The ratio of moisture in the air compared to how much moisture the air can contain.
Relative Humidity Details
Have you ever traveled to a different city with the same temperature as your initial place, but the new city feels significantly different in terms of temperature? That's because these places have different levels of humidity. High humidity makes the air 'wetter,' preventing your sweat from evaporating and making you sweat even more. Low humidity has arid air that's ready to contain moisture, which makes your sweat evaporate faster.
To deepen our understanding of relative humidity, let's look at three important factors that determine an area's relative humidity.
The concentration of water vapor is the primary factor in determining humidity levels. Water vapor is the gaseous state of water where the atoms that make up water (H2O) float freely around the air. To convert water from its denser liquid state to gas, we need to apply energy to it by boiling it. Some atoms in the boiled water will have so much energy that they overcome the vapor pressure, escape into the air, and become water vapor.
Even though water vapor's density varies wildly, it always falls close to the surrounding air's specific density, making it rise or drop very slowly. As a result, the water vapor mingles with the air near the earth's surface, where we live, breathe, and feel. Water vapor can be converted back to its liquid state by adsorption onto a solid surface. This phenomenon happens naturally every dawn, where the water vapor adheres to surfaces such as plant leaves and house roofs, condensing into water droplets.
A hot atom will shake, spin, and move more erratically than atoms with a lower temperature. A hot atom allows air molecules to expand, allowing it to contain more water molecules. Hot air can also evaporate liquid water molecules, slowly turning them into water vapor, thus, increasing the relative humidity.
Think of it as a cup of tea. The cup is the overall temperature, which means the bigger the cup gets, the more tea it can hold. The tea is the relative humidity, which means the cup contains the tea equivalent to the relative humidity percentage. You could get the biggest cup ever, but if there's only a mouthful of tea, it's not going to feel humid.
Air pressure is a measure of how dense the air is in an area. The air pressure and density across civilization are typically the same because we mostly live in low-height areas. However, the air pressure rapidly decreases the higher we go.
This decrease in air pressure is why climbing Mount Everest is such a daunting task. The air pressure is so low that it's hard to inhale air into your lungs. The low concentration of air prevents it from holding moisture, regardless of temperature.
Example of Relative Humidity
We express the relative humidity in percentages. It's usually shown together with temperature and wind speed in weather apps or weather prediction TV programs. Here are two examples of real-world places with significantly varying levels of humidity.
There's a widespread misconception that we call deserts "deserts" because they have an extreme temperature. We define a desert because it receives a tiny amount of rainfall, resulting in extremely low humidity. The low humidity amplifies the effects of extreme temperature. High temperatures feel like they're sucking all of the moisture out of your body, while low temperatures feel like they're freezing every inch of your body down to the bone.
The Atacama Desert, located in the west of the Andes Mountains, is regarded as the driest desert on earth. The Atacama Desert has a total area of 105 square kilometers with an average height of 3000 meters above sea level. This extreme height prevents moisture from flowing to the Atacama Desert from the Pacific Ocean located just beside it. It also keeps the temperature low enough that it can't carry as much moisture.
Humidity's primary source is water vapor. What's the biggest water source on earth? The ocean. That's why we find most humid areas next to the ocean. Temperature also plays a significant role in humidity since higher temperatures allow the air to contain more moisture.
Miami, Dublin, Singapore, Jakarta, Tokyo are a few of many coastal metropolitans that constantly receive high humidity, especially during summer. In contrast, other coastal cities like Vancouver, Melbourne, Saint Petersburg, Stockholm, and Oslo have less humidity because they're far from the earth's equator line, resulting in much lower temperatures.
Significance of Relative Humidity
Weather conditions have a massive impact on human comfort. Humidity is one of the core weather measurements alongside temperature, precipitation, wind speed, and dew point. It's advisable to check the weather prediction at the start of your day, so you know what to wear and carry.
A relative humidity number above 70% on a hot temperature (around 24-30 °C) means that it's going to be sweltering hot. Bring fresh cold drinks and wear minimal clothes to help your body cool down faster. Avoid prolonged sunlight to prevent sunburns.
Relative humidity below 30% indicates that it's going to be dry. Bring lip balm and extra water to hydrate properly. Maybe wear additional layers of clothing since low humidity could make mild snow feel like a frigid blizzard.
Water vapor is a greenhouse gas, much like Carbon Dioxide (CO2) and Methane (CH4). Water vapor regulates the global temperature by trapping heat and cooling the surface. The unique property of water vapor lets green light pass through but absorbs red light, preventing heat from escaping the planet earth. Water vapor cools the surface by absorbing heat from the environment, carrying the heat into the air by evaporation.