Lesson 3: Humidity and Clouds

Rule 1: Moisture and the air of a place — Words like hot, moist and cool describe the air. They tell us about the moisture in it, which is invisible but a major part of weather. For precipitation (rain or snow) moisture must be present. Rajasthan: hot, dry air (loose cotton clothes). Kashmir valley: cold, dry air (warm clothes). Mumbai: hot, humid air, even more when dark clouds cover the sky.

In the figure : In the figure Rajasthan is hot and dry, the Kashmir valley is cold and dry and Mumbai is hot and humid.

• Easy : Easy: Rajasthan air is dry.
• Medium : Medium: Mumbai is hot and humid.
• Exam-type : Exam: Name the place that is cold and dry.

Rule 2: What decides evaporation — Evaporation depends on the dryness of the air, its temperature and the speed of the wind. In dry, hot air it is fast; it continues even in dry, cold air; in moist air it is very slow. High wind speed and temperature make it rapid; slow wind and cooler air make it slow. If the air near the water is saturated, evaporation does not occur. A breeze increases it.

In the figure : In the figure evaporation is fast in hot, dry and windy air and slow in cool, calm and moist air; it does not occur if the air near the water is saturated.

• Easy : Easy: Clothes dry fast in sun and wind.
• Medium : Medium: A wet cloth dries in 1 hour in summer and 6 hours in the monsoon, which is 6 times slower.
• Exam-type : Exam: Why does a breeze help drying?

Spot the difference

Rule 1: Holding capacity — Air can hold only a certain amount of vapour at a certain temperature. Warm air holds more than cold air; when air cools, its holding capacity falls. When capacity equals the vapour present, the air is saturated. If vapour exceeds capacity, precipitation occurs. Capacity of 1 m³ of air (g/m³): −5 °C 3.26; 0 °C 4.85; 5 °C 6.80; 10 °C 9.40; 15 °C 12.83; 20 °C 17.30; 30 °C 30.37; 40 °C 51.12. Higher up, the air is cooler, so capacity decreases.

In the figure : In the figure warm air holds more vapour than cold air: 1 m³ holds 4.85 g at 0 °C, 17.30 g at 20 °C and 51.12 g at 40 °C.

• Easy : Easy: At 20 °C air holds 17.30 g/m³.
• Medium : Medium: From 20 °C to 0 °C the capacity falls by 17.30 − 4.85 = 12.45 g/m³.
• Exam-type : Exam: If air at 20 °C has 17.30 g and cools to 10 °C, how much vapour must leave?

Rule 2: Absolute and relative humidity — Absolute humidity (निरपेक्ष आर्द्रता) is the vapour in 1 m³ of air, in g/m³; dry air has 0. It is higher near coasts than the interior, higher at the equator and lower towards the poles. Relative humidity (R.H.) is the vapour as a percentage of the amount needed for saturation at the same temperature: R.H. (%) = absolute humidity ÷ capacity × 100. Example: 9 g/m³ at 30 °C (capacity 30.37) → 9 ÷ 30.37 × 100 = 29.6%. Example: 8.65 g/m³ at 20 °C → 8.65 ÷ 17.30 × 100 = 50%. At saturation R.H. = 100%.

In the figure : In the figure relative humidity = absolute humidity ÷ holding capacity × 100: 20 ÷ 30 × 100 = 66.7%; when the vapour equals the capacity the air is saturated (100%).

• Easy : Easy: Saturated air has R.H. 100%.
• Medium : Medium: 6.80 g at 5 °C (capacity 6.80) gives R.H. 100%.
• Exam-type : Exam: Find R.H. for 15 g at 40 °C. 15 ÷ 51.12 × 100 = 29.3%.

Rule 3: Daily changes — R.H. is more in the morning and at night. In the afternoon, as the temperature rises, it falls. It is high near coasts and low in deserts.

In the figure : In the figure relative humidity is higher in the mornings and at night and lower in the afternoon, when the temperature is the highest.

• Easy : Easy: R.H. is lowest in the afternoon.
• Medium : Medium: Vapour same, capacity rises with heat, so R.H. falls.
• Exam-type : Exam: Why is R.H. low in deserts?

Spot the difference

Rule 1: Condensation — The change of vapour (gas) into a solid is sublimation (ऊर्ध्वपातन). When R.H. becomes 100%, vapour starts condensing; the air is then at the dew point. Condensation needs low temperature and high R.H. In the open it occurs around fine particles such as dust and salt. Hot air rises and cools; its capacity falls and R.H. keeps rising until the dew point, where vapour turns to water or snow.

In the figure : In the figure condensation: warm moist air rises and cools, the relative humidity reaches 100% at the dew point and the vapour condenses around dust particles.

• Easy : Easy: Dew point means R.H. is 100%.
• Medium : Medium: Dust gives drops something to form on.
• Exam-type : Exam: State two conditions for condensation.

Rule 2: Dew, frost, fog, smog — Dew, frost and fog are condensation at ground level; clouds form at a higher level. Dew: tiny water drops on cooled surfaces like grass and leaves. Frost: vapour turns to ice on leaves and grass in cold weather. Fog: tiny water or ice particles floating near the ground; visibility falls. Smog = smoke + fog, in polluted cities.

In the figure : In the figure dew, frost, fog and smog are condensation at ground level; smog is smoke mixed with fog.

• Easy : Easy: Dew forms on grass.
• Medium : Medium: Fog lowers visibility.
• Exam-type : Exam: Smog = smoke + fog.

Rule 3: Two easy activities — Activity 1: steam from a cooker cools on the cooler lid and condenses into drops. Activity 2: vapour in the air condenses on the cold outer surface of a glass with ice cubes.

In the figure : In the figure steam turns to drops on the cooler lid of a cooker, and the vapour in the air condenses on the cold outside of a glass with ice cubes.

• Easy : Easy: Drops form on a cold glass.
• Medium : Medium: Warm room air cools near the glass to the dew point.
• Exam-type : Exam: Explain the glass activity.

Spot the difference

Rule 1: How clouds float and three heights — Fine particles gather around dust particles and grow. They are almost weightless, so the vertical flow of wind keeps them floating, like a kite. High clouds are made of very fine snow particles. Vapour is more near the earth, so low clouds are larger. By height of the base above mean sea level: High clouds 7000–14000 m; Medium 2000–7000 m; Low below 2000 m.

In the figure : In the figure by the height of the base above mean sea level: high clouds 7000–14000 m, medium clouds 2000–7000 m and low clouds below 2000 m.

• Easy : Easy: There are 3 main types.
• Medium : Medium: High clouds span 14000 − 7000 = 7000 m.
• Exam-type : Exam: Which clouds are largest and why?

Rule 2: Ten types — The international classification has 10 types. High: Cirrus (wispy), Cirro-cumulus (small waves), Cirro-stratus (like a wrinkled sheet, often with a halo). Medium: Alto-cumulus (wave-like layers, white with grey), Alto-stratus (thin; sun seen as through milky glass). Low: Strato-cumulus (round clusters), Stratus (ash-coloured uniform layers), Nimbostratus (thick grey layers; continuous rain or snow), Cumulus and Cumulonimbus.

In the figure : In the figure the ten types: cirrus, cirro-cumulus, cirro-stratus (high); alto-cumulus, alto-stratus (medium); strato-cumulus, stratus, nimbostratus, cumulus, cumulonimbus (low).

• Easy : Easy: Cirrus is a high cloud.
• Medium : Medium: 3 high + 2 medium + 5 low = 10.
• Exam-type : Exam: Name the five low clouds.

Spot the difference

Rule 1: Cumulus and cumulonimbus — Cumulus: found between 500 and 6000 m; huge, dome-shaped, grey; they indicate pleasant weather. When their vertical extent grows a lot they become cumulonimbus and bring rain. Cumulonimbus: they indicate a thunderstorm; look like huge mountains, dense and dark, with an anvil-shaped top. They bring storm rain, sometimes hailstones, but not for long.

In the figure : In the figure cumulus clouds indicate pleasant weather; cumulonimbus clouds indicate a thunderstorm and have an anvil-shaped top.

• Easy : Easy: Cumulus = pleasant weather.
• Medium : Medium: Cumulus can grow into cumulonimbus.
• Exam-type : Exam: Describe the shape of a cumulonimbus.

Rule 2: Lightning, thunder, hail — The upper end of the cloud is positive, the lower end negative, and the land below is negative. The difference in charges causes lightning (वीज). The air around lightning is heated and rises, causing thunder (गडगडाट). Drops move up and down many times and collect more water; when too heavy they fall as rain. If the air is very cold, drops freeze and fall as hail (गारा).

In the figure : In the figure in a cumulonimbus cloud the upper end is positive and the lower end is negative; the land below is negative. The difference in charges causes lightning and the heated air causes thunder.

• Easy : Easy: Lightning comes before we hear thunder.
• Medium : Medium: Sound 340 m/s takes 3 s for 1020 m.
• Exam-type : Exam: Why does hail form?

Rule 3: Cloudburst — Strong vertical winds stop the raindrops inside the cloud and they turn into hail. The cloud becomes heavier and the winds cannot bear the weight. Then heavy rain with large hail falls: more than 100 mm in a small area. It occurs mainly in mountain regions, in states through which the Himalayan ranges pass. Nimbostratus gives continuous rain. Precipitation occurs when moisture exceeds holding capacity.

In the figure : In the figure a cloudburst: strong vertical winds stop the drops, they turn into hail and the heavy cloud gives more than 100 mm of rain in a small area.

• Easy : Easy: A cloudburst gives over 100 mm.
• Medium : Medium: 100 mm = 10 cm.
• Exam-type : Exam: Where do cloudbursts occur mainly?

Spot the difference