The value of water vapour is just two per cent in the total composition of the atmosphere even though it shows spatial and temporal variation from near zero to five per cent. This two per cent water vapour plays a vital role in the weather of a particular area. The content of water vapour in the air is referred to as humidity.
WATER VAPOUR IN THE ATMOSPHERE
Water vapour is an important greenhouse gas and an important factor in weather. The water vapour originates from land, and the ocean to the atmosphere and back to land and ocean, called a Hydrological cycle. If we calculate the mass of water vapour in the atmospheric air, it is about 13 million million tonnes or 13 teratonnes. This 2 per cent is enough to provide a rainfall of twenty-five millimetres over the entire earth. However, due to the balance in the hydrological cycle, it happens in a controlled manner.
The process of transforming liquid into a gas is called evaporation, and the process of converting gas into water is called condensation. Water has a high specific heat. The heat flow needed to raise the temperature of 1 gram of a substance by 1 degree celsius is 15 degrees Celsius or fifty-nine degrees Fahrenheit.
What is the reason for this extraordinary feature? The particular arrangement of the two hydrogen atoms and one oxygen atom in each water molecule is responsible for this distinctive feature.
PROCESS OF TRANSFORMATION
There are six possible transformations of a substance from one to another
Out of 6 now, we will see the four possible transformations.
- Gas to liquid – condensation.
- Liquid to solid – freezing.
- Solid to gas – sublimation.
- Solid to liquid – melting.
In the case of clouds, the conversion of gas to solid is called deposition. The frost on a cold winter night is an example. One more concept is important when we look at the hydro cycle. It is Ablation, which is the simultaneous melting and sublimation.
WHAT IS LATENT HEAT?
The heat energy required to change a substance from one state or form to another is called latent heat. The energy lost by water during evaporation is present in it.
If you examine a small drop of water. Due to evaporation, molecules escape from its surface. The more energetic, faster-moving molecules escape easily. This tendency decreases with a decrease in temperature. So evaporation is a cooling process because energy is required for it. Energy is required to change its phase, which may come from water and other sources, including the air around it.
After evaporation, the vapour contains the energy absorbed during the change and contains latent heat. Latent heat means hidden heat, which was first proposed by Joseph Black. The hidden heat cannot be felt or measured with a thermometer. Any change of state involves the loosening of bonding that lies between molecules. This change is because of thermal vibrations.
The heat energy released when water vapour changes to liquid droplets is called latent heat of condensation. The heat energy needed to convert liquid into vapour at the same temperature is called latent heat of evaporation.
To change a kilogram of ice at zero degrees Celsius to water vapour, we need at least 2830 kilojoules. The resulting molecular motion disrupts the bonds entirely and produces vapour.
Yes, the statement seems to be confusing. In simple words, we need more energy to convert solid into vapour, or we need more energy during sublimation.
The evaporation of liquid requires less energy than sublimation. Here, we need only about 2460 kilojoules because liquid has weaker bonds than solid.
Latent heat is an important source of atmospheric energy.
VAPOUR PRESSURE
Let’s consider an air parcel near sea level, with 1000 millibars of air pressure inside. Now, how will you define the reason for the total air pressure inside the parcel? Naturally, it is due to the collision of all molecules due to the to and for movement against each other and with the walls of the parcel. The process mentioned above occurs at the same temperature. This model of colliding molecules is based on the Kinetic theory of Gases proposed by Rudolf Clasusis.
Since the total pressure is 1000 millibars, the nitrogen, oxygen, and one to two per cent water vapour effectively play their roles. The partial pressure exerted by nitrogen will be 780 millibars, oxygen will be 210 millibars, and water vapour will be 10 to 20 millibars. The partial pressure exerted by water vapour molecules is called the actual vapour pressure. An increase in water molecules will increase total vapour pressure. So, the actual vapour pressure can clearly measure the total amount of water vapour in the air.
If the actual vapour pressure is high, then a large amount of water vapour is present. Low actual pressure means a small amount of vapour is present compared with high vapour pressure.
When the condensation rate matches the evaporation rate, the space above the liquid is said to be saturated. The amount of water vapour required to make the air saturated at any given temperature is called saturation vapour pressure.
At high air temperatures, more water vapour is required to saturate the air. In other words, it depends on temperature. Suppose at ten degrees celsius, the saturation vapour pressure is 12 millibars and if the saturation vapour pressure is about 42 millibars.
WHAT IS RELATIVE HUMIDITY?
You may have noticed relative humidity in weather alerts on TV and radio. Relative humidity is commonly used to describe atmospheric moisture. The concept of relative humidity doesn’t indicate the actual amount of water vapour in the atmosphere. Instead, it shows us how near the air is to being saturated.
Relative humidity is the ratio of the air’s vapour pressure to the saturated vapour pressure at the air’s temperature or the ratio of water vapour in the air to the maximum amount of water vapour required for saturation at that particular temperature.
If described in simple terms, it is the ratio of the air’s water vapour content to its capacity.
Relative humidity = Water vapour content / Water vapour capacity
So, the relative humidity can be expressed as actual vapour pressure/saturation vapour pressure * 100.
Air with a 100 per cent relative humidity is said to be saturated, and if it is greater than 100 per cent, it is known as supersaturated.
What are the primary ways that result in relative humidity change?
Two primary ways can make a change
- A change in air’s water vapour content.
- A change in air temperature.
If the water vapour in the air increases, the molecules may stick together and result in condensation. As more and more water vapour molecules are added, the air gradually becomes saturated, and relative humidity increases. If the water vapour decreases, vice versa will happen, or a decrease in water vapour in the air lowers the relative humidity.
If the temperature increases, the speed of molecular motion also increases. Thus, the chance of reaching the saturation level is low. In short, if the water vapour content doesn’t change, an increase in air temperature lowers the relative humidity.
Now, we will explore the reason for the highest relative humidity in the morning.
In most places, air temperature is responsible for the daily variation of relative humidity. The air cools at night, so the relative humidity increases and the highest relative humidity occurs in the early morning. After sunrise, the air temperature will increase, and the relative humidity will decrease.
How relative humidity influences our daily life?
- A relative humidity above 85 per cent may result in deterioration of stored cotton lint. To keep cotton intact, we should control relative humidity.
- An increase in relative humidity can promote plant diseases like potato blight.
- High relative humidity can make ironwork rust.
- Low relative humidity causes rapid moisture evaporation from exposed skin, which causes skin to itch and crack. It can also irritate the mucous membranes of the nose and throat.
- To properly store documents, relative humidity should be within 50 to 65 per cent. Humidity affects paintings, and the best way to store them is at 45 to 55 per cent relative humidity and 8 to 22 degrees Celsius.
- Relative humidity is important in film preservation. Nitrate films are stored at a temperature between 10 and 12 degrees Celsius and 40 to 50 percent relative humidity. Black-and-white films are stored at a temperature between 10 and 12 degrees Celsius and 40 to 50 per cent relative humidity. Colour films are stored at a temperature between 0 and 12 degrees celsius and 40 to 50 per cent relative humidity.
WHAT IS SPECIFIC HUMIDITY ?
The mass of water vapour in grams contained within a parcel of air is called specific humidity.
Specific humidity = mass of water vapour/mass of total air.
It is seldom affected by changes in air pressure or air temperature because it is measured in units of weight and expressed as grams of water vapour per kilogram of air (g/kg).
This measurement describes the amount of water vapour in a large mass of air. It is directly proportional to vapour pressure, which is the partial pressure exerted by water vapour in the air, and is inversely proportional to air pressure.
Specific humidity is highest in the warm equatorial zones and falls off rapidly towards the colder polar region.
WHAT IS ABSOLUTE HUMIDITY ?
The total weight of water content per volume of air at a specific temperature is called absolute humidity. If additional vapour is not added, absolute humidity does not change with an increase or decrease in temperature. If we observe closely, we can notice a decrease in absolute humidity from the equator towards the poles and from the ocean to the continents.
There will be a decrease in absolute humidity from the equator towards the poles and from the ocean to the continents. The absolute humidity largely influences the precipitation.
WHAT IS SATURATION DEFICIT?
The difference between the saturation vapour pressure and the actual vapour pressure is called saturation deficit, which serves as a better index of air moisture.
What is the difference between relative humidity and saturation deficit? Relative humidity is the ratio of the two, whereas saturation deficit is the difference between them.
The saturation deficit can be related to evaporation in climates, which depends more on advection than radiation and flora growth.
MIXING RATIO
The ratio of water vapour in a unit mass of air without vapour is termed the mixing ratio. The ratio observed is minimal because the atmosphere consists mostly of nitrogen and oxygen. The value of the mixing ratio is expressed in units of grams of water per kilogram of dry air—for example, 7.6 grams per kilogram at 10 degrees celsius.
It remains unchanged even if a parcel of air rises or falls in the atmosphere, so long as there is no condensation. Due to this unfluctuating characteristic, the mixing ratio is the preferred measure of humidity when considering cloud formation and precipitation.
DEW POINT TEMPERATURE
When the air is slowly chilled, its saturation-specific humidity decreases and reaches a stage where saturation-specific humidity equals the specific humidity. At this stage, the air has reached saturation due to the presence of a maximum amount of water vapour. If the temperature lowers further, condensation on surfaces will start. So, the temperature at which saturation occurs is known as dew point temperature or the temperature at which dew forms by condensation.
The pressure at the earth’s surface varies only slightly. Dewpoint can be considered a good indicator of the air’s actual water vapour content. The addition of water vapour increases the dew point, and removing water lowers the dew point. Warm, moist wind can help to maintain a higher dew point than cold, dry wind. For example, the Gulf of Mexico maintains a higher dew point due to warm, moist air.
When the temperature and dew point are different, the relative humidity will be low. The relative humidity will be high when they are close to the same value. If both are equal, the air is saturated, and the relative humidity will be 100 per cent.
Why is polar air described as dry even when the relative humidity is high?
In polar areas, dew point and air temperature are normally close. However, the low dew point temperature means little water vapour in the air. Consequently, the air is dry even though the relative humidity is high.
MEASURING AIR’S HUMIDITY
1.Psychrometer
A psychrometer is a common instrument used to measure dew point and relative humidity. It consists of two thermometers, one of which has a piece of wet wick or fabric around the bulb. This fabric-covered one is called a wet-bulb thermometer. The difference between the values of both thermometers is known as wet-bulb depression. A large value indicates a higher rate of evaporation and low relative humidity. A small value indicates a lower rate of evaporation and high relative humidity.
After a couple of minutes, the fabric-wrapped wet thermometer will cool to the lowest possible value. This value is known as wet bulb temperature. In other words, it is the lowest temperature that can be achieved by evaporating water into the atmosphere.
Before taking the values, both thermometers are ventilated for a few minutes. Two methods are used to ensure ventilation.
The first one is whirling the instrument. This type is known as the Sling psychrometer.
The second method involves drawing air past it with an electric fan. This type is known as an Aspirated psychrometer.
2.Hygrometer
In the hygrometer, human or horse hair is used to measure relative humidity. Horace de Saussure found that hair extends by about two point five per cent between extremes of dryness and wetness. This feature is employed to measure humidity. This principle is simple. The oil-removed hair is attached to a system of levers, which can magnify small changes and display them in detail.
FACTORS THAT INFLUENCE HUMIDITY
- When the latitude increases, vapour pressure decreases. This decrease is due to lower temperature.
- Values in coastal areas are higher than inland areas. Warm seas and oceans create water vapour through evaporation, increasing vapour pressure.
- There will be a decrease in vapour or moisture with an increase in altitude due to the colder temperatures and greater distance from the sea or ocean surface.
- Factors like daytime convection and sea breezes can bring moisture-laden winds, which can increase vapour content and vapour pressure.
- Urban heating can reduce surface humidity. Moreover, in cities, rainwater is drained away and not re-evaporated into the air.
