Overview On Humidity

Humidity denotes the probability of precipitation (rain), dew, or fog. The higher the water vapour in the air, the higher the humidity. It impacts surface life significantly. High humidity slows the rate of moisture evaporation from skin surfaces, lowering heat exchange efficiency in animals that use perspiration (sweating) to regulate their internal body temperature. A heat index table, often known as a humidex, can be used to calculate this effect.

Absolute, relative, and specific humidity measurements are the most commonly used. The mass of water vapour per mass of dry air is used to calculate absolute humidity. The difference between the current level of absolute humidity and the maximal humidity at the same temperature is called the relative humidity, and the ratio of water vapour mass to total moist air parcel mass is known as the specific humidity.

Primary Measurements of Humidity

The most common primary humidity measurements are absolute, relative and specific humidity. Let’s discuss each of them in detail –

• Absolute Humidity – Absolute humidity means the whole mass of water vapour present in a given volume or mass of the air that does not take temperature into its consideration. It can be expressed as –

                                         AH = m (H₂O) / V

Where, m (H₂O) is the mass of water vapour while V is the volume of air. However, absolute humidity in chemical engineering is mass of water vapour per unit mass of dry air because the temperature or pressure is highly varied which affects the overall calculations. It is often referred to as the humidity ratio or mass mixing ratio.

When the air is saturated at 30 °C, absolute humidity in the atmosphere ranges from near zero to around 30 g (1.1 oz) per cubic metre and is represented in g/kg or kg/kg.

• Relative Humidity –  When you see weather reports, humidity is explained in terms of relative humidity. Relative humidity means the actual amount of water vapour in the air as a percentage of the maximum amount of water vapour the air can hold at the same temperature. It can be expressed as –

                                     RH = p(H₂O) / p₁(H₂O)

Now, colder air holds less vapour, which means that a change in the temperature can change the relative humidity, with absolute humidity being constant. In short, chilled air increases the relative humidity while hot air decreases it. 

Relative humidity is devoid of mists, clouds, fogs, and aerosols of water. The higher the percentage of relative humidity, the higher the humidity, which means that at 100%, the air will be saturated and will be close to the dew point. 

• Specific Humidity – Specific humidity means water vapour mass per unit mass of moist air and is expressed as grams of vapour per kilogram of air. It is called specific because as long as moisture is not added to or taken away from a body of air, it does not change as the temperature or pressure of the air varies.

This property makes it useful for measuring moving air mass. And also, it is an important tool for the meteorology department.

Humidity Measurement 

The device used to measure humidity is called a hygrometer which can measure the amount of water vapour in air and soil. Now, the question is whether these measurements are accurate enough to rely upon? Meteorology faces a hard time relying upon hygrometers since it measures relative humidity rather than the absolute amount of water present; however, because relative humidity is a function of both temperature and absolute moisture content, small temperature changes will result in relative humidity changes. Though condensation temperature or dew point is used in modern electronic devices to measure the humidity. 

Relationship between Humidity and Temperature

The relative humidity rises when the water vapour concentration remains constant and the temperature falls. The relative humidity drops when the water vapour content remains constant while the temperature rises. This is due to the fact that colder air requires less moisture to become saturated than warmer air.

Now, because it is not a percentage dependent on temperature, dew point is a better measure of humidity than relative humidity. Dew point is the temperature at which air would become saturated if it were chilled to that degree. For example, if you wake up early in the morning and touch the grass surfaces, they must have been covered with water because the temperature might have fallen below dew point at night.

Effects of Humidity

High humidity and heat result in more water in the air, which can carry scent molecules longer, resulting in a strong odour in the summer near bacteria sources like garbage. In the temperate zones of the hemisphere, influenza activity rises in the winter as temperature and humidity have a direct impact on them. 

Cardiovascular death rates increased in low-temperature, high-humidity circumstances. This could be because of the effects of high humidity on thrombotic risk, as well as the human body’s different cold-stress reactions. Also, condensation issues, such as misting of windshields and shorting of electrical components, can occur when there is a high level of humidity within a car.

Conclusion

So far, we have learnt how the mere amount of water vapour in the air affects our lives. Though there is no direct relationship between temperature and humidity, it does affect people who are sensitive to these factors.

Relative humidity is frequently cited in weather forecasts and reports because it is a predictor of dew or fog. It also raises the apparent temperature of humans (and other animals) in hot summer weather by preventing perspiration from evaporating from the skin as the relative humidity rises. This relative humidity is measured by a hygrometer, which is subjected to accuracy.