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What is Relative Humidity? - Definition, Equation & Calculation

Instructor: Dominic Corsini
This lesson explains the concept of relative humidity. It helps you understand why thunderstorms occur in the late afternoon or why dew forms overnight. A summary and brief quiz is also included.

What is Relative Humidity?

It seems as though every day someone inquires about the weather. What's the weather forecast for today? Is it going to rain or snow? Inevitably, on a hot summer day, someone will make the statement that it's very humid outside. Well, what exactly does that mean; to be humid? I ask this question to my students when we begin our investigation into meteorology. Meteorology is the study of our atmosphere. And while the range of answers varies from course to course, eventually a student will make the assertion that humidity is the amount of water in the atmosphere. At first, this might seem logical, on humid days there does appear to be more water in the atmosphere than on less humid days. However, this is largely an inaccurate statement. Take a moment and ponder what makes this claim that humidity is the amount of water in the air inaccurate.

Have an idea? Let's investigate the assertion. On any given day, the weather forecast may call for 80% humidity. If humidity is really the amount of water in the air, does that mean the air is actually 80% water? Of course not! Everyone would likely drown. In fact, what the weather forecast is referring to is relative humidity. Relative humidity is a measure of how close the air is to reaching saturation. Saturation is the point when the air can hold no more water vapor and is essentially full. Think of it as a water balloon being filled with water. At some point the balloon can hold no more water before bursting, the same is true of air. The air reaches a point when it can hold no more water vapor and must release it, not by bursting, but as liquid water (rain, snow, sleet, hail, dew, frost, or fog). Let's take a moment and break this concept down even further.

Saturation

The first thing we need to understand is that there are different measures of humidity. There's the aforementioned relative humidity (how close the air is to saturation), and there is specific humidity. Specific humidity is the actual amount of water vapor in the air and can be measured using a scientific instrument called a psychrometer or hydrometer. As an example, let's assume a body of air can hold a maximum of 50 grams of water vapor. If that air is currently holding 40 grams of water vapor, then it is nearly full. As the day wears on, and more water evaporates into the atmosphere, our hypothetical air body fills up more and more. Eventually, it will hold its maximum of 50 grams of water vapor. At that point, the air is considered saturated.

The interesting thing is that the maximum amount of water vapor the air can hold changes with respect to temperature. As temperature increases, the maximum amount of water vapor held also increases. And as temperature decreases, the maximum amount of water vapor decreases as well.

So, let's go back to our example. Even if no evaporation occurs and our body of air never holds more than 40 grams of water vapor, eventually it will reach saturation because as the temperature drops, so does the maximum amount of held water vapor. Overnight, when the air is cooler, that maximum might be 40 grams and at this point, the air would be saturated. And what happens when the air is saturated (or full)? It releases its moisture as liquid water. This is why dew forms overnight.

Calculating Relative Humidity

Now that you have an understanding of saturation, let's connect that concept to relative humidity. Remember that relative humidity is a measure of how close air is to saturation. So, let's look at another example.

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