Clouds, Fog and Forms of Condensation
Geography › Pressure, Winds and Atmospheric Moisture · Topic 7.7
Condensation takes different forms depending on height and temperature. Near the ground it forms dew, frost, mist and fog, and aloft it forms clouds, collections of tiny water droplets or ice crystals. Clouds are classed by height and shape into high clouds such as cirrus, middle clouds such as altostratus, low clouds such as stratus, and vertical clouds such as cumulus and cumulonimbus.
6 min read · UPSC Prelims · Geography · Pressure, Winds and Atmospheric Moisture
At a Glance
- High clouds Above 6 km: cirrus, cirrostratus, cirrocumulus
- Middle clouds 2 to 6 km: altostratus, altocumulus
- Low clouds Below 2 km: stratus, stratocumulus, nimbostratus
- Vertical clouds Cumulus, cumulonimbus
- Fog types Radiation, advection, upslope, steam, frontal
- Exam link UPSC Prelims, Geography: Pressure, Winds and Atmospheric Moisture
Where Does It Fit?
This is the seventh topic of the category on pressure, winds and atmospheric moisture. The previous topic explained how condensation happens, and this one describes the forms it takes. The next and last topic of the category covers precipitation and types of rainfall. Previous topic: Water Vapour, Evaporation and Condensation. Next topic: Precipitation and Types of Rainfall.
What Is a Cloud and How Is It Classified?
In short: a cloud is a mass of tiny water droplets or ice crystals formed where rising air cools below its dew point.
A cloud forms when air rises, expands and cools to its dew point, and the vapour condenses on dust, salt and other particles, called condensation nuclei, as in Water Vapour, Evaporation and Condensation. The droplets are so small, about 0.01 to 0.02 mm, that they float in the air. Clouds are classified by their height and form, following the system proposed by Luke Howard in 1802. The Latin names tell the type: cirrus means a curl of hair, cumulus a heap, stratus a layer and nimbus rain-bearing.
| Group | Height | Clouds | Features |
|---|---|---|---|
| High clouds | Above about 6 km | Cirrus, cirrostratus, cirrocumulus | Thin, wispy, made of ice crystals; cirrus often shows an approaching warm front; cirrostratus can cause a halo around the Sun or Moon |
| Middle clouds | About 2 to 6 km | Altostratus, altocumulus | Greyish sheets or patches of water droplets, may bring light rain |
| Low clouds | Below about 2 km | Stratus, stratocumulus, nimbostratus | Grey, dull layers; stratus brings drizzle and fog-like cover; nimbostratus brings steady rain or snow |
| Clouds of great vertical extent | From low levels to the top of the troposphere | Cumulus, cumulonimbus | Cumulus is a fair-weather heap; cumulonimbus is a towering storm cloud with an anvil top, bringing thunder, lightning, hail and heavy rain |
Which Clouds Are Most Important for Weather?
- Cirrus: thin and feathery at great height; a sign of changing weather.
- Cumulus: puffy, with a flat base; formed by convection on warm days; small ones mean fair weather, but they can grow.
- Cumulonimbus: the thundercloud, which can be 10 to 15 km tall and brings lightning, hail, heavy showers and sometimes tornadoes, as in Thunderstorms, Lightning and Hailstorms.
- Nimbostratus: a thick dark layer that gives long, steady rain, typical of warm fronts, as in Air Masses and Fronts.
- Stratus: low uniform grey layer, like fog lifted off the ground.
Satellites such as INSAT and the Indian weather satellites give images of cloud cover that forecasters at the India Meteorological Department use to track monsoon and cyclone cloud systems.
What Are Dew, Frost, Mist, Fog and Haze?
| Form | How it forms |
|---|---|
| Dew | On a clear, calm night the ground and objects lose heat and cool below the dew point of the air touching them; vapour condenses as droplets on grass and leaves |
| Frost | If the dew point is below freezing, vapour turns directly into ice crystals on surfaces; harmful to crops |
| Mist | Thin cloud at the ground with visibility of more than 1 km |
| Fog | Cloud at ground level, in which visibility is less than 1 km, from condensation on nuclei near the surface |
| Haze | Dry particles of dust and smoke in the air, reducing visibility |
| Smog | Fog mixed with smoke, formed where pollution provides abundant nuclei |
Fog is a cloud that touches the ground. It differs from a cloud only in its height, and it is not rain. It is a significant hazard to road, rail and air transport, and in winter the Indo-Gangetic plain regularly has dense fog that delays flights and trains.
What Are the Types of Fog?
- Radiation fog: forms on clear, calm nights when the ground cools by radiation and chills the air above, as in a temperature inversion, as in Vertical Distribution of Temperature and Temperature Inversion. Common in valleys and plains in winter, and it lifts in the morning. It is the typical fog of the Indo-Gangetic plain.
- Advection fog: forms when warm, moist air moves over a colder surface and is cooled. It is common over coasts and cold currents; the dense fogs of the Grand Banks near Newfoundland occur where the warm Gulf Stream meets the cold Labrador Current.
- Upslope fog: forms when moist air is forced up a slope and cools; common on hills.
- Steam (evaporation) fog: cold air passes over warmer water, as on lakes and rivers in cold mornings in the Arctic.
- Frontal fog: forms along fronts where warm rain falls into cooler air.
As an analytical point, winter fog in north India worsens when the number of pollution particles is large, because more nuclei means more small droplets and thicker fog, so cleaner air can reduce the fog hazard as well as the health hazard.
How Are Clouds Used in Weather Modification?
Cloud seeding is the introduction of particles such as silver iodide, sodium chloride or dry ice into suitable clouds so that they produce more rain or snow. It works only when clouds with enough moisture are already present, and its overall effect is modest and still debated by scientists, so it cannot create rain from clear skies and it is not a substitute for water management. The formation of rain from clouds is discussed in Precipitation and Types of Rainfall.
Why Is This Topic Important?
- Classification Cloud types asked
- Hazards Fog and thunderstorms
- Weather reading Clouds signal weather
What Came Next?
The next topic describes precipitation and the types of rainfall. Next topic: Precipitation and Types of Rainfall.
Key People and Terms
- Cirrus High feathery cloud
- Cumulus Heap cloud
- Stratus Layer cloud
- Cumulonimbus Thundercloud
- Radiation fog Fog from ground cooling
- Advection fog Fog from warm air over cold surface
Exam Corner
Points to Remember
- Cirrus is the highest cloud.
- Cumulonimbus brings thunderstorms and hail.
- Nimbo means rain-bearing.
- Radiation fog is common in the Indo-Gangetic plain in winter.
- Fog has visibility below 1 km.
Do Not Confuse With
- Fog and mist Below versus above 1 km visibility
- Dew and frost Above versus below freezing
- Stratus and cumulus Layer versus heap
Memory Hook
Cirrus curl high, cumulus heap, stratus spread; nimbus pours, cumulonimbus thunders.
Mains Angle
Questions ask for cloud and fog types. These points add depth.
- Height High, middle, low
- Fog Radiation and advection
- Hazard Transport disruption
A Question You May Face
An original practice question, not a past paper question.
“Classify clouds and describe the formation of radiation and advection fog with examples.”
How to Answer
- Clouds By height and form.
- Fog Types.
- Examples Indo-Gangetic plain, Grand Banks.
- Impact Transport and health.
GS Relevance
Frequently Asked Questions
How are clouds classified?
Clouds are classified by height and form into high clouds, such as cirrus, cirrostratus and cirrocumulus, above about 6 km; middle clouds, such as altostratus and altocumulus; low clouds, such as stratus and nimbostratus; and clouds of great vertical extent, cumulus and cumulonimbus.
What is cumulonimbus cloud?
Cumulonimbus is a tall, dense, towering cloud that can reach 10 to 15 km, usually with an anvil-shaped top. It forms by strong convection and brings thunderstorms, lightning, heavy rain, hail and sometimes tornadoes and squalls.
What is the difference between fog and mist?
Both are clouds at ground level formed of tiny water droplets, but fog is dense, with visibility of less than 1 km, while mist is thinner, with visibility of more than 1 km. Haze is made of dry dust and smoke particles and not water droplets.
What is radiation fog?
Radiation fog forms on clear, calm nights when the ground loses heat by radiation and cools the air above to its dew point, so water vapour condenses near the surface. It is common in valleys and in the Indo-Gangetic plain in winter, and often clears after sunrise.
What is advection fog?
Advection fog forms when warm, moist air moves horizontally over a cold surface and is cooled below its dew point. It is common on coasts and over cold ocean currents, as on the Grand Banks near Newfoundland where the warm Gulf Stream meets the cold Labrador Current.
PYQ Practice — Statement Analysis
1 Cirrus clouds are high clouds made of ice crystals.
They lie above about 6 km.
2 Nimbostratus clouds are fair-weather clouds.
They bring steady rain.
3 Radiation fog forms on clear, calm nights.
The ground cools by radiation.
4 Fog and haze are both made of water droplets.
Haze is made of dry dust and smoke particles.
Related Topics
Browse the full Geography index