- The hydrological cycle, also known as the water cycle, describes the continuous movement and exchange of water between the Earth’s surface, atmosphere, and oceans.
- This cycle is driven by solar energy, which causes evaporation of water from oceans, lakes, and rivers, as well as transpiration from plants.
Water Vapour in Atmosphere
- Water vapor in air varies from zero to four percent by volume of the atmosphere (averaging around 2% in the atmosphere). The amount of water vapor (Humidity) is measured by an instrument called a Hygrometer.
Significance of Atmospheric Moisture
- Water vapor is a potent greenhouse gas that absorbs and emits radiation, including both incoming solar radiation and terrestrial radiation emitted by the Earth’s surface.
- The amount of water vapor present in the atmosphere is a critical factor in determining the energy available for the development of storms and cyclones.
- The sensible temperature is the temperature perceived by the human body based on a combination of air temperature, humidity, and air movement.
Evaporation
- The oceans cover about 71% of the Earth’s surface and hold about 97% of all the Earth’s water reserves. The remaining 3% of the Earth’s water is found in glaciers, lakes, rivers, and the atmosphere.
- The oceans contribute about 84% of the annual total evapotranspiration, while the continents contribute about 16%.
- The highest annual evaporation rates occur in the subtropics of the western North Atlantic and North Pacific oceans due to the influence of the warm Gulf Stream and Kuroshio Current, respectively, and in the trade wind zone of the southern oceans.
- On land, the maximum evaporation rates occur in the equatorial region, where the high levels of insolation and abundant vegetation lead to high rates of transpiration.
Humidity
- Humidity refers to the amount of water vapor present in the air. It is typically expressed as either relative humidity or specific humidity.
- Specific humidity is the mass of water vapor present in a unit mass of air. It is typically expressed in grams of water vapor per kilogram of dry air.
- Measuring humidity requires specialized instruments such as a hygrometer or psychrometer.
Absolute Humidity
- Absolute humidity refers to the actual amount of water vapor present in the atmosphere, expressed as the weight of water vapor per unit volume of air, typically in grams per cubic meter.
- It is a measure of the actual amount of water vapor present in the air, regardless of temperature or pressure.
- The absolute humidity varies from place to place on the surface of the Earth and can also change over time due to changes in temperature, pressure, and atmospheric circulation patterns.
- The ability of air to hold water vapor, or its water-holding capacity, depends mainly on its temperature.
Relative Humidity
- The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as relative humidity.
- Relative Humidity = [Actual amount of water vapor in the air (absolute humidity)/humidity at the saturation point (the maximum water vapor air can hold at a given temperature)] X 100.
- With the change of air temperature, the capacity to retain moisture increases or decreases, and the relative humidity is also affected.
- The relative humidity is generally higher over oceans and lower over continents because the availability of water for evaporation is greater over the oceans.
- Relative humidity is an important climatic factor as it determines the amount and rate of evaporation, which is a crucial component of the water cycle and influences various aspects of weather and climate.
- The relative humidity of the saturated air is 100% because the actual amount of water vapor in the air is equal to the maximum amount of water vapor that the air can hold at that temperature.
- The percentage of moisture present in the atmosphere as compared to its full capacity at a given temperature is known as relative humidity.
- Relative Humidity = [Actual amount of water vapor in air (absolute humidity)/humidity at the saturation point (the maximum water vapor air can hold at a given temperature)] X 100
- With the change in air temperature, the capacity to retain moisture increases or decreases, and the relative humidity is also affected.
- The relative humidity is greater over the oceans and least over the continents (absolute humidity is greater over oceans because of the greater availability of water for evaporation).
- The relative humidity determines the amount and rate of evaporation and hence it is an important climatic factor.
- Air containing moisture to its full capacity at a given temperature is said to be ‘saturated’. At this temperature, the air cannot hold any additional amount of moisture. Thus, the relative humidity of the saturated air is 100%.
- If the air has half the amount of moisture that it can carry, then it is unsaturated and its relative humidity is only 50%.
Relative humidity can be changed in either of the two ways:
- By adding moisture through evaporation (by increasing absolute humidity): if moisture is added by evaporation, the relative humidity will increase and vice versa.
- By changing the temperature of air (by changing the saturation point): a decrease in temperature (hence, a decrease in moisture-holding capacity/decrease in saturation point) will cause an increase in relative humidity and vice versa.
Consider 1 m3 of air at a temperature ‘T’.
- Let us assume that saturation occurs when 0.5 kg of water vapor is present in 1 m3 of air.
- That is, relative humidity will be 100% if 1 m3 of air contains 0.5 kg of water vapor at temperature T (saturation temperature or saturation point).
- Assume that 1 m3 of air at a given time consists of 0.2 kg of water vapor at a temperature ‘T’.
- Now the relative humidity = 40 % => 0.2 kg of water vapor per 1 m3 of air => the air can still hold 0.3 kg of water vapor since saturation occurs at 0.5 kg.
Here,
Absolute Humidity = 0.2 kg/ m3 and
Relative Humidity = 40 %
- So, relative humidity is expressed as % whereas absolute humidity is expressed in absolute terms.
- Now to make the air saturated (100 % relative humidity), we can add that additional 0.3 kg of water vapor by evaporation. OR we can decrease the temperature. If we decrease the temperature, the saturation point will come down.
- Let us assume that the temperature of 1 m3 of air is decreased by 2 °C. The water holding capacity will fall due to a decrease in temperature. Let us assume that the water holding capacity decreases by 0.1 kg per m3 of air per 1 °C fall in temperature.
- So, for a 2 °C fall in temperature, the fall in water holding capacity is 0.2 kg/m3 of air (0.1 kg/m3 x 2). Hence the new saturation point occurs at 0.3 kg/m3 of air [0.5 kg/m3 – 0.2 kg/m3].
- That is, the ‘new saturation point’ (relative humidity = 100%)” occurs when the water vapor content is 0.3 kg per 1 m3 of air. So now we can saturate 1 m3 of air by adding just 0.1 kg instead of 0.3 kg as in the earlier case. [because, initially, we assumed that 1 m3 of air at a given time consists of 0.2 kg of water vapor at a temperature ‘T’.]
Dew point
- The air containing moisture to its full capacity at a given temperature is said to be saturated.
- It means that the air at the given temperature is incapable of holding any additional amount of moisture at that stage.
- The temperature at which saturation occurs in a given sample of air is known as dew point.
- The dew point occurs when Relative Humidity = 100%.
Specific Humidity
- It is expressed as the weight of water vapor per unit weight of air.
- Since it is measured in units of weight (usually grams per kilogram), the specific humidity is not affected by changes in pressure or temperature.
- Absolute Humidity and Relative Humidity are Variable whereas Specific Humidity is a constant.
Frequently Asked Questions (FAQs)
1. What is the Hydrological Cycle (Water Cycle)?
The Hydrological Cycle, commonly known as the Water Cycle, is a natural process that describes the continuous movement and circulation of water on, above, and below the Earth’s surface. It involves the transformation of water between its various states—liquid, solid (ice), and gas (water vapor)—as it moves through the atmosphere, land, and oceans. The key processes in the water cycle include evaporation, condensation, precipitation, infiltration, runoff, and transpiration.
2. How does Evaporation contribute to the Hydrological Cycle?
Evaporation is a crucial component of the Hydrological Cycle as it represents the process through which liquid water on the Earth’s surface transforms into water vapor and enters the atmosphere. This occurs primarily due to solar energy heating the Earth’s surface, causing water molecules to gain enough energy to break free from the liquid state. As water vapor rises into the atmosphere, it eventually cools and condenses to form clouds. Subsequently, the condensed water droplets may lead to precipitation, completing one cycle of the water cycle.
3. What role do Rivers and Oceans play in the Hydrological Cycle?
Rivers and oceans are integral components of the Hydrological Cycle, serving as major reservoirs and pathways for water movement. Precipitation, in the form of rain or snow, replenishes rivers and oceans. Runoff from the land, which includes water from rainfall or melting snow, flows into rivers and eventually reaches the oceans. Oceans are vast reservoirs of water and also contribute to the cycle through evaporation, releasing water vapor into the atmosphere. This water vapor may later condense to form clouds and return to the Earth’s surface as precipitation, completing the continuous cycle of water movement on our planet.
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