Humidity

1. With reference to “water vapour”, which of the following statements is

With reference to “water vapour”, which of the following statements is/are correct?

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  • It is a gas, the amount of which decreases with altitude.
  • Its percentage is maximum at the poles.

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1 only
2 only
Both 1 and 2
Neither 1 nor 2
This question was previously asked in
UPSC IAS – 2024
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Statement 1: Water vapour is the gaseous state of water and is a greenhouse gas. Its concentration in the atmosphere varies greatly but is generally highest near the Earth’s surface due to evaporation from water bodies and plants. As altitude increases, temperature and pressure decrease, limiting the amount of water vapour the air can hold (dew point decreases). Therefore, the amount of water vapour typically decreases significantly with altitude. This statement is correct.
Statement 2: The amount of water vapour the air can hold is strongly dependent on temperature. Warmer air can hold much more water vapour than cold air. The poles are very cold regions. Consequently, the absolute amount and percentage of water vapour in the atmosphere are generally lowest at the poles and highest in warm, humid tropical and equatorial regions. This statement is incorrect.
Water vapour concentration is inversely related to altitude and directly related to temperature; it is highest near the surface and in warm regions.
Although its percentage is low at the poles, water vapour still plays a role in the polar climate, and changes in Arctic water vapour are being studied in the context of Arctic amplification (warming). Globally, water vapour is the most abundant greenhouse gas.

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2. Relative humidity

Relative humidity

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increases with increased temperature
decreases with increased temperature
decreases with decreased temperature
has no relation with increase or decrease of temperature
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UPSC CAPF – 2011
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Relative humidity decreases with increased temperature, assuming the absolute amount of water vapour in the air remains constant.
Relative humidity is defined as the ratio of the partial pressure of water vapour to the equilibrium vapour pressure of water at a given temperature. The equilibrium vapour pressure (the maximum amount of water vapour the air can hold) increases significantly with increasing temperature.
If the amount of water vapour in the air stays the same, but the temperature rises, the air’s capacity to hold water vapour increases. This means the actual vapour content becomes a smaller percentage of the maximum possible content, resulting in a decrease in relative humidity.

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3. Which of the following statements regarding humidity is/are correct ?

Which of the following statements regarding humidity is/are correct ?

  • 1. It is a ratio between the actual amount of water vapour present in the atmosphere and the maximum amount it can hold at a given temperature.
  • 2. When the relative humidity is high, more water evaporates from the skin.
  • 3. Higher the air temperature, lower the relative humidity of the air.

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1 only
2 and 3 only
1 and 3 only
1, 2 and 3
This question was previously asked in
UPSC NDA-1 – 2024
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Statements 1 and 3 are correct, while statement 2 is incorrect.
Statement 1 is correct. Relative humidity is indeed the ratio, usually expressed as a percentage, of the amount of water vapour currently in the air compared to the maximum amount of water vapour the air can hold at that specific temperature and pressure (its saturation capacity). Statement 2 is incorrect. When relative humidity is high, the air is already close to saturation, meaning it has little capacity to absorb more moisture. Therefore, evaporation from surfaces like skin is *reduced*, making it feel hotter and more uncomfortable. Statement 3 is correct. For a fixed amount of water vapour in the air, if the air temperature increases, the maximum amount of water vapour that air can hold also increases. Since relative humidity is the ratio of actual to maximum capacity, the relative humidity will decrease.
Absolute humidity is the mass of water vapour in a unit volume of air. Specific humidity is the mass of water vapour per unit mass of moist air. Dew point is the temperature at which air becomes saturated and condensation begins.

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4. Dew point temperature represents that

Dew point temperature represents that

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thin layer of air near the ground surface has become cold enough to condensate
there is thorough mixing of air
evaporation has exceeded condensation
the night temperature is warm
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UPSC Geoscientist – 2022
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The dew point temperature is defined as the temperature to which air must be cooled at constant pressure and humidity for water vapor to condense into liquid water (dew). When the air near the ground surface cools down to this temperature, it becomes saturated, and any further cooling leads to condensation, forming dew.
– Dew point is the temperature of saturation for water vapor.
– Condensation occurs when air cools to or below the dew point.
– Dew forms when surface air cools to the dew point temperature.
Relative humidity is 100% at the dew point. The difference between the air temperature and the dew point indicates how close the air is to saturation. A smaller difference means higher relative humidity.

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5. How can one determine the relative humidity on a particular day at a g

How can one determine the relative humidity on a particular day at a given place?

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If temperature is given
If absolute humidity is given
If both temperature and absolute humidity are given
If percentage of absolute humidity is given
This question was previously asked in
UPSC CDS-1 – 2024
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Relative humidity on a particular day at a given place can be determined if both temperature and absolute humidity are given.
Relative humidity is defined as the ratio of the partial pressure of water vapour in the air to the equilibrium vapour pressure of water at the same temperature. Equivalently, it can be expressed as the ratio of absolute humidity (mass of water vapour per unit volume of air) to the saturation humidity (maximum possible mass of water vapour per unit volume of air) at that temperature.
To calculate relative humidity, you need the actual amount of water vapour in the air (like absolute humidity) and the maximum amount of water vapour the air can hold at that specific temperature (saturation humidity). The maximum capacity of air to hold water vapour (saturation humidity/vapour pressure) is highly dependent on temperature. Therefore, knowing only temperature or only absolute humidity is insufficient; both are required. Percentage of absolute humidity is not a standard meteorological term used to determine relative humidity without reference to saturation capacity.

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6. Atmospheric conditions are well-governed by humidity. Which one among

Atmospheric conditions are well-governed by humidity. Which one among the following may best define humidity?

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Form of suspended water droplets caused by condensation
Deposition of atmospheric moisture
Almost microscopically small drops of water condensed from and suspended in air
The moisture content of the atmosphere at a particular time and place
This question was previously asked in
UPSC CDS-1 – 2019
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The correct answer is D. Humidity is a term used to describe the amount of water vapor present in the atmosphere. It is a measure of the moisture content of the air at a specific time and location. Humidity can be expressed in various ways, such as absolute humidity (mass of water vapor per unit volume or mass of air) or relative humidity (ratio of actual water vapor content to the maximum possible water vapor content at a given temperature and pressure).
Humidity is defined as the moisture content of the atmosphere.
Options A and C describe phenomena like fog or clouds, which are results of condensation when the air becomes saturated with water vapor (i.e., reaches 100% relative humidity). Option B describes deposition processes like dew or frost. None of these directly define humidity itself, which is the presence of water vapor in the air.

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