What is relative humidity and why does it matter on a hot day?

Introduction:

• Relative humidity (RH) is an essential weather parameter that affects human comfort and health.
• It is the amount of moisture present in the air compared to the maximum amount that can be held at a particular temperature.
• In this analysis, we discuss the significance of relative humidity and how it impacts human health.

Three Ways to Track Humidity

• Absolute humidity is the mass of water vapor in a given volume of air, expressed as kg/m3.
• Specific humidity is the mass of moisture divided by the mass of air, expressed as a dimensionless number.
• Relative humidity factors in the amount of vapor that air can hold at different temperatures and is the vapor density of the air divided by the saturation vapor density at the dry-bulb temperature.

How Relative Humidity Affects Cooling

• Sweat evaporates to cool the body on a hot day.
• When air is already filled with moisture, sweat won’t easily evaporate, which can be dangerous.

Ideal Relative Humidity Levels

• 30-60% relative humidity is generally considered comfortable.
• Lower levels typically use humidifiers to increase humidity.
• Higher levels use fans to help sweat evaporate better.

The Relationship Between Temperature and Humidity

• Warmer air can hold more moisture than cooler air.
• At the same absolute humidity, the relative humidity of warmer air will be lower than cooler air.

Ways to Measure Relative Humidity

• Conduct a simple experiment with household items.
• Use math.
• Use an app or device like a psychrometer.

Experiment to Measure Relative Humidity

• Use two mercury thermometers, cotton, rubber bands, and cold water.
• Record the dry-bulb and wet-bulb temperatures.
• Subtract the wet-bulb temperature from the dry-bulb temperature to get the temperature difference.
• Use a relative humidity chart to get the relative humidity value.

Conclusion

• The lower the wet-bulb temperature, the drier the air is, and the less relatively humid it is.