Maarg Manthan · Topic 5.15

Formation of Dew, Frost, Fog and Mist

Geography › Climatology · Topic 5.15

Dew, frost, fog and mist are forms of condensation near the ground, produced when air is cooled to its dew point. Dew forms on cold surfaces on clear, calm nights. Frost forms when the dew point is below freezing and vapour changes directly into ice. Fog is a cloud at ground level with visibility below one kilometre, and mist is a thinner layer. They form by radiation cooling at night, by moist air moving over a cold surface, or by air rising up a slope, and are common in winter in the Indo-Gangetic plain.

6 min read · UPSC Prelims · Geography · Climatology

At a Glance

Dew Water drops on cold surfaces
Frost Ice deposits when the dew point is below 0 °C
Fog Cloud at ground, visibility under 1 km
Mist Thinner than fog
Common type Radiation fog in winter
Exam link UPSC Prelims, Geography: Climatology

Where Does It Fit?

This topic follows the topic on humidity and condensation and applies it to the ground. It leads to the topic on clouds, which are the same process happening higher in the air. Previous topic: Humidity, Saturation, Dew Point, Evaporation and Condensation. Next topic: Types of Clouds: Cirrus, Cumulus, Stratus, Nimbus and Their Combinations.

How Do Dew and Frost Form?

In short: dew and frost form on cold surfaces when the air next to them is cooled to its dew point.

Form	Conditions	What happens
Dew	Clear, calm, cool night; moist air; temperature of the surface falls to the dew point above 0 °C	The ground loses heat by radiation and cools the air touching it; water vapour condenses as small drops on grass, leaves and stones
White frost (hoar frost)	Dew point below freezing, so condensation occurs directly as ice	Vapour turns directly into ice crystals (deposition), with no liquid stage
Black frost	Air so dry that the temperature falls below freezing but vapour does not condense	Plant tissues freeze and turn black without any visible frost, which is the most damaging kind
Rime	Fog droplets freeze on contact with cold surfaces	A rough, white deposit on poles, wires and trees

Dew needs a surface that cools well, so metal and grass get it first. A cloudy sky or wind prevents dew because clouds return heat to the ground and wind mixes the cold layer away. Frost harms crops such as potato, mustard, peas and fruit trees, and farmers in the north Indian plains in December and January use irrigation or smoke to guard their crops.

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 Inversion of Temperature. 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 Do Mist, Fog and Haze Differ?

Term	Meaning	Visibility
Mist	Thin cloud of water droplets at the surface; lighter than fog	Reduced, but above about 1 km
Fog	Dense cloud at ground level, with tiny water droplets suspended in air	Below 1 km; Indian forecasters class very dense fog as below 50 m
Haze	Dry particles of dust, smoke and salt suspended in the air	Reduced, but humidity is lower than for mist
Smog	Mixture of smoke and fog, or polluted haze	Poor, with health risks

Fog is more common than mist when air is cooled quickly over a larger area. Fog is more frequent in areas where there are many particles in the air, since they supply nuclei for droplets.

What Are the Effects of Fog and Frost?

Transport: dense winter fog on the Indo-Gangetic plain from December to January delays trains, closes airports and increases road accidents. Airports use instrument landing systems for such conditions.
Agriculture: frost damages standing crops, but fog and dew supply moisture in dry seasons. In arid coastal strips, such as the Namib Desert, fog is the main source of water for plants and animals, and fog-harvesting nets are used to collect it.
Pollution: the winter fog mixes with smoke from industry, traffic and crop burning to form smog, which is a health hazard in north India. The same condition is related to temperature inversion in Inversion of Temperature.
Weather forecasting: dew point and fog data indicate how close the air is to saturation, as in Humidity, Saturation, Dew Point, Evaporation and Condensation.

As an analytical point, all these forms of condensation are results of cooling near the ground. Their larger relatives, the clouds, form higher up and are covered in Types of Clouds: Cirrus, Cumulus, Stratus, Nimbus and Their Combinations.

Why Is This Topic Important?

Concepts Dew point and condensation
Hazards Fog and frost
India Winter fog and smog

What Came Next?

The next topic covers the types of clouds. Next topic: Types of Clouds: Cirrus, Cumulus, Stratus, Nimbus and Their Combinations.

Key People and Terms

Dew point Temperature at which air is saturated
Hoar frost Ice crystals deposited from vapour
Radiation fog Fog from ground cooling at night
Advection fog Fog from moist air over a cold surface
Smog Smoke plus fog
Haze Dry particles in the air

Exam Corner

Points to Remember

Dew forms when the temperature falls to the dew point above freezing.
White frost is a deposit of ice crystals.
Fog has visibility below 1 km.
Radiation fog forms on clear, calm winter nights.
Black frost is dry freezing without visible ice.

Do Not Confuse With

Fog and mist Dense versus thin
Fog and haze Water droplets versus dry particles
Dew and frost Liquid drops versus ice

Memory Hook

Dew by cooling, frost by freezing; fog is cloud on the ground, mist is thin.

Mains Angle

Questions on winter fog and smog in north India are common. These points add depth.

Cause Cooling, moisture and nuclei
Effects Transport and crops
Remedy Forecasting and pollution control

A Question You May Face

An original practice question, not a past paper question.

“Explain the formation of dew, frost and fog and discuss the effects of winter fog in the Indo-Gangetic plain.”

How to Answer

Process Cooling to dew point.
Types Dew, frost, fog, mist.
Effects Transport and crops.
Link Smog and inversion.

GS Relevance

Prelims: conditions for dew and frost, difference between fog, mist and haze, types of fog. GS Paper 1: atmospheric moisture.

Frequently Asked Questions

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.

How is dew formed?

Dew forms on clear, calm nights when the ground loses heat by radiation and cools the air touching it to its dew point, which is above freezing. The water vapour in this air condenses as small drops on cold surfaces such as grass, leaves and stones. Clouds and wind prevent dew.

What is the difference between white frost and black frost?

White frost, also called hoar frost, forms when the dew point is below freezing, so vapour changes directly to ice crystals. Black frost occurs when the air is so dry that the temperature falls below freezing without any condensation, freezing and blackening plant tissues without visible ice.

PYQ Practice — Statement Analysis

1 Cirrus clouds are high clouds made of ice crystals.

True

They lie above about 6 km.

2 Nimbostratus clouds are fair-weather clouds.

False

They bring steady rain.

3 Radiation fog forms on clear, calm nights.