How does humidity increase heat?

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In the case of a weather forecast, more emphasis is placed on mentioning the temperature than needed. This means that if today's temperature is forecasted to be 40 degrees Celsius, it will likely feel like 43 or 44 degrees Celsius. Now the question arises, why is this the case?

To understand the answer, we need to grasp the concept of humidity. Understanding humidity allows us to comprehend how weather can be both beneficial and detrimental. So, what exactly is humidity? The simple answer is that humidity is the presence of water vapor in the air. Due to the heat of the sun and the influence of air, various sources on Earth, such as bodies of water, vegetation, and soil, release water vapor into the air. In this context, the temperature does not have to be 100 degrees Celsius. Evaporation occurs when the temperature is below 100 degrees Celsius, where liquid water turns into water vapor in the air. For example, think about drying clothes in the sun. When we hang clothes to dry, even though the temperature of the clothes' water may not reach 100 degrees Celsius, the water evaporates and dries out, following the process of evaporation.

Humidity refers to the presence of water vapor in the air in any process. The three types of humidity measurement methods are absolute humidity, specific humidity, and relative humidity.

Absolute humidity is the amount or quantity of water vapor present per unit volume of air or gas at a given temperature and pressure. For example, if there are 10 grams of water vapor in 1 cubic meter of air at a certain location, then the absolute humidity of that location will be 10 grams per cubic meter.

On the other hand, specific humidity is the ratio of the mass of water vapor to the total mass of air including that water vapor at a given temperature and pressure. For example, if the specific humidity is 10 grams of water vapor in a mixture of air and vapor weighing 500 grams, the specific humidity will be 10 grams / 500 grams or 0.02 degrees Celsius.

Relative humidity, on the other hand, expresses the ratio of the amount of water vapor actually present in the air compared to the maximum amount of water vapor that the air can hold at that temperature. For example, at a temperature of 20 degrees Celsius, if there is 20 grams of water vapor in the air and the air can hold a maximum of 70 grams of water vapor, then the relative humidity will be 28.57%. This means that relative humidity tells us what percentage of the air's maximum water vapor capacity is currently being held.

In most cases, relative humidity is mentioned because it gives a better understanding of how much water vapor is present in the air and thus has a significant impact on our weather forecast, including on social media.

Now you may wonder, how can we determine the amount of water vapor in the air? To understand this, let's go back a bit. In 1783, Swiss physicist and geologist Horace Bénédict de Saussure invented a device to measure humidity using human hair, which is called a Hair Tension Hygrometer. On days when there is more humidity in the air, you'll notice that the hair tends to become slightly curled, and when there is less humidity, the hair becomes straighter. This happens because the hydrogen atoms in the proteins of human hair form bonds with the hydrogen atoms in the water vapor present in the air, causing the hair to become slightly curly.

Human hair was used in early humidity measurement devices because it reacts sensitively to changes in humidity. Later, similar principles were applied to create hygrometers using different substances instead of hair. Nowadays, humidity is often measured using a Wet Bulb Thermometer.

We typically encounter dry bulb thermometers in our daily lives or use them for various purposes. On the other hand, under a wet bulb thermometer, there is a cloth soaked in water, and the thermometer measures the airflow of the air. In this case, the reading on the thermometer is lower than that of a dry bulb thermometer.

Whether a day will be comfortable or uncomfortable depends largely on humidity. When the body temperature rises, people sweat to cool down, and the sweat evaporates into water vapor in the air. Now, when there is already excess humidity in the air, the sweat cannot evaporate fully into the water vapor. As a result, our body temperature doesn't decrease enough, and we feel uncomfortable. Therefore, if both the temperature and humidity are high, we will experience extreme discomfort and feel hotter than the actual temperature.

This feeling of experiencing a higher temperature than what is actually recorded is known as the heat index. It occurs when the combined effect of temperature and humidity makes it feel hotter than the actual temperature. For example, if the temperature for a day is 40 degrees Celsius and the relative humidity is 55%, we will feel like it's 59 degrees Celsius. There is a lengthy formula to calculate the heat index, but you can easily find online calculators where you input the temperature and humidity to get the heat index.

Humidity also plays a crucial role in cold weather. In winter, due to the low temperature, the air can hold less water vapor. Therefore, if there is low humidity in the air during winter, it can make the air feel drier. Conversely, during the night in winter, the humidity can increase because the water vapor formed during the day remains in the air. This can lead to condensation on surfaces like windows and the formation of frost.

The maximum amount of water vapor the air can hold depends on its temperature. So, if the same amount of water vapor is present in air of different temperatures, their relative humidity will not be the same but will vary according to the temperature. For example, if 10 grams of water vapor is present in air of different temperatures, their relative humidity will vary according to the temperature. The air will have less relative humidity if the temperature is higher, and more relative humidity if the temperature is lower.

At a certain point, when the air's humidity reaches the maximum level it can hold, it becomes saturated with water vapor. This level of humidity is called 100% relative humidity. Beyond this point, any additional water vapor will start to condense, meaning the water vapor will turn into liquid form. You can easily observe this phenomenon in the morning in winter when dew forms on grass. This happens because the humidity has exceeded 100%. Similarly, clouds form precipitation when the humidity in them exceeds 100% and the excess water vapor condenses into liquid form, returning to the Earth's surface as rain. However, it's not necessary to exceed exactly 100% humidity for this to occur, but rather to approach or be close to 100% humidity.

In this context, it's essential to mention another topic: the dew point. Imagine a place where the air can hold a maximum of 20 grams of water vapor per cubic meter at a temperature of 40 degrees Celsius, but there's only 10 grams of water vapor present in the air. Now, if we lower the temperature of this location, the maximum water vapor holding capacity will decrease. The temperature at which the maximum water vapor holding capacity in the air equals the amount of water vapor present is called the dew point.

Now, if we consider an environment where the temperature is lowered to 20 degrees Celsius, the maximum water vapor holding capacity of the air becomes 10 grams, matching the amount of water vapor in the air. Therefore, in this case, the dew point will be 20 degrees Celsius. As a result, the water vapor in the air will start to condense into liquid form, just like dew forming on a cold water glass. The dew point is not a specific temperature; it depends on the air pressure and humidity. Due to the variability of this dew point, sometimes dew forms while at other times frost forms.

The amount of water vapor in the air is a crucial factor. On extremely hot days, as humidity increases, discomfort also increases. So, to reduce discomfort in hot weather, humidity must be kept low. For instance, if you spray water during scorching heat, temporary relief might be felt, but overall discomfort will increase. Because by spraying water, it can easily evaporate into water vapor, thus increasing humidity and discomfort. Consequently, the higher humidity will lead to more discomfort.

This phenomenon can be observed easily when there's a light rain on a scorching hot day. Immediately after a light rain on a scorching hot day, intense humidity is felt. This is because the amount of water vapor in the air increases due to rainwater, resulting in a feeling of increased humidity from the evaporating water. Another everyday example is bathing in a hot bathroom. Just after finishing a bath, the bathroom feels warmer, but be aware that the moment you step out of the bathroom, relative coolness is felt. The reason for this is that during bathing, the water splashed increases the humidity inside the bathroom, thereby increasing the perceived temperature.

Ultimately, our weather experience depends on humidity. Excess humidity creates discomfort, but too little humidity can also lead to discomfort.

Md. Jumman Bhuiyan: Science writer, content creator, and teacher.