Humidity Explained: Measuring Moisture in the Air

Building a weather station involves tracking several key atmospheric conditions. While temperature and pressure often get a lot of attention, humidity is crucial for a complete picture of the weather. This article explains what humidity is, why it matters, and how to measure it with your DIY weather station.

What is Humidity?

Humidity refers to the amount of water vapor present in the air. Water constantly evaporates from bodies of water, plants, and even the ground, adding moisture to the atmosphere. The warmer the air, the more moisture it can hold. However, there are different ways to express humidity, which can be confusing!

Types of Humidity

There are a few common terms used when discussing humidity. Understanding these distinctions is important for accurate weather monitoring.

• Absolute Humidity: This is the actual mass of water vapor present in a given volume of air. It's typically expressed in grams per cubic meter (g/m³). While it sounds straightforward, absolute humidity changes rapidly with temperature and air movement, making it less practical for everyday weather reporting.
• Specific Humidity: This is the mass of water vapor per unit mass of dry air. It’s a bit more stable than absolute humidity and often used in meteorological calculations.
• Relative Humidity (RH): This is the most commonly reported type of humidity. It represents the amount of water vapor present in the air compared to the maximum amount the air *could* hold at that specific temperature. It's expressed as a percentage. For example, 50% relative humidity means the air holds half of the moisture it could potentially hold at that temperature. This is the value you’re most likely to see on weather reports.

Why is Humidity Important?

Humidity impacts many aspects of weather and our daily lives. Here's why it's worth measuring:

• Comfort: High humidity makes the air feel hotter because it reduces our ability to cool ourselves through sweating. Low humidity can dry out skin and mucous membranes.
• Forecast Accuracy: Humidity plays a vital role in predicting precipitation, fog formation, and storm development.
• Plant Health: Understanding humidity levels can help monitor the health of plants and crops.

Measuring Humidity: Using a Hygrometer

The instrument used to measure humidity is called a hygrometer. There are several types of hygrometers suitable for a DIY weather station:

• Psychrometer: This classic instrument uses two thermometers – one dry and one wet. The wet thermometer is covered with a wet wick. Evaporation from the wick cools the wet bulb, and the difference in temperature between the dry and wet bulbs is used to calculate relative humidity. Requires ventilation for accurate readings.
• Capacitive Hygrometers: These are electronic sensors that change their capacitance based on the amount of water vapor in the air. They're compact and provide accurate readings.
• Hair Hygrometers: These use the property of some materials (like human hair) to change length with changes in humidity. This length change is mechanically linked to a pointer indicating the relative humidity.

Integrating Humidity Measurement into Your Weather Station

When building your DIY weather station, consider the type of hygrometer that best suits your budget and skill level. Capacitive hygrometers offer excellent accuracy and ease of integration with microcontrollers. Even a simple psychrometer can provide valuable data with careful observation. Remember to calibrate your hygrometer periodically for the most reliable readings. The data gathered will be a valuable component of your complete weather monitoring system.