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Summary
Moisture in the air gives it a different character. Water enters the air when it heats up and becomes water vapor. This is evaporation. When water vapor cools, it turns into tiny water droplets. This is condensation. When these droplets fall, it is precipitation. Humidity is the amount of water vapor in the air.
During evaporation, heat is absorbed by water and becomes hidden, called latent heat. When condensation occurs, this latent heat is released, warming the air slightly. This explains why rainy seasons can feel warm. Dry air helps more water evaporate. More heat and strong winds also increase evaporation.
Absolute humidity is the actual amount of water vapor in a specific air volume. Relative humidity compares this amount to the maximum water vapor the air can hold at that temperature. It is given as a percentage. Warm air can hold more water vapor than cold air. Saturated air is full of water vapor, with 100% relative humidity. The dew point is the temperature at which air becomes saturated.
A hygrometer measures humidity, often using a dry and a wet bulb thermometer. The temperature difference indicates relative humidity.
Condensation is the reverse of evaporation. Water vapor changes into water droplets or ice. This happens when air with much water vapor cools below its dew point. Tiny particles like dust, called condensation nuclei, are needed for water vapor to condense on.
Clouds are visible water droplets or ice crystals in the air. Cirrus clouds are feathery and high. Cumulus clouds are puffy, like cauliflower. Stratus clouds are layered. Dew is water droplets on cool surfaces. Frost is ice crystals on surfaces in freezing temperatures. Fog is a thick cloud near the ground. Mist is like fog but less dense.
Precipitation is water falling from clouds. Rain is liquid water. Snow is ice crystals. Hail is ice lumps.
Relief rainfall happens when moist air rises over mountains. It cools, causing rain on the windward side. The leeward side, or rain shadow area, gets little rain. Convectional rainfall occurs in hot areas. The ground heats the air, which rises, cools, and causes heavy rain, often in the afternoon. This is sometimes called a “4 O’clock shower” because of its regular timing. Cyclonic rainfall occurs when warm and cold air masses meet. The warmer air rises over the colder air along a front, leading to rain.
Textbook solutions (Morning Star/Total)
Multiple-Choice Questions
1. The process by which water vapour enters the atmosphere on heating:
(a) Water Vapour
(b) Evaporation
(c) Humidity
(d) Precipitation
Answer: (b) Evaporation
2. The process by which water vapour forms water droplets on cooling:
(a) Condensation
(b) Evaporation
(c) Humidity
(d) Precipitation
Answer: (a) Condensation
3. The amount of water vapour present in air:
(a) Condensation
(b) Evaporation
(c) Humidity
(d) Precipitation
Answer: (c) Humidity
4. Name the heat that is lost during evaporation and passes into water vapour in a hidden form and is released in the atmosphere during condensation.
(a) Condensed heat
(b) Humidity
(c) Latent heat
(d) Relative Humidity
Answer: (c) Latent heat
5. Which, amongst the following, is NOT one of the factors favouring evaporation?
(a) Humidity
(b) Supply of heat
(c) Winds
(d) Precipitation
Answer: (d) Precipitation
6. 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:
(a) Humidity
(b) Dew Point
(c) Relative humidity
(d) Latent heat
Answer: (c) Relative humidity
7. The temperature at which air gets fully saturated:
(a) Humidity
(b) Dew Point
(c) Relative humidity
(d) Latent heat
Answer: (b) Dew Point
8. The measure of the actual amount of water vapour in the air, regardless of temperature:
(a) Absolute Humidity
(b) Humidity
(c) Dew Point
(d) Relative Humidity
Answer: (a) Absolute Humidity
9. The process by which products of condensation, viz water droplets, ice crystals, sleet etc. fall to the ground:
(a) Evaporation
(b) Humidation
(c) Precipitation
(d) Condensation
Answer: (c) Precipitation
10. Which of the following is not a type of a cloud:
(a) Cirrus
(b) Cumulus
(c) Strata
(d) Stratus
Answer: (c) Strata
11. Which of the following type of cloud has a cauliflower-like shape?
(a) Cirrus
(b) Cumulus
(c) Strata
(d) None of the above
Answer: (b) Cumulus
12. Condensation nuclei are
(a) impurities that support condensation.
(b) water particles that help in precipitation.
(c) droplets formed as a result of condensation.
(d) droplets that fall to the ground.
Answer: (a) impurities that support condensation.
13. Which of the following is NOT correct
(a) Frontal rainfall : cyclones
(b) Convectional rainfall : 4 o’clock showers
(c) Cyclonic rainfall : mountains
(d) Relief rainfall: mountain barriers
Answer: (c) Cyclonic rainfall : mountains
14. In the month of August after many rainy days there was a clear Sunday. We went for a picnic to a park. We felt so hot and uncomfortable that we changed plans and went to watch a movie instead. What caused us this discomfort?
(a) Latent heat released as a result of condensation raised the temperature.
(b) High humidity due to the rains of the previous day.
(c) Heat released from the surface of the earth raised the temperature.
(d) None of the above.
Answer: (b) High humidity due to the rains of the previous day.
15. Jyoti and her family had gone to the Little Andaman Island for a holiday. It was a clear day when the sky became dark with clouds and there was heavy rain late in the afternoon. This was the case every day that they spent there. What type of rainfall is this?
(a) Relief
(b) Convectional
(c) Cyclonic
(d) Frontal
Answer: (b) Convectional
Short Answer Questions
1. What determines the amount of water vapour in the air?
Answer: The amount of water vapour present in air is known as humidity. Humidity is related to temperature; the higher the temperature, the more is the water vapour present in it. When the atmospheric temperature is low, water vapour is less.
2. How is relative humidity determined?
Answer: 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 in percentage and can be determined using the formula:
Relative Humidity (RH) = (Actual amount / Water holding Capacity) × 100.
3. Name four chief forms of condensation.
Answer: Four chief forms of condensation are:
(i) Clouds
(ii) Dew
(iii) Frost
(iv) Fog and Mist
4. Mention the different forms of precipitation.
Answer: Rainfall, snowfall, drizzle, sleet and hail are the chief forms of precipitation. Rain is the most common form of precipitation; raindrops of smaller size and less intensity are known as drizzle. Snow occurs when water droplets rise higher and freeze on account of a drop in temperature. Hail forms when vertical air currents push water droplets or ice particles higher, where they form into solid ice and fall.
5. Give one difference between fog and mist.
Answer: One difference between fog and mist is that mist is less dense than fog and the visibility is relatively better. Visibility during fog extends to less than one kilometre but under conditions of mist, it may go up to two kilometres.
6. What do you understand by ‘condensation nuclei’?
Answer: Condensation nuclei are minuscule particles of dust, salt, and even smoke that act as particles around which water vapour condenses. These impurities in air are known as condensation nuclei, and without them, condensation does not take place.
7. State the three types of rainfall.
Answer: The three types of rainfall are Convectional Rainfall, Orographic Rainfall and Cyclonic Rainfall.
8. Why is orographic rainfall also called relief rainfall?
Answer: Orographic rainfall is also called relief rainfall because 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. Since it is caused by the relief of the land, it is called relief rain.
9. What is cyclonic rainfall? What are fronts?
Answer: Cyclonic rainfall is a type of rainfall due to cyclones (in Tropical Latitudes) and depressions (in Temperate Latitudes or the mid-latitudes), irrespective of relief or convection. It is caused by convergence (meeting) of two different air masses with different temperatures and other physical properties. When warm and cold air masses confront each other, the warmer (lighter) air generally climbs above the colder (heavier) air. The rising air is cooled while undergoing a frontal lift, which causes precipitation. Such precipitation or rainfall is called Cyclonic or Frontal Rainfall.
Fronts are the boundary zones of two different air masses with different temperatures and other physical properties, where the warmer (lighter) air generally climbs above the colder (heavier) air when they confront each other.
Explain the Following Terms
(a) Precipitation
Answer: Precipitation is the process by which droplets fall to ground in liquid, solid or frozen form. More specifically, it is the process by which products of condensation, viz water droplets, ice crystals, sleet, etc., fall to ground. Deposition of moisture from the atmosphere on the earth’s surface is called precipitation.
(b) Evaporation
Answer: Evaporation is the process by which water vapour enters the atmosphere on heating. The heat passes into water vapour in a hidden form, known as latent heat.
(c) Condensation
Answer: Condensation is the process by which water vapour forms water droplets on cooling. It is the reverse process of evaporation. In condensation, water vapour in the atmosphere gets converted into water droplets or ice. During condensation, the latent heat locked in water vapour is released back into the atmosphere.
(d) Humidity
Answer: The amount of water vapour present in air is known as the humidity. Water is added to the atmosphere by the process of evaporation, changing from liquid to gaseous form. The water vapour in air is called humidity. Humidity is related to temperature; the higher the temperature, the more is the water vapour present in it. When the atmospheric temperature is low, water vapour is less.
Structured Questions
1. (a) What is ‘humidity’? How is humidity measured?
Answer: ‘Humidity’ is the amount of water vapour present in air.
The humidity of the atmosphere is measured with the help of a Hygrometer, also known as Dry and Wet Bulb Thermometer.
(b) Distinguish between Absolute Humidity and Relative Humidity.
Answer: The differences between Absolute Humidity and Relative Humidity are as follows:
| Absolute Humidity | Relative Humidity |
| It is the measure of the actual amount of water vapour in the air, regardless of temperature. | It measures water vapour in air but relative to the maximum vapour that the air can hold at that temperature. |
| It is expressed as grams of moisture per cubic meter of air (g/m³). | It is expressed as the ratio (percentage value) 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. |
| The higher the amount of water vapour, the higher is the absolute humidity. | Warm air possesses more water vapour than cold air, so with the same amount of absolute humidity, air will have a different relative humidity depending on temperature. |
| In weather calculations, absolute humidity is generally not taken into account. | It is the essential characteristic of weather forecasts, which indicates the likelihood of precipitation, dew or fog. |
(c) Give a geographical reason for each of the following:
(i) The total volume of water in the oceans and seas remains constant.
Answer: The total volume of water in the oceans and seas remains constant because all the water that evaporates from the earth’s water bodies is eventually returned to it directly by the process of condensation and precipitation; and indirectly by stream and overflow from land surfaces.
(ii) In Equatorial regions the sky often remains overcast with clouds.
Answer: In Equatorial regions, the high temperature leads to the rapid heating of air. Such heated air rises in convectional currents, leading to the development of clouds at about 10 km height. Ascending currents of hot and humid air cause condensation of the clouds, which results in the sky often remaining overcast.
(iii) Human comfort depends on humidity.
Answer: Human comfort depends on humidity because the human body dissipates heat through perspiration and its evaporation. Under conditions of high relative humidity, the rate of evaporation of sweat from the skin decreases and the human being feels warm and uncomfortable. Conversely, very dry air can also cause discomfort. Highly humid air is more oppressive.
(d) Draw a well labelled diagram showing orographic rainfall.
Answer: See Fig. 15.6 of Morning Star/Total Geography book or click here.
2. (a) What is condensation? Explain briefly the process of condensation.
Answer: Condensation is the process by which water vapour forms water droplets on cooling; it is the reverse process of evaporation. In condensation, water vapour in the atmosphere gets converted into water droplets or ice.
During the process of condensation, the latent heat locked in water vapour is released back into the atmosphere. Condensation can take place only when water vapour is added to saturated air or when the temperature falls below the temperature at which air becomes saturated. For example, at 25°C temperature, air can hold 30 gm of water vapour per cubic metre. If the temperature falls to 20°C, it can hold only 24 gm of water vapour. The remaining 6 gm of water vapour will get condensed.
(b) Explain the atmospheric conditions that favour condensation.
Answer: Condensation takes place when the following atmospheric conditions exist:
(i) There should be a high amount of water vapour present in atmosphere.
(ii) Minuscule particles of dust, salt, and even smoke act as condensation nuclei (i.e., particles around which the water vapour condenses).
(iii) The temperature of air must be below dew point temperature so as to encourage condensation.
(c) Give a geographical reason for each of the following:
(i) Condensation is the reverse process of evaporation.
Answer: Condensation is the reverse process of evaporation because evaporation is the process by which water vapour enters the atmosphere on heating, changing from liquid to gaseous state, and it involves the absorption of latent heat. Condensation is the process by which water vapour forms water droplets or ice on cooling, changing from gaseous state to liquid or solid, and it involves the release of this latent heat.
(ii) Coasts receive more rainfall than the interior of the continents.
Answer: Heating of water over oceans, lakes and rivers causes water to evaporate, and this water vapour is essential for condensation and precipitation. Coastal areas are in closer proximity to oceans, which are vast sources of water for evaporation. This provides more moisture in the air over coastal regions, which can lead to higher rainfall compared to interior regions that are further from these large water bodies and may have less available moisture. For example, orographic rainfall is especially heavy in hot and humid areas bounded by tropical oceans.
(iii) The windward sides of mountains receive more rainfall than the leeward sides.
Answer: The windward sides of mountains receive more rainfall because warm moist air ascends above the mountain barrier, and this sudden ascent causes cooling of air, leading to condensation and precipitation on the windward side. 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 this rain shadow area.
(d) Draw a well labelled diagram showing convectional rainfall.
Answer: See Fig. 15.7 of Morning Star/Total Geography book or click here.
3. (a) How is dew formed? What is the difference between fog and mist?
Answer: Dew is formed when water vapour condenses on the surface, forming tiny droplets of water. Dew commonly occurs during winter on account of cooling of air below dew point.
The difference between fog and mist is that mist is less dense than fog and the visibility is relatively better. Visibility during fog extends to less than one kilometre but under conditions of mist, it may go up to two kilometres.
(b) Describe briefly the orographic and convectional rainfall.
Answer: Orographic Rainfall: Also known as Relief Rainfall, it occurs from the cooling of warm moist air which ascends above a 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. Since it is caused by the relief of the land, it is called relief rain. 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 no precipitation in the rain shadow area.
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 the development of clouds at about 10 km height. Ascending currents of hot and humid air cause condensation of the clouds, resulting in heavy rainfall. This type of rainfall occurs in the afternoon at about 4 O’clock and is known as the ‘4 O’clock Shower.’ Such rainfall is also accompanied by thunder and lightning and occurs daily in the Equatorial regions, which receive an annual rainfall of more than 200 cm.
(c) Give a geographical reason for each of the following:
(i) A rain shadow area is generally dry.
Answer: A rain shadow area is generally dry because as air descends on the leeward slope of a mountain, a decrease in altitude increases both the pressure and the temperature. This leads 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.
(ii) Frontal rain is common in mid-latitudes.
Answer: Frontal rain, also known as cyclonic rainfall, is due to depressions in Temperate Latitudes or the mid-latitudes. In temperate depressions, such as those of Western Europe, this type of rainfall occurs, making it common in mid-latitudes.
(iii) Convectional rainfall is called 4 o’clock rainfall.
Answer: Convectional rainfall is called 4 o’clock rainfall because the high temperatures during the day lead to the rapid heating of air. This heated air rises in convectional currents, leading to the development and condensation of clouds. This entire process typically culminates in heavy rainfall in the afternoon, at about 4 O’clock, especially in Equatorial regions.
(d) Define precipitation. Explain briefly the three forms of precipitation.
Answer: Precipitation is the process by which products of condensation, viz water droplets, ice crystals, sleet, etc., fall to ground. It is also defined as the deposition of moisture from the atmosphere on the earth’s surface.
Three chief forms of precipitation are:
- Rain: It is the most common form of precipitation. Raindrops of smaller size and less intensity are known as drizzle.
- Snow: Water droplets which rise higher and freeze on account of drop in temperature form snow. Snowfall usually occurs in winter in cold climates or on high mountains.
- 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.
Thinking Skills
1. Condensation is regarded as the reverse process of evaporation. Imagine if this reverse process does not happen, what would be its consequences on climate and human beings? Also state the conditions under which this reversal may not happen.
Answer: If condensation does not take place, clouds would not form and there would be no rainfall. This would disrupt the water cycle and severely affect agriculture, as crops rely on rainfall. Rivers, lakes and groundwater would not be replenished, leading to water scarcity. Moreover, the absence of precipitation would increase aridity in many regions, possibly leading to desertification. For humans, it would lead to extreme discomfort, particularly in humid regions, as the cooling effect of rainfall would be absent.
Condensation may not happen if:
- There is not enough water vapour in the atmosphere.
- The air does not cool below the dew point.
- There are no condensation nuclei like dust or salt particles around which vapour can condense.
2. Name two physical objects in your home which are affected by humidity. Also state the reason for such an effect.
Answer: (i) Wooden furniture – High humidity causes wood to absorb moisture from the air, leading to swelling and warping.
(ii) Metal items (like tools or appliances) – Humid conditions can lead to condensation on metal surfaces, which may cause rusting over time.
3. List two places which receive two types of rainfall. Give reasons to support your answer.
Answer: (i) Western Ghats, India – This region receives relief rainfall due to the monsoon winds hitting the mountain barrier, and also convectional rainfall during the afternoons in summer due to intense heating.
(ii) Eastern United States – This area experiences cyclonic rainfall during temperate depressions and convectional rainfall in summer due to rising warm air and high humidity.
Project Work/Map Work
List the forms of precipitation that occur in your city along with the time of year when they occur.
Answer: Do it yourself. Here are the instructions:
- First, understand what precipitation is: it means water falling from the sky in different forms such as rain, snow, or hail.
- Look around in your city throughout the year and note down what type of precipitation you observe (examples: rain, snow, drizzle, hail).
- Use your daily experiences or ask elders at home about the usual weather in different months.
- For example:
- Rain – Mostly in the monsoon season (June to September).
- Drizzle – During light rain in cooler months or start/end of monsoon.
- Hail – Sometimes in summer or pre-monsoon thunderstorms (March to May).
- Snow – Only if your city is in a mountainous area (like Shimla or Manali) and usually during winter (December to February).
- Write your answer in a list format, like:
- Rain – June to September
- Hail – March to May
- Drizzle – November and February
- Snow – December to February (only if applicable)
- Keep it simple and stick to your city or nearby places.
- Optional: Use the weather forecast on your phone or Google to confirm the rainy or snowy months for your city.
Extras
Additional MCQs
1. Which process describes water vapour entering the atmosphere on heating?
A. Condensation
B. Evaporation
C. Precipitation
D. Humidity
Answer: B. Evaporation
45. In a dry and wet bulb thermometer, why is the wet bulb cooler than the dry bulb?
A. Conduction
B. Radiation
C. Evaporation
D. Condensation
Answer: C. Evaporation
Additional Assertion and Reason
1. Assertion (A): During the rainy season, one often feels more heat than when the skies are clear.
Reason (R): Latent heat is released back into the atmosphere when condensation occurs.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R does not explain A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a) Both A and R are true and R is the correct explanation of A.
15. Assertion (A): A rain shadow area is generally dry.
Reason (R): On the leeward slope, air descends, gets compressed and warms, causing relative humidity to drop and little to no precipitation.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R does not explain A.
(c) A is true but R is false.
(d) A is false but R is true.
Answer: (a) Both A and R are true and R is the correct explanation of A.
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