunlit parts of these clouds are mostly brilliant white while their bases are relatively dark and horizontal.
- The thunderstorm cloud, this is a heavy and dense cloud in the form of a mountain or huge tower. The upper portion is usually smoothed, fibrous or striated and nearly always flattened in the shape of an anvil or vast plume.
- Under the base of this cloud which is often very dark, there are often low ragged clouds that may or may not merge with the base.
- Cumulonimbus clouds also produce hail and tornadoes.