Effect Of humidity

Humidity: What Does It Mean?

The amount of water vapour present in the atmosphere, or specifically in the gaseous state, is called humidity. The higher the percentage of humidity in the atmosphere, the more we feel wetter and warmer, or ‘humid’. Due to the levels of high humidity, we feel hotter than the actual temperature outside. It can be explained with a simple example. A dry summer day in Delhi, with a 0% humidity at a temperature of 30℃, is much more bearable than a day with the same 30℃ temperature but with a humidity of 100%, which indicates the onset of rainfall. 

The relative humidity is depicted by weather reports. It is used for determining the actual amount of water vapour present in the atmosphere. When there is high humidity outside, our sweat does not evaporate usually, and hence, it becomes difficult to cool off. It is because the air is so clogged with the amount of water vapour that it leaves little room for anything else.

How does humidity affect the speed of sound in air?

The speed of sound varies according to the medium it travels through and the environment surrounding it. With heat, the sound wave particles acquire more kinetic energy; thereby, the particles vibrate more through the medium. With an increase in temperature, the heat provided makes the sound waves travel faster.

Explanation:

A sound wave, when travelling from a denser medium to the rarer (less dense) medium, bends away from the denser-rarer medium boundary due to refraction. On chilly days, the cold air is closer to the ground and the warm air, being less dense, rises above the layer of cold air. In case you call out to a neighbour standing down the street or hear the arrival of your bus, the sound waves produced by you would bend away by the warm air. As sound travels faster in warm air (less dense) than cold air (denser), the waves refract away from that warm air above and then move towards the ground surface. Hence, it can be inferred that sound travels farther on cold days, owing to the density of air as well as the atmospheric temperature.

Formula to Determine the relation between humidity and the speed of sound:

Even though there is no direct formula to derive a relation between relative humidity and the speed of sound in air, we can indirectly correlate the two using various formulae as discussed below.

In a gas, the speed of sound is

v =√ γRT,  where

 = Ratio of specific heats Cp ⁄ Cv

R = Specific gas constant

T = Temperature

Here,  and R are dependent on the gas composition, including the humidity levels in the air.

Further, molar specific heat at constant pressure Cp is given by

Cp = 1.005 + 1.82H, where 

1.005 kJ/kg℃  = Heat capacity of dry air

1.82 kJ/kg℃ = Heat capacity of water vapour

H = Absolute humidity

Therefore, the specific heat capacity of dry air < humid air.

Also, Cp = Cv – R, where 

Cv = molar specific heat at constant volume

R = Runi ⁄ Mgas, where

Mags = Molecular weight of the gas

Conclusion:

In this article, we saw an overview of humidity and its effects on the speed of sound in air. The amount of water vapour present in the atmosphere, or specifically in the gaseous state, is called humidity. 

The higher the percentage of humidity in the atmosphere, the more we feel wetter and warmer, or ‘humid’. The term relative humidity is used for weather forecasts. It is used for determining the actual amount of water vapour present in the atmosphere. A sound wave, when travelling from a denser medium to the rarer (less dense) medium, bends away from the denser-rarer medium boundary due to refraction. Further, we also saw the relevant formulae for determining the relation between humidity and speed of sound in air.