humidity & precipitation and concept of airmasses

Humidity & Precipitation and Concept of Airmasses

By Sudarshan Gurjar

Current Events-Climatology

Increasing instances of lightning strikes: WMO

The World Meteorological Organization(WMO) has released its findings on lightning strikes which had happened around the world.

Key Points

➢ Brazil ➔ recorded the longest lightning strike thattravelled a distance of 709 kilometres in 2018.

➢ Argentina ➔ recorded a lightning strike that lastedcontinuously for 16.73 seconds in 2019.

➢ India ➔ Uttar Pradesh recorded the maximumnumber of deaths due to lightning in India. It wasfollowed by Bihar.

➢ Lightning:


◎ What is it? ➔ It is a very rapid and massive discharge of electricity in theatmosphere, some of which is directed towards the Earth. These dischargesare generated in giant moisture-bearing clouds that are 10-12 km tall.

◎ How is it formed?➔

○ Air generally acts as an insulator between the positive and negative charges in the cloud and between the cloud and the ground.

○ But when the opposite charges build up enough, this insulatingcapacity of the air breaks down and there is a rapid discharge of electricity that we know as lightning.


➢ Effects of Lightning:

1. Direct strike➔ Victims may sustain a direct strike, which is often fatal.

2. Contact injury ➔ This occurs when lightning strikes an object, such as acar or metal pole that the victim is touching.

3. Ground current➔ This occurs when lightning strikes the ground near avictim and the ground current passes from the strike point through theground and into the victim among others.


◎ The World Meteorological Organization (WMO) is an intergovernmental organization with a membership of 192 Member States and Territories.

◎ Headquarters location ➔ Geneva, Switzerland

◎ Founded ➔ 23 March 1950

◎ What does WMO do? ➔ WMO coordinates the activities of NationalMeteorological and Hydrological Services in 192 States and Territories sothat basic weather, climate and water services are made available toanyone who needs them, when they need them.

◎ Reports➔ Greenhouse Gas Bulletin, Status of the World Climate.


HINDU-KUSH-HIMALAYAN (HKH) REGION

➢Why in News = The India Meteorological Department (IMD) will collaborate withmeteorological agencies in China and Pakistan to provide climate forecast services tocountries in the Hindu-Kush-Himalayan (HKH) region.

➢About• The Hindu-Kush-Himalayan(HKH) region spans Afghanistan,

Bangladesh, Bhutan, China, India, Kyrgyzstan,Mongolia, Myanmar, Nepal, Pakistan, Tajikistan,and Uzbekistan.

• The HKH region is considered the Third Pole and hassignificant implications for climate.

• The Third Pole, which contains vast cryospheric zones, is also the world’s largeststore of snow and ice outside the polar region, and the source of 10 major rivers,and, therefore, particularly sensitive to climate change.

➢World Meteorological Organization (WMO):

It is an intergovernmental organization with a membership of 192 Member

States and Territories.

It is a specialized agency of the United Nations(UN) for meteorology, operational

hydrology and related geophysical sciences.

• It was established by the ratification of the WMO Convention on 23

March 1950.

• It is headquartered at Geneva, Switzerland.

Humidity

Water vapour present in the air is known as Humidity.

Absolute Humidity

The actual amount of the water vapour present in the atmosphere is known asthe absolute humidity.

It is the weight of water vapour per unit volume of air and is expressed in terms of grams per cubic metre.

The absolute humidity differs from place to place on the surface of theearth.

The ability of the air to hold water vapour depends entirely on its temperature (Warm air can hold more moisture than cold air).

By Sudarshan Gurjar

By Sudarshan Gurjar

Relative Humidity The percentage of moisture present in the atmosphere as compared to its full

capacity at a given temperatureis known as the relative humidity.

Relative Humidity = [Actual amount of water vapor in air (absolutehumidity)/humidity at saturation point (the maximum water vapor air can hold at agiven temperature)] X 100

With the change of air temperature, the capacity to retain moistureincreases or decreases and the relative humidity is also affected.

Relative humidity is greater over the oceans and least over the continents (absolutehumidity is greater over oceans because of greater availability of water forevaporation).

The relative humidity determines the amount and rate of evaporation andhence it is an important climatic factor.

By Sudarshan Gurjar

By Sudarshan Gurjar

Air containing moisture to its full capacity at a given temperature is said tobe ‘saturated’.

At this temperature, the air cannot hold any additional amount ofmoisture. Thus, relative humidity of the saturated air is 100%.

If the air has half the amount of moisture that it can carry, then it isunsaturated and its relative humidity is only 50%.

By Sudarshan Gurjar

Dew point The air containing moisture to its full capacity at a given temperature is said

to be saturated.

It means that the air at the given temperature is incapable of holdingany additional amount of moisture at that stage.

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

Dew point occurs when Relative Humidity = 100%.

By Sudarshan Gurjar

Condensation The transformation of water vapour into water is called condensation.

Condensation is caused by the loss of heat (latent heat of condensation,opposite of latent heat of vaporization).

When moist air is cooled, it may reach a level when its capacity to holdwater vapour ceases (Saturation Point = 100% Relative Humidity = Dew Pointreached).

Then, the excess water vapour condenses into liquid form. If it directlycondenses into solid form, it is known as sublimation.

By Sudarshan Gurjar

In free air, condensation results from cooling around very small particlestermed as hygroscopic condensation nuclei. Particles of dust, smoke, pollenand salt from the ocean are particularly good nuclei because they absorbwater.

Condensation also takes place when the moist air comes in contact with somecolder object and it may also take place when the temperature is close to thedew point.

Condensation, therefore, depends upon the amount of cooling and therelative humidity of theair.

After condensation the water vapour or the moisture in the atmosphere takes one of the following forms — dew, frost, fog and clouds.

Forms of Condensation

The forms of condensation can be classified on the basis of temperature at which the dew point is reached.

Condensation can take place when the dew point is

lower than the freezing point,

higher than the freezing point.

White frost, snow and some clouds (cirrus clouds) are produced when thetemperature is lower than the freezing point.

Dew, fog and clouds result even when the temperature is higher than the freezing point.

Forms of condensation may also be classified on the basis of their location,i.e. at or near the earth’s surface aBynSdudianrshfarneGeurjaarir.

By Sudarshan Gurjar

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By Sudarshan Gurjar

By Sudarshan Gurjar

By Sudarshan Gurjar

Dew, white frost, fog and mist come in the first category,whereas clouds are in the second category.

Dew When the moisture is deposited in the form of water droplets on

cooler surfaces of solid objects (rather than nuclei in air above thesurface) such as stones, grass blades and plant leaves, it is known asdew.

The ideal conditions for its formation are clear sky, calm air, highrelative humidity, and cold and long nights.

For the formation of dew, it is necessary that the dew point is abovethe freezing point.

By Sudarshan Gurjar

White Frost Frost forms on cold surfaces when condensation takes place below freezing

point (0° C), i.e. the dew point is at or below the freezingpoint.

The excess moisture is deposited in the form of minute ice crystals insteadof waterdroplets.

The ideal conditions for the formation of white frost are the same as thosefor the formation of dew, except that the air temperature must be at orbelow the freezing point.

By Sudarshan Gurjar

By Sudarshan Gurjar

Fog When the temperature of an air mass containing a large quantity of water

vapour falls all of a sudden, condensation takes place within itself on fine dustparticles.

So, the fog is a cloud with its base at or very near to the ground.Because of the fog and mist, the visibility becomes poor to zero.

In urban and industrial centers smoke provides plenty of nuclei which help theformation of fog and mist. Such a condition when fog is mixed with smoke, isdescribed as Smog

By Sudarshan Gurjar

Mist The difference between the mist and fog is that mist contains more

moisture than fog.

In mist each nuclei contains a thicker layer of moisture.

Mists are frequent over mountains as the rising warm air up the slopes meets a cold surface.

Mist is also formed by water droplets, but with less merging orcoalescing. This means mist is less dense and quicker to dissipate.

Fogs are drier than mist and they are prevalent where warm currents of air come in contact with cold currents.

In mist visibility is more than one kilometre but less than two kilometres

By Sudarshan Gurjar

Haze Haze is traditionally an atmospheric phenomenon where dust, smoke

and other dry particles obscure the clarity of the sky (Nocondensation. Smog is similar to haze but there is condensation insmog).

Sources for haze particles include farming (ploughing in dryweather), traffic, industry, and wildfires.

By Sudarshan Gurjar

Nimbus Clouds Nimbus clouds are black or dark gray. They form at middle levels or very near

to the surface of the earth.

These are extremely dense and opaque to the rays of the sun.

Sometimes, the clouds are so low that they seem to touch the ground.

Nimbus clouds are shapeless masses of thick vapour.

By Sudarshan Gurjar

A combination of these four basic types can give rise to the following types ofclouds:

High clouds – cirrus, cirrostratus, cirrocumulus;

Middle clouds – altostratus and altocumulus;

Low clouds – stratocumulus and nimbostratus (long duration rainfall cloud)and

Clouds with extensive vertical development – cumulus and cumulonimbus (thunderstorm cloud)

By Sudarshan Gurjar

By Sudarshan Gurjar

Clouds Cloud is a mass of minute water droplets or tiny crystals of ice formed by the

condensationof the water vapour in free air at considerable elevations.

Cirrus Clouds Cirrus clouds are formed at high altitudes (8,000 – 12,000m). They are thin and

detached clouds having a feathery appearance. They are always white incolour.

By Sudarshan Gurjar

Cumulus Clouds Cumulus clouds look like cotton wool. They are generally formed at a height of

4,000 -7,000m. They exist in patches and can be seen scattered here and there. They have a flat base.

Stratus Clouds As their name implies, these are layered clouds covering large portions of the

sky.

These clouds are generally formed either due to loss of heat or the mixing of air masses with different temperatures.

By Sudarshan Gurjar

Precipitation The process of continuous condensation in free air helps the condensed particles to grow

in size.

When the resistance of the air fails to hold them against the force of gravity, they fallon to the earth’s surface.

So after the condensation of water vapour, the release of moisture is known asprecipitation. This may take place in liquid or solid form.

Precipitation in the form of drops of water is called rainfall, when the drop size is morethan 5 mm.

It is called virage when raindrops evaporate before reaching the earth while passingthrough dry air.

By Sudarshan Gurjar

Drizzle is light rainfall with drop size being less than 0.5 mm, and when evaporationoccurs before reaching the ground, it is referred to as

When the temperature is lower than the 0° C, precipitation takes place in the form offine flakes of snow and is called snowfall. Moisture is released in the form of hexagonalcrystals.

These crystals form flakes of snow. Besides rain and snow, other forms of precipitationare sleet and hail.

Sleet is frozen raindrops and refrozen melted snow-water. When a layer of air with thetemperature above freezing point overlies a subfreezing layer near the ground,precipitation takes place in the form of sleet.

By Sudarshan Gurjar

Raindrops, which leave the warmer air, encounter the colder air below.

As a result, they solidify and reach the ground as small pellets of ice notbigger than the raindrops from which they are formed.

Sometimes, drops of rain after being released by the clouds becomesolidified into small rounded solid pieces of ice and which reach the surfaceof the earth are called hailstones.

These are formed by the rainwater passing through the colder layers.Hailstones have several concentric layers of ice one over the other.

Rainfall: drop size more than 0.5mm.

Virage: raindrops evaporate beforereaching the earth.

By Sudarshan Gurjar

Drizzle: light rainfall; drop size less than 0.5mm.

Mist: evaporation occurs before reaching the ground leading to foggyweather.

Snowfall: fine flakes of snow fall when the temperature is less than0°C

Sleet: frozen raindrops and refrozen melted snow; mixture of snow and rainor merelypartially melted snow.

Hail: precipitation in the form of hard rounded pellets is known as hail; 5 mmand 50 mm.

By Sudarshan Gurjar

Types of Rainfall On the basis of origin, rainfall may be classified into three main types –

the convectional, orographic or relief and the cyclonic or frontal.

Conventional Rainfall

The, air on being heated, becomes light and rises up in convection currents.

As it rises, it expands and loses heat and consequently, condensation takesplace and cumulous clouds are formed.

This process releases latent heat of condensation which further heats the airand forces the air to go further up.

By Sudarshan Gurjar

Conventional Rainfall

Convectional precipitation is heavy but of short duration, highly localised andis associated with minimum amount of cloudiness.

It occurs mainly during summer and is common over equatorial doldrums in theCongo basin, the Amazon basin and the islands of south-east Asia.

By Sudarshan Gurjar

Orographic Rainfall When the saturated air mass comes across a mountain, it is forced to ascend

and as it rises, it expands (because of fall in pressure); the temperature falls,and the moisture is condensed.

This type of precipitation occurs when warm, humid air strikes an orographicbarrier (a mountain range) head on.

Because of the initial momentum, the air is forced to rise.

As the moisture laden air gains height, condensation sets in, and soonsaturation is reached.

By Sudarshan Gurjar

The surplus moisture falls down as orographic precipitation along the windward slopes.

The chief characteristic of this sort of rain is that the windward slopes receivegreater rainfall.

After giving rain on the windward side, when these winds reach the other slope, theydescend, and their temperature rises.

Then their capacity to take in moisture increases and hence, these leeward slopesremain rainless and dry.

The area situated on the leeward side, which gets less rainfall is known as the rain-shadow area (Some arid and semi-arid regions are a direct consequence of rain-shadow effect. Example: Patagonian desert in Argentina, Eastern slopes of WesternGhats).

It is also known as the relief rain.

By Sudarshan Gurjar

By Sudarshan Gurjar

By Sudarshan Gurjar

Example: Mahabaleshwar, situated on the Western Ghats, receivesmore than 600 cm of rainfall, whereas Pune, lying in the rain shadowarea, has only about 70 cm.

Frontal Precipitation

When two air masses with different temperatures meet, turbulent conditionsare produced. Along the front convection occurs and causes precipitation

By Sudarshan Gurjar

humidity & precipitation and concept of airmasses

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