[Physics Class Notes] on Humidity Pdf for Exam

Humidity is the amount of moisture or water vapour or water molecules present in the atmospheric gas. The more water in the vapour, the higher the humidity. Humidity arises from water evaporating from places like lakes and oceans. Warm water evaporates quickly. That’s why, you may find the most humid regions near to warm water bodies in places like the Red Sea, the Persian Gulf, and Miami.

 

Here, we will learn about the humidity and its types viz: specific, relative, and absolute humidity.

 

Types of Humidity

• Relative humidity

• Specific humidity

• Absolute humidity

Relative Humidity

A meteorologist uses the term ‘relative humidity’. The relative humidity is a comparison of the amount of moisture present in the air to the amount of moisture air can hold. The amount of moisture the atmosphere can hold totally depends on the temperature. 

 

The formula for the relative humidity is given by:

Relative Humidity =  Actual amount of water in the air/saturated amount of moisture in the air can hold at that temperature

 

The relative humidity is the function of both water content (moisture) and the temperature.

 

Point to Remember

The relative humidity is 100% when the air is saturated with water vapour and 0% when no vapour is present in the atmosphere.

 

Relative Humidity Explained

Think of the atmosphere as a sponge and it is capable of absorbing a fixed amount of water, i.e., a mug of water. Now, think of a rise in temperature as an increase in the sponge size. 

 

When the sponge has no water, it means the relative humidity is zero. Now, pour a half bucket of water on the sponge, the relative humidity reaches 50%. 

 

We know that a sponge saturated with a half mug of water has 50% humidity, on increasing the size of the sponge (increasing the temperature) without adding water further, the relative humidity decreases because the sponge becomes bigger and is capable to take on water vapour; however, the amount of water remains the same.

 

Soaking a sponge (atmosphere) with water more than the capacity it can hold can lead to dripping; however, it doesn’t symbolize rainfall.  So, how does rainfall occur?

 

Rainfall

Rainfall occurs when the rising air cannot hold enough water molecules that are gathered in the form of clouds in the sky.

 

Specific Humidity

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Absolute Humidity

We define the absolute humidity in the two following sentences:

• Absolute humidity is equal to the mass of water vapour per unit of volume of air, i.e., grams of water/cm³ of air. The formula for the absolute humidity is given by:

             Absolute humidity = Mass of water/volume in cm³

Absolute humidity in the atmosphere is between zero and approximately 30 gm/m³ when the air is saturated at 300C

 

Dew point is frequently cited as a more accurate way of evaluating the humidity and comfort of the air than relative humidity because it is an absolute measurement, unlike relative humidity.

When the dewpoint and temperature are the same, the relative humidity is 100 percent. Condensation will occur if the temperature falls lower, and liquid water will form.

If the relative humidity is 100 percent, which means the dew point temperature and absolute air temperature are both the same, precipitation is unlikely. It simply indicates that the maximum amount of moisture in the air is present at the current temperature. Fog on the surface and clouds in the sky, which are made up of microscopic water droplets suspended in the air, can develop from saturation.

While dew point provides a quick indication of air moisture content, relative humidity does not since humidity is proportional to air temperature. It can be put another way. One can calculate relative humidity based on the dew point. Also, the actual air temperature must be known in that case.

The actual saturation vapour pressure ratio is also known as relative humidity, where the real vapour pressure is a measurement of the amount of water vapour in a volume of air that rises with the amount of water vapour.

At any given temperature, saturated vapour pressure is the greatest Vapor Pressure that can exist.

Water vapour, whose VP is its SVP at the given temperature, exists in the air with relative humidity (RH) of 100 percent. This is equivalent to air in a state of equilibrium with liquid water. RH is the percentage representation of the VP/SVP ratio. ‘dry’ air will contain water vapour at the given temperature with a VP smaller than the SVP.

 

Effects of Humidity

Humidity can cause various uncomfortable conditions, let’s discuss these one-by-one:

• Bacteria and viruses spread easily in humid conditions because of which people often fall sick, especially from respiratory issues. 

• When the relative humidity goes above 60%, the viruses spread among people, and they get ill.

• The rise in the moisture content of air leads to the rise in temperature because of which the evaporation rate of sweat from our bodies slows. This slowdown leads to the following problems:

1. Overheating in our bodies

2. Exhausts easily, and

3. Consequences are lethal to health like Altered blood circulation, increased respiration rate, and sweating.

4. During humidity, higher levels of dust mites and fungi lead to allergies among people.

5. The spread of airborne chemical contaminants.

6. Detrimental to asthma sufferers.

What Method is Used to Determine Humidity?

A hygrometer is mainly used to measure relative humidity. The basic hygrometer is a sling psychrometer consisting of two thermometers connected by a chain and a handle. A single thermometer is standard. The other is a wet-bulb thermometer, a cotton wick over its bulb. The temperature of the air is measured with a dry bulb thermometer.

On the other hand, the wet-bulb thermometer has a moist cloth at the tip. As water molecules evaporate from the wet bulb’s surface, they carry heat with them, decreasing the thermometer’s reading. The vapour pressure, or the amount of water vapour in the air, determines the evaporation rate. No water will evaporate from the wet bulb at 100 percent relative humidity, and the readings on both thermometers will be the same. In a graph, comparing
the two temperatures yields relative humidity results.

Fun Facts

• In 1783, a person named Horace Bénédict de Saussure was the person to build the first hygrometer with hair. A hygrometer is a humidity measuring device.

• Humidity provides us with a tonal language

Our vocal cords consist of a pair of mucus membranes that stretch across the larynx (our voice box). These membranes vibrate, controlling the air from the lungs that flow as we speak or sing. It means the level of moisture present in the air affects the elasticity of our vocal cords. 

 

Singers can tell us how hard it is to carry a tune in a dry environment.

• As per the latest research, we are better at detecting smells in a humid environment because, in heat and high humidity, there are more water molecules in the air to bind and carry odorous particles into our nostrils.