Relative Humidity 

When we think about the weather the first thing that comes to mind is the temperature. We take a look at the weather app on our phones and dress according to the number we see. However, anyone from Houston, Miami or any tropical environment knows there's another element that plays a crucial role in how we feel when we go outside⎯ humidity. More specifically, what we are affected by most is something called relative humidity, a term meteorologists use to describe the amount of moisture (water vapor) present in the air relative to the maximum amount the air can hold at a given temperature.

What is Relative Humidity?

Air has a capacity to hold water vapor. That capacity will change depending on the temperature. Warm air, for example, can hold more moisture, while cold air holds less. Relative humidity is a measurement that compares the actual amount of water vapor in the air to the maximum amount the air could contain. For example, when air has 100% relative humidity, it is fully saturated with moisture—think of a foggy morning when the air feels thick with moisture. At 0% humidity, the air is completely dry.

Relative humidity significantly impacts how we perceive heat. When humidity is high, sweat evaporates more slowly from our skin, making us feel hotter than the actual temperature. On the other hand, in low humidity, sweat evaporates quickly, which cools us down more efficiently.

How is Relative Humidity Measured?

The instrument used to measure humidity is called a hygrometer. These can range from simple analog devices to more complex digital ones, but one of the most common types, especially in classroom settings, is the sling psychrometer. This device uses two thermometers to measure the relative humidity—one "dry bulb" thermometer and one "wet bulb" thermometer.

Real-World Applications of Relative Humidity

Relative humidity plays a vital role in various industries and daily life. In weather forecasting, meteorologists rely on humidity data to predict not only how warm or cold it will feel but also the likelihood of precipitation, fog, or storms. In agriculture, controlling humidity helps farmers ensure optimal growing conditions, as excessive moisture can encourage mold growth while too little can stress crops. In indoor climate control, managing humidity is essential for comfort and health—low humidity can lead to dry skin and respiratory issues, while high humidity fosters mold and allergens. Additionally, in manufacturing, maintaining proper humidity levels is critical in processes like textile production and electronics assembly, where too much or too little moisture can compromise product quality. In the end understanding and measuring relative humidity can have far-reaching impacts, from personal comfort to large-scale industrial operations.

Understanding relative humidity not only helps us appreciate how weather affects our comfort, but it also reveals the deeper connections between climate, technology, and everyday life. Whether it's managing indoor environments, supporting agriculture, or improving industrial processes, knowing how to measure and apply humidity data is an essential skill with real-world impacts. For more exciting activities that explore the science of energy and its effects on our daily lives, visit Watt Watchers of Texas. You'll discover a wealth of interactive energy lessons, fun experiments, and insightful blogs that make learning about energy engaging and practical. Explore now and start your journey towards energy awareness!

Activity Overview: Temperature is the weather metric most obviously associated with human comfort, but humidity is the real key. Humidity is the amount of moisture in the form of water vapor in the air all around us, and it really affects how we feel in the weather. Meteorologists use the term relative humidity, which refers to the ratio of water vapor actually in the air compared to how much water vapor air can hold. In other words, fog is 100% relative humidity; the air contains as much water it can hold.  The instrument used to measure relative humidity is the hygrometer. A variety of different digital and analog models exist, but you can build a simple version with your class. Known as the sling psychrometer, this hygrometer takes a measurement with a "wet bulb" thermometer and a "dry bulb" thermometer simultaneously. The difference between the measurements can be interpreted to determine the relative humidity, because evaporation over the wet bulb is directly related to how much moisture is in the air. Materials: 2"x2" cotton gauze rubber band 2 analog thermometers water folder/magazine/piece of cardboard Procedure: First, wet the cotton gauze with water. The gauze should be damp, but not dripping. Cover the bulb of one thermometer with the wet gauze, and secure the gauze to the thermometer with the rubber band. [NOTE: Some thermometers use liquid mercury, which is toxic. Take special care when working with mercury-based thermometers. If you only have these thermometers to work with, consider performing this part of the procedure as the instructor and then only allowing students to observe.] The gauze must be moist with water in order for this activity to work. Place both thermometers next to each other under the same conditions. Use a folder, magazine, or piece of cardboard to blow air over the thermometers. The fanning will not significantly change the temperature of the air, but it will catalyze evaporation, if possible. Wait several minutes for the thermometers to adjust to the temperature of the air. Water will evaporate from the gauze if relative humidity allows, slightly cooling the wet bulb thermometer and allowing a temperature difference to show between the two.  Ask students to observe the temperature of the two thermometers and record the results. Then, consult the following chart to interpret the measurements from the thermometer in order to determine the relative humidity of your environment. For example, if the dry-bulb temperature is 24°C and there is a 2°C temperature difference between wet-bulb and dry-bulb temperature, then your environment has 84% relative humidity.