Meteorologists and weather enthusiasts are well aware of the critical role humidity plays in the creation of different meteorological events. But what is humidity, how does it form, and what are its characteristics?

At any given time, no matter where we are (indoors or outdoors), we are constantly surrounded by air containing a certain amount of humidity.

There is no dispute about the vital role water plays in meteorological activities. After all, it is the primary driving force of almost any weather occurrence.

But humidity plays an essential role in the transport of water between different regions. This is clearly illustrated when one looks at the water cycle.

It is also crucial for air to contain the right amount of moisture to maintain a balanced environment for all living organisms. (Too little or too much humidity can negatively impact the environment and even human health.)

In the following sections, we take a closer look at how humidity forms, how it is measured, and its impact on weather events.

Before we take a closer look at how humidity is formed, its characteristics, and the different types of humidity, we first need a clear definition of what exactly humidity is:

Humidity Definition

Humidity is defined as the amount of water vapor or water in its gaseous state present in the air at a given time. It is typically expressed as relative humidity. Although invisible to the naked eye, high humidity levels can be felt and are often associated with growing levels of discomfort.

Humidity is caused when water turns from its liquid or solid state into its gaseous form (water vapor). This occurs due to evaporation and transpiration (known as evapotranspiration), typically resulting from heating caused by solar radiation.

As radiation from the sun heats the planet’s surface, water evaporates from bodies of water (oceans, lakes, dams, puddles) and also from surfaces that are rich in water content, like soil. The evaporation process turns water into its gaseous state.

Vegetation in the form of trees and plants also releases water into the atmosphere through their leaves in a process called transpiration. (The combined processes of evaporation and transpiration are called evapotranspiration.)

The evaporation of water is not only the result of solar radiation, though. When precipitation takes place, raindrops, hail, or snow start falling towards the ground and accelerate.

As it gains speed, the water starts interacting with the surrounding air. In drier atmospheric conditions, some of the precipitation can evaporate before reaching the ground, turning into water vapor.

All these processes are responsible for increasing the humidity in the air.

In earlier sections, the definition and cause of humidity were already established. The following list, though, highlights the key facts and characteristics surrounding this element of weather:

1. 1. Humidity refers to the amount of water vapor present in the atmosphere at any given time.
   2. Water vapor is nothing more than water in its gaseous state.
   3. Humidity is caused when water is turned from its liquid or solid state into its gaseous state.
   4. Although humidity and its effects can usually be felt, it is normally invisible to the naked eye.
   5. Meteorologists use the dew point to indicate the amount of water vapor present in a body of air.
   6. Humidity is a crucial element necessary for the formation of snowflakes and hailstones in subzero temperatures, which you can read more about in this article.
   7. Air contains a certain percentage of humidity at all times.
   8. Relative humidity is a more accurate reflection of humidity than absolute humidity.

This is a concise and cryptic list of the most important characteristics and facts that define humidity, but the details about specific aspects are highlighted in other sections throughout this report.

Before explaining how humidity is measured, we need to address the difference between relative humidity and absolute humidity, as there is some confusion about the difference between the two.

Absolute humidity is the measurement of the amount of water vapor in the air. It is a very rigid form of measurement, as it does not take variables like temperature into consideration. It is normally indicated as grams of moisture per cubic meter of air (g/m3).

Relative humidity is the measurement of the amount of water vapor in the air. Unlike absolute humidity, though, it is measured relative to the temperature of the air.

In other words, relative humidity measures the percentage of water vapor present in the air relative to the maximum amount of vapor that can be held at a given temperature.

This is important, as warm air can hold much more water vapor than cold air.

If absolute humidity is used to measure the water vapor in both warm and cold air, and it measures identical water vapor percentages in both, it is misleading and not representative of the actual atmospheric conditions.

(In this case, the relative humidity in the cold air is much higher than that of the warm air since warm air holds more moisture than cold air. Even with the same absolute humidity, the relative humidity will be higher in cold air.)

For this reason, relative humidity is the best and most widely used way of measuring moisture levels as it most accurately reflects the actual atmospheric conditions.

The instrument used to measure the humidity in the air is called a hygrometer. There are a variety of these instruments that have been used throughout the years.

The psychrometer is probably the most well-known early example of a humidity-measuring device. It basically consists of two thermometers (one being covered with a wet cloth) used to measure the humidity.

The bulb of the thermometer covered by the wet cloth measures a lower temperature as a result of the evaporation of the moisture in the cloth. By using the difference between the two temperature readings, the humidity is measured.

Obviously, this is not a very accurate and reliable way of measuring humidity.

A capacitive or resistance hygrometer uses a much more reliable way to measure humidity. A material able to absorb moisture is used. The amount of moisture influences the material’s ability to carry an electrical current.

An electrical current is then sent through the material and measured. Based on the strength of the current (influenced by the amount of moisture absorbed by the material), the amount of humidity in the air can be measured.

Humidity is one of the main driving forces of almost all weather systems around the world. The combination of humidity and temperature is the impetus behind many weather systems and occurrences.

The moist air in a warm front gently moves over a cold front and cools down as it rises. It results in condensation and cloud formation, which leads to the gentle precipitation that is welcoming to the agricultural sector. (Similar to the weather produced by a stationary front.)

On the other side of the scale, the warm, moist air over the oceans of the tropics rises and forms powerful low-pressure systems. As the air keeps rising and rotating winds are pulled in and build up around the low-pressure center, the warm, humid air keeps feeding the growing system.

What was a tropical depression can now quickly build into a tropical storm. If enough humid air builds up in this system and stays over the warm ocean waters long enough, hurricanes of varying strength can be very destructive when they make landfall.

And this whole process got started by some humid air rising up from the ocean’s surface. That is why these warm tropical waters are called the engine rooms of big storm systems and the fuel that drives these massive systems.

Similarly, the torrential rains falling during the Monsoon Season over India and Southeast Asia are all part of the circulation pattern that brings huge amounts of humid air from the warm Indian and Western Pacific Oceans during the warm summer season.

As is the case with hurricanes and tropical storms, moisture-filled air is once again one of the main driving forces of a major weather system. This is also another example of how humidity and temperature work together in the creation of a major storm system.

There are obviously many more processes involved in the formation of all these weather systems. If you want to read in detail how hurricanes and monsoons are formed, as well as the role humidity plays in all this, you can read the in-depth article here. 

Humidity doesn’t just influence the weather, but it also has a direct impact on how we experience it.

When humidity levels are high, the air already contains a large amount of moisture. As a result, sweat on your skin evaporates more slowly, making it harder for your body to cool down. This is one of the primary reasons why hot and humid conditions often feel much warmer and more uncomfortable than dry heat.

On the other hand, low humidity allows moisture to evaporate more quickly. This helps the body cool more efficiently, but it can also lead to dry skin, irritated eyes, and dehydration over time.

Meteorologists often use the dew point as a more accurate way of describing how humid it actually feels. The higher the dew point, the more moisture is present in the air, and the more uncomfortable the conditions will feel.

Although it cannot be seen, humidity plays a crucial role in the creation of almost all weather occurrences, as this article clearly illustrates. It highlights what humidity is, how it forms, and also describes the different processes involved.

The powerful effect humidity has on the weather on a global scale should also be evident, and we just touched the surface. There is so much more to humidity than is covered in this article, but the focus of this post was to capture its definition and illustrate its formation.

Until next time, keep your eye on the weather!