Water In The Atmosphere

Water is present in the atmosphere mainly in the form of water vapour.

Water vapour is important because it controls humidity, condensation, cloud formation and precipitation. Even though water vapour forms only a small part of the atmosphere, it plays a major role in weather and climate.

Water Vapour And Humidity

Water present in the atmosphere in gaseous form is called water vapour.

The amount of water vapour present in the air is called humidity.

Humidity is generally measured by comparing dry and wet bulb temperatures.

Types Of Humidity

There are three main types of humidity:

• Absolute Humidity
• Specific Humidity
• Relative Humidity

Absolute Humidity

Absolute humidity is the actual amount of water vapour present per unit volume of air at a certain temperature in a particular area.

It tells how much moisture is present in a given volume of air.

Specific Humidity

Specific humidity is the mass of water vapour present per unit mass of air.

It is a more stable measure than relative humidity because it does not change directly with temperature in the same way.

Relative Humidity

Relative humidity is the ratio between the actual amount of water vapour present in the air and the maximum amount of water vapour the air can hold at that temperature.

When relative humidity is 100%, the air is saturated.

This means the air contains moisture to its full capacity.

Dew Point

The temperature at which saturation occurs in a given sample of air is called the dew point.

At the dew point, air becomes saturated and condensation may begin.

Condensation

Condensation is the process in which water vapour in the air changes into liquid water.

Condensation takes place in two situations:

• When air is cooled to its dew point.
• When air becomes saturated with water vapour.

Types Of Condensation

The important forms of condensation discussed in the chapter are:

• Dew
• Frost
• Fog and Mist
• Clouds

Dew

When atmospheric moisture condenses and gets deposited as water droplets on solid objects such as leaves, grass and other surfaces, it is called dew.

Favourable Conditions For Dew Formation

Dew forms under the following conditions:

• Clear sky and calm night
• Rapid terrestrial radiation or cooling of surface
• Dew point above freezing point

Frost

When the dew point is below freezing point, water vapour directly changes into ice crystals.

This is called frost.

Frost forms when surface temperature falls below freezing point and water vapour deposits directly as ice.

Fog And Mist

Fog is a cloud formed very close to the ground.

It forms when condensation occurs around dust particles, salt particles or other aerosols near the ground surface.

The only difference between fog and mist is the amount of moisture present.

Mist contains more moisture than fog.

In mist, each nucleus contains a thicker layer of moisture.

Clouds

Clouds are visible aggregates of tiny water droplets or ice particles.

They are formed by condensation of water vapour around hygroscopic nuclei such as dust particles or aerosols.

A cloud may also be described as a mass of water droplets or tiny ice crystals formed by condensation of water vapour in free air at considerable heights.

Clouds are classified on two bases:

• Physical form
• Height at which clouds are formed

Clouds Based On Physical Form

On the basis of physical form, clouds are grouped into:

• Cirrus
• Cumulus
• Stratus
• Nimbus

Clouds Based On Height

On the basis of height, clouds are classified into:

• High-level clouds
• Mid-level clouds
• Low-level clouds

High-Level Clouds

High-level clouds include:

• Cirrus
• Cirrocumulus
• Cirrostratus

These clouds are generally thin and white.

They may appear in different colours when the Sun is low on the horizon.

Cirrus Clouds

Cirrus clouds are detached clouds in the form of white, delicate filaments.

They are mostly seen in patches or narrow bands.

They may have a fibrous, hair-like or silky appearance.

Cirrus clouds are always composed of ice crystals.

When these clouds cross the Sun’s disk, they generally do not reduce its brightness much.

Before sunrise and after sunset, cirrus clouds may appear bright yellow or red.

Near the horizon, they often appear yellowish because light travels through a greater thickness of the atmosphere at a low angle.

Cirrocumulus Clouds

Cirrocumulus clouds are thin and white clouds.

They look like a patchy sheet or layer arranged into grains or ripples without shading.

Most elements of cirrocumulus have an apparent width of less than one degree.

They are mainly made of ice crystals.

They often form in association with cirrus or cirrostratus clouds and are usually short-lived.

Cirrostratus Clouds

Cirrostratus clouds are transparent, whitish, veil-like clouds.

They may have a fibrous or smooth appearance.

A sheet of cirrostratus may cover the whole sky.

During the day, when the Sun is high, cirrostratus is usually not thick enough to prevent shadows on the ground.

Cirrostratus can look like a thin veil of fog, but it is identified by the halo phenomenon around the Sun or Moon.

Mid-Level Clouds

High-level clouds include:

• Altocumulus
• Altostratus
• Nimbostratus

These clouds are mainly composed of water droplets.

They may also contain ice crystals when temperatures are low.

The word alto means high in Latin, but altocumulus and altostratus are classified as mid-level clouds.

Altocumulus Clouds

Altocumulus clouds are white or grey patchy, sheet-like or layered clouds.

They are generally made of plates, rounded masses or rolls.

They may be partly fibrous or diffuse.

Their small elements have an apparent width of one to five degrees.

When a thin patch of altocumulus passes in front of the Sun or Moon, a coloured ring called a corona may appear.

The corona has red on the outside and blue on the inside.

Altostratus Clouds

Altostratus clouds are grey or bluish cloud sheets or layers.

They may cover the sky totally or partially.

They are thin enough to reveal the Sun faintly, as if seen through ground glass.

Altostratus clouds do not produce a halo phenomenon.

Shadows of objects on the ground are not visible under altostratus clouds.

Sometimes virga, or streaks of rain, may hang from altostratus clouds and occasionally reach the ground as very light precipitation.

Nimbostratus Clouds

Nimbostratus clouds are formed by the thickening of altostratus clouds.

They appear as a dark grey cloud layer diffused by falling rain or snow.

They are thick enough to block out the Sun.

Low ragged clouds often occur below nimbostratus clouds and may merge with their base.

Low-Level Clouds

Low-level clouds include:

• Cumulus
• Stratocumulus
• Stratus
• Cumulonimbus

These clouds are mainly composed of water droplets.

Cumulonimbus clouds have strong vertical development and may extend into middle and high levels of the atmosphere.

Cumulus Clouds

Cumulus clouds are detached and generally dense clouds with sharp outlines.

They develop vertically in the form of rising mounds, domes or towers.

Their upper parts often look like cauliflower.

The sunlit parts are usually bright white, while the base is relatively dark and horizontal.

Over land, cumulus clouds develop on clear days due to diurnal convection.

They usually appear in the morning, grow during the day and dissolve towards evening.

Cumulonimbus Clouds

Cumulonimbus clouds are thunderstorm clouds.

They are heavy and dense clouds in the form of a mountain or huge tower.

Their upper portion is usually smooth, fibrous or striated and flattened like an anvil or a vast plume.

Their base is often very dark.

Low ragged clouds may be present under the base and may or may not merge with it.

Cumulonimbus clouds may produce:

• Heavy rain
• Hail
• Lightning
• Thunder
• Strong winds
• Occasionally tornadoes

Stratocumulus Clouds

Stratocumulus clouds are grey or whitish patchy, sheet-like or layered clouds.

They often show dark tessellations or a honeycomb appearance.

They may also appear as rounded masses or rolls.

Except for virga, they are non-fibrous.

Their small elements have an apparent width of more than five degrees.

Stratus Clouds

Stratus clouds form a generally grey cloud layer with a uniform base.

If thick enough, they may produce:

• Drizzle
• Ice prisms
• Snow grains

When the Sun is visible through stratus clouds, its outline can be clearly seen.

When a layer of stratus breaks and dissipates, patches of blue sky may become visible.

Stratus clouds may appear as ragged sheets.

They rarely produce halo phenomena, except at very low temperatures.

They are often dense and opaque to sunlight when thick.

Precipitation

Precipitation is water falling on the Earth in solid or liquid form.

It occurs when continuous condensation within a body of air helps water droplets or ice crystals grow in size and weight.

When the air can no longer hold them, they fall to the ground under the influence of gravity.

Types Of Precipitation

The main types of precipitation are:

• Drizzle and rainfall
• Snowfall
• Sleet
• Hail
• Graupel

Drizzle

Drizzle is light precipitation made of numerous fine water droplets.

The diameter of drizzle droplets is less than 0.5 mm.

It is generated from low-level stratiform clouds or stratocumulus clouds.

Sometimes drizzle evaporates before reaching the ground because the droplets are very small.

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Rainfall

When condensation occurs above freezing point, water droplets grow larger and fall to the ground as liquid precipitation.

This is called rainfall.

Rainfall is classified into three main types based on origin:

• Convectional rainfall
• Orographic or relief rainfall
• Cyclonic or frontal rainfall

Convectional Rainfall

Convectional rainfall occurs when air gets heated, becomes lighter and rises upward in convection currents.

As the air rises, it expands and cools.

This causes condensation and the formation of cumulonimbus clouds.

Heavy rainfall with lightning and thunder may occur, but it usually lasts for a short time.

Convectional rainfall usually occurs during the hotter part of the day, especially in summer.

It is common in:

• Equatorial regions
• Interior parts of continents

Orographic Or Relief Rainfall

Orographic rainfall is also known as relief rainfall.

It occurs when a moist air mass meets a mountain and is forced to rise along the slope.

As the air rises, it expands and cools.

Moisture condenses and forms clouds.

The windward slopes receive heavy rainfall.

After precipitation on the windward side, the air descends on the leeward side.

As it descends, it warms and becomes dry.

The leeward side receives very little rainfall.

This dry region is called the rain-shadow area.

Cyclonic Or Frontal Rainfall

Cyclonic rainfall occurs due to the movement of cyclones and the interaction of air masses along a front.

When two air masses of different temperature, density and humidity meet, a front is formed.

The boundary separating them is called a front.

Two common fronts are involved:

• Warm front
• Cold front

At a warm front, warm and lighter air rises gradually over denser cold air.

As the warm air rises, it cools and condensation occurs.

This forms nimbostratus clouds and sometimes cumulonimbus clouds.

Cyclonic rainfall usually continues steadily for several hours or even days.

Snowfall

Snowfall occurs when condensation and freezing take place below freezing point, or 0°C.

Water vapour directly changes into ice crystals.

These ice crystals fall to the ground as snowflakes.

This form of precipitation is called snowfall.

Sleet

Sleet is frozen rain.

It forms when raindrops pass through a layer of cold air and freeze before reaching the ground.

This produces small solid particles of clear ice.

Hail

Hail is precipitation in the form of balls or irregular lumps of ice called hailstones.

Their diameter usually ranges from about 5 mm to 50 mm.

Larger hailstones may sometimes occur.

Graupel

Graupel is an opaque, white, soft and rounded form of precipitation.

It is also called:

• Snow pellets
• Soft hail

It forms when supercooled water droplets freeze instantly onto falling snowflakes.

Graupel is different from hail and sleet.

It is usually less than 5 mm in diameter and can be easily crushed.

Graupel is often observed in high-altitude or cold climates, especially during spring and autumn.

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