Humidity

The process of conversion of water into its gaseous state is called evaporation. It may also be termed as vaporization.

The factors affecting evaporation are:

1. Air Pressure — The rate of evaporation is higher when pressure is low.
2. Availability of water — The rate of evaporation is higher where large water bodies are present.
3. Temperature — Evaporation is directly proportional to temperature. The warmer the evaporating surface, the higher the rate of evaporation.
4. Relative humidity — The rate of evaporation in dry air is more than in moist air.
5. Area of evaporating surface — Larger area of evaporating surface offers more evaporation.

A process of change of matter in the gaseous state (water vapour) to the liquid state (water) or solid water state (ice) is known as condensation.

Condensation occurs when:

1. Air is cooled below its dew point.
2. The dew point temperature falls below the freezing point.

As the warm air rises up, the air pressure drops and the gases expand. The temperature is lower at higher altitudes which results in cooling of the air as well.

The different forms of condensation are:

1. Clouds
2. Dew
3. Frost
4. Fog
5. Mist

Precipitation is defined as water in liquid or solid forms falling to the Earth. It can be termed as deposition of atmospheric moisture and is the most important phase of the hydrological cycle. Precipitation cam be in the form of hail, snow, sleet,rain etc.

Following are the conditions necessary for precipitation:

1. The air must be saturated.
2. The temperature at which condensation takes place must be present.
3. The air must contain small particles of matter such as dust around which droplets form.
4. The air must be cooled below its dew point.

The different forms of precipitation are:

1. Rain
2. Snow
3. Hail
4. Sleet
5. Drizzle
6. Fog
7. Frost

The two factors necessary to cause convectional rainfall are intense heating of the surface and abundant supply of moisture. Due to intense heating of the surface, the surface air gets heated and expands and rises up conventionally holding moisture. It is thereby cooled adiabatically and its temperature falls below the dew point, forming Cumulonimbus cloud, which gives heavy rains. Such rainfall is called convectional rainfall.

Two characteristic features of the convectional rainfall are:

1. This type of rain mostly occurs at 4 o'clock in the afternoon.
2. It is always torrential accompanied by lightening and thunder.

The area which commonly experiences this type of rainfall is equatorial zone as it experiences direct sun rays throughout the year.

The chief requirements to experience orographic rainfall are:

1. moisture laden winds.
2. mountain in the path of the winds.

It is experienced in the western Ghats. Monsoon winds bring rainfall to the windward side of the ghats while the leeward side remains dry.

The area on the leeward slope of a mountain range, in the shadow of the mountain where precipitation is greatly reduced compared to windward slope is called rain shadow.

The leeward side of the Western Ghats is an example of rain shadow region.

Cyclonic rainfall occurs when a mass of warm air (warm front) meets the mass of cold air (cold Front). The warm air is forced up above the cooler air as it is lighter. Condensation takes place and clouds are formed, followed by rains.

Northwest India experiences this type of rainfall in winters.

The principal factors affecting the distribution of rainfall are:

1. Latitude — The latitudinal belt of maximum precipitation is found in the equatorial Zone between 8° N to 8° S along with the seasonal shifting of the Doldrums. The shifting of the Trade Winds causes winter rains in Mediterranean Regions in Europe. Trade Winds blow from North-East to South-West, they shed maximum moisture on the Eastern margins of the continents and western margins are left dry.
2. Continents and oceans — Places which are located away from the sea in the interior receive relatively less precipitation than coastal areas.
3. Mountain Barriers — They play a great role in the world distribution of precipitation by causing orographic rainfall on the windward side while the leeward side remains dry or receives little rainfall.

The latitudinal belt between 8° N to 8° S is the wettest region of the world.

Two examples of the areas that receive less than 25 cm of rainfall are:

1. Desert area of Rajasthan
2. Desert of China(Gobi desert).

The formation of clouds by condensation takes place due to adiabatic cooling. For precipitation air must be saturated and temperature below dew point must be reached. The air must contain small particles of matter such as dust around which droplets form. When the cold droplets, ice pellets or ice crystals grow to such a large size as to overcome the normal buoyancy, precipitation occurs.

The three types of rainfall are:

1. Convectional rainfall
2. Orographic rainfall
3. Cyclonic Rainfall

Two characteristics of orographic rainfall are:

1. When the moisture laden wind rises along the windward slope of the mountain the pressure on it decreases, the air expands and cools.
2. On the leeward side, as the dry air descends the mountain slopes, the pressure decreases.

The factors necessary for convectional rainfall are:

1. Intense heating of the surface.
2. Abundant supply of moisture

Cyclonic rainfall is common throughout the Doldrums where the Trade winds meet.

Canada is located in the higher latitudes, where temperatures often remain below freezing point for a large part of the year. As a result, precipitation there usually falls in the form of snow. In contrast, India lies in the tropical zone, where the climate is warmer. Therefore, the primary form of precipitation in India is rainfall.

Absolute humidity is the actual amount of water vapour present in a given volume of air, regardless of temperature. It is expressed as grams of moisture per cubic metre of air (g/m³). The amount of absolute humidity decreases as we go towards the poles. The maximum absolute humidity of warm air at 30°C is approximately 30 gm of water vapour — 30 g/m³.

Relative Humidity refers to the water vapour present in the air as compared to the maximum amount of water the air can hold at that temperature. It can be defined as "a ratio between the actual amount of water vapour present in the air and the maximum amount of water the air can hold at that temperature." It is always expressed in percentage.

The temperature at which the air becomes fully saturated with water vapour is known as dew point. It is used to predict dew, frost, fog etc. It can also indicate air's actual water vapour content.

Clouds are defined as a visible aggregation of minute water droplets/ice particles in the air above ground level. Clouds can be classified into Low, medium and High, on the basis of their height.

Hailstones are small round balls of ice with frosted appearance. They are formed of concentric layers of ice much like an onion. Hail stones range from 0.5 to 5 cm in diameter. They occur only from the Cumulonimbus cloud type.

Condensation of water vapour near the surface of earth leads to formation of fog. Fog is formed when warm and moist air moves along the surface of colder region. It affects visibility. Fog makes it difficult to see the objects beyond 1 km.

Snow consists of crystals or grains of ice which grow directly from the water vapour. Snow displays beautiful forms of symmetrical patterns that are formed in winter when the temperature is below freezing point.

Precipitation is defined as water in liquid or solid forms falling to the Earth. Hail, snow, sleet, rain and drizzle are the common forms of precipitation.

Convectional currents generally lead to rain as due to intense heating of the surface, the surface air gets heated and expands and rises up conventionally holding moisture. It is thereby cooled adiabatically and its temperature falls below the dew point, forming Cumulonimbus clouds, which give heavy rains.

The sudden ascent of warm moist air on the windward sides causes cooling of air, leading to condensation and precipitation.
On the contrary, on descending the leeward slope, a decrease in altitude increases both the pressure and the temperature, leading the air to get compressed and warm. Consequently, the relative humidity drops and there is evaporation and little or no precipitation in the rain shadow area. Hence, a rain shadow area is generally dry.

Cyclonic rainfall occurs when cold and warm fronts meet. The westerlies blow from the the warmer Horse latitudes towards the poles in north and south hemisphere. This results in the convergence of warm westerlies and cold polar winds, which further results in Cyclonic rainfall. Therefore, Cyclonic rainfall is common in the belt of westerlies.

Areas of trade winds are usually dry because trade winds blow from dry mainland. They lack moisture and hence cannot cause rainfall.

Cyclonic rainfall is also known as frontal rainfall because it occurs when warm front (a mass of warm air) meets the cold front (a mass of cold air).

Convectional rainfall is called 4 o'clock rainfall as this type of rainfall occurs in the afternoon at about 4 o'clock. This is because the heat from the sun builds up throughout the day, causing the air to rise and form clouds that eventually bring rain in late afternoon around 4 o'clock.

Below diagram shows Convectional rainfall:

Below diagram shows Orographic rainfall:

Below diagram shows Cyclonic rainfall:

Relative humidity is measured in degrees.

Evaporation

All the above

Orographic

Equatorial zone

All the above

All the above

Rain

Dew point

All of the above

Winter season

Stratus

1. The specific type of rainfall experienced by this region is Convectional rainfall.

2. Rainforests are situated near the equator, where the sun's rays strike directly throughout the year, causing intense surface heating. This heat warms the moist air, which then rises rapidly in the form of convection currents. As the moist air ascends, it cools and condenses to form thick clouds, resulting in frequent and heavy rainfall. This continuous cycle supports the lush, multi-layered vegetation and rich biodiversity found in rainforest regions.

1. Foggy weather.

2. During winter, long and clear nights lead to rapid loss of heat from the Earth's surface through terrestrial radiation. This causes the air near the ground to cool quickly. When this cool air comes in contact with moisture, the water vapour condenses into tiny droplets that remain suspended in the air, forming dense fog.

3. Fog significantly reduces visibility, often to less than 1 kilometre. This low visibility creates hazardous conditions for transportation, making it difficult for pilots and train operators to navigate safely. As a result, many flights and trains are delayed, postponed, or even cancelled.

Both A and R are true and R is the correct explanation of A.

Explanation — The warmer the air, the more moisture it can hold, until saturation point reaches and condensation takes place. As summers are hotter than winters, humidity is also higher in summers in comparison to winters.

A is true but R is false.

Explanation — Cloud is formed when the water vapour cools and condenses into tiny water droplets.