Humidity

Latent heat

Reason — Latent heat is the heat absorbed during evaporation and stored in water vapour without causing a rise in temperature. It is released into the atmosphere during condensation, warming the surrounding air.

Precipitation

Reason — Precipitation is not a factor favouring evaporation. The factors favouring evaporation are humidity, supply of heat and winds. Precipitation is instead the process by which the products of condensation fall to the ground.

Relative humidity

Reason — Relative humidity is the ratio between the absolute humidity of a given mass of air and the maximum amount of water vapour that it can hold at the same temperature. It is generally expressed as a percentage and is the essential characteristic of weather forecasts.

Dew Point

Reason — Dew point is the temperature at which air gets fully saturated. Saturated air will not hold any more water vapour at that temperature and its relative humidity becomes 100%.

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

Explanation — The human body cools itself through perspiration and the evaporation of sweat. When the relative humidity is high, the air already contains a large amount of water vapour, so evaporation of sweat slows down. As a result, the body loses heat less efficiently, making a person feel warm and uncomfortable.

Absolute Humidity

Reason — Absolute humidity is the amount of water vapour held by air at a certain temperature and volume, regardless of temperature. It is expressed as grams of moisture per cubic metre of air (g/m³).

Precipitation

Reason — Precipitation is the process by which the products of condensation, viz. water droplets, ice crystals, sleet, etc., fall to the ground. Rainfall, snowfall, drizzle, sleet and hail are its chief forms.

Cumulus

Reason — Cumulus clouds have a cauliflower-like appearance with a flat base and rounded, dome-shaped tops. They usually indicate fair weather, though when they grow into cumulonimbus clouds, they bring heavy rain and thunderstorms. Cirrus clouds are thin and feathery, while stratus clouds form uniform layers.

impurities that support condensation

Reason — Condensation nuclei are the impurities present in air, such as minuscule particles of dust, salt and smoke, around which water vapour condenses. Without condensation nuclei, condensation does not take place.

Cyclonic rainfall: mountains

Reason — Cyclonic or frontal rainfall is caused by the convergence of two air masses with different temperatures, irrespective of relief or convection. Hence, it is not associated with mountains. It is relief or orographic rainfall that is caused by mountain barriers.

High humidity due to the rains of the previous day.

Reason — After several rainy days, the air contains a high amount of water vapour (high relative humidity). Under such conditions, sweat evaporates slowly, reducing the body's ability to lose heat through perspiration. As a result, the air feels hot, sticky, and uncomfortable.

Convectional

Reason — Little Andaman Island, being close to the Equator, experiences intense heating during the day. The warm, moist air rises by convection, cools, condenses into clouds, and produces heavy afternoon rainfall. This type of rainfall, which commonly occurs around 4 p.m. in equatorial regions, is known as convectional rainfall or the "4 O'clock Shower."

The amount of water vapour in the air is determined by temperature. The hotter the air, the more the water vapour present in it. When the atmospheric temperature is low, water vapour is less.

Relative humidity is expressed as the ratio between the absolute humidity of a given mass of air and the maximum amount of water vapour that it can hold at the same temperature.

Four chief forms of condensation are-

1. Clouds
2. Dew
3. Frost
4. Fog and mist

The different forms of precipitation are-

1. Rain
2. Snow
3. Hail

Condensation occurs only around tiny solid particles like dust or carbon dioxide in smoke. These impurities in air are known as condensation nuclei.

The three types of rainfall are-

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

Orographic rainfall is also called relief rainfall because it is caused by the relief (mountain barriers) of the land. When moist air is forced to rise over mountains, it cools, condenses, and produces rainfall on the windward side.

Cyclonic rainfall is the rainfall caused by the meeting (convergence) of warm and cold air masses in cyclones or depressions, irrespective of relief or convection. Since warm air is lighter, it rises over the cold air, cools, condenses, and produces rainfall.

The boundary between two air masses of different temperatures and other physical properties is called a front. The uplift of warm air along this boundary is known as frontal lifting, which results in cyclonic rainfall.

The process by which the droplets fall to ground in liquid, solid or frozen form is known as precipitation.

The process by which water vapour enters the atmosphere on heating is known as evaporation.

The process by which water vapour forms water droplets on cooling is known as condensation.

The amount of water vapour present in air is known as humidity.

The amount of water vapour present in air is known as humidity.

Humidity of the atmosphere is measured with the help of a hygrometer.

(i) The total volume of water in the oceans and seas remains almost constant because the water lost through evaporation is returned by precipitation directly into the oceans and indirectly through rivers and surface run-off. This continuous water cycle maintains the overall balance.

(ii) In the Equatorial region, intense heating causes warm, moist air to rise by convection. As it rises, it cools and condenses to form cumulonimbus clouds, keeping the sky overcast and producing frequent afternoon rainfall.

(iii) When the relative humidity is 100%, the air is fully saturated with water vapour. Since no evaporation takes place from the wet bulb, there is no cooling, and both the wet-bulb and dry-bulb thermometers record the same temperature.

Below labelled diagram shows orographic rainfall:

Condensation is the process by which water vapour changes into tiny water droplets on cooling.

Condensation occurs when moist air cools to its dew point or when additional water vapour is added to saturated air. During this process, the latent heat stored in water vapour is released into the atmosphere, and the water vapour condenses around dust or salt particles (condensation nuclei) to form tiny water droplets, leading to the formation of clouds, fog, dew, or rain.

The atmospheric conditions that favour condensation are-

1. There should be a high amount of water vapour present in atmosphere.
2. Minuscule particles of dust, salt and even smoke act as condensation nuclei.
3. The temperature of air must be below dew point temperature so as to encourage condensation.

(i) Condensation is the reverse process of evaporation because evaporation changes liquid water into water vapour by heating, whereas condensation changes water vapour back into liquid water by cooling.

(ii) Coastal areas receive more rainfall because moist winds from the sea contain abundant water vapour. As these winds move inland, they gradually lose their moisture through rainfall, so the interior regions receive less rainfall.

(iii) The windward side receives more rainfall because moist air is forced to rise over the mountain, where it cools, condenses, and produces rainfall. On the leeward side, the air descends, becomes warmer and drier, reducing relative humidity and resulting in little or no rainfall (rain shadow region).

Below labelled diagram shows convectional rainfall:

When water vapour condenses on the surface, it forms tiny droplets of water called dew.

The difference between fog and mist is as follows-

Orographic rainfall — It occurs from the cooling of warm moist air which ascends above the mountain barrier lying in the direction of the prevailing winds. The presence of mountains causes humid air to rise. The sudden ascent causes cooling of air, leading to condensation and precipitation.

Convectional rainfall — This type of rainfall is common in equatorial regions. The high temperature leads to the rapid heating of air. Such heated air rises in convectional currents, leading to development of clouds at about 10 km height. Ascending currents of hot and humid air causes condensation of the clouds, resulting in heavy rainfall.

(i) The windward side of a mountain receives rainfall because moist air is forced to rise, cool, condense, and produce precipitation. As the air descends the leeward side, it becomes warmer and drier due to compression, reducing its relative humidity. Therefore, there is little or no rainfall on the leeward side, forming a rain shadow area.

(ii) Frontal rainfall is common in the mid-latitudes because warm tropical air masses meet cold polar air masses. The lighter warm air rises over the cold air along a front, cools, condenses, and produces rainfall.

(iii) Convectional rainfall is called "4 O'clock Rainfall" because intense heating during the day causes warm, moist air to rise by convection. The air cools, condenses, and produces heavy rainfall, which typically occurs in the late afternoon (around 4 p.m.), especially in equatorial regions.

The process by which products of condensation, water droplets, ice crystals, sleet, etc., fall to ground is known as precipitation.

The three forms of precipitation are-

1. Rain — It is the most common form of precipitation. Raindrops of smaller size and less intensity are known as drizzle.

2. Snow — Water droplets which rise higher and freeze on account of drop in temperature is called snow. Snowfall usually occurs in winters in cold climates or on high mountains.

3. Hail — Sometimes, vertical air currents may push water droplets or ice particles higher. They form into solid ice and fall as hail. Hailstones cause great damage to crops.

If condensation did not occur, it would have serious consequences for both climate and human beings.

Climate consequences —

1. No cloud formation — Without condensation, clouds would not form.
2. No or very little rainfall — The absence of condensation would disrupt precipitation, leading to droughts.
3. Disrupted water cycle — Water vapour would not return to the Earth's surface, disturbing the natural water cycle.
4. Hotter climate — The atmosphere would retain more water vapour, leading to changes in weather and climate.

Human consequences —

1. Acute water scarcity for drinking, irrigation, and industries.
2. Crop failure and food shortages due to lack of rainfall.
3. Damage to ecosystems and loss of biodiversity because plants and animals depend on rainfall for survival.

Conditions under which condensation may not occur

1. The air is not cooled to its dew point (saturation point).
2. The air has very low humidity (insufficient water vapour).
3. There are no condensation nuclei (such as dust, smoke, or salt particles) for water vapour to condense upon.

Two physical objects in my home which are affected by humidity are:

1. Wooden furniture — High humidity causes the wood to absorb moisture and swell, while low humidity may cause it to shrink and crack
2. Electronic devices (TV, computer, or smartphone) — High humidity causes moisture to accumulate inside the devices, which can corrode electrical components and affect their functioning.

Two places which receive two types of rainfall are:

Western Ghats receive both orographic rainfall and convectional rainfall.

1. Moist winds from the Arabian Sea rise over the mountains, causing heavy orographic rainfall on the windward side.
2. High temperatures and local conditions lead to localized thunderstorms and convectional rainfall.

Northeastern states of Meghalaya and Assam also receive both orographic rainfall and convectional rainfall.

1. Moisture-laden winds from the Bay of Bengal encounter the Eastern Himalayas, resulting in significant orographic rainfall on the windward side.
2. Warm and humid conditions contribute to convective cloud formation, leading to heavy convectional showers and thunderstorms.

The forms of precipitation that occur in my city are:

1. Rain — Rainfall is most common during the monsoon season, which extends from June to September. The heaviest rainfall occurs in July and August.
2. Hail — Hailstorms are relatively rare in my city, but they can occur during the pre-monsoon months of March to May. These storms are more sporadic and less frequent compared to rainfall.
3. Fog, Mist and Dew — In winters, mornings are foggy in my city and there is a lot of dew at night.