KDKA meteorologist Ray Petelin is back with another science lesson!By Ray Petelin

PITTSBURGH (KDKA) — We are now in the time of year where our furnaces roar back to life, and once the furnaces come on, you may notice the air in our homes becomes much drier.

(Photo Credit: Ray Petelin)

So what is happening? To understand this, we need to understand the dew point and relative humidity.

The dew point is the temperature at which water in the atmosphere begins to condense.

The dew point cannot be higher than the actual temperature.

(Photo Credit: KDKA Weather Center)

The relative humidity is the measure of the actual amount of water vapor in the air compared to the total amount of vapor that can exist in the air at its current temperature.

(Photo Credit: KDKA Weather Center)

Simply, the relative humidity is how close the air is to being saturated with water, and it is given as a percent.

When it is colder outside, the dew point is much lower.

Remember, the dew point can’t get above the air temperature, so lower temperatures will obviously mean lower dew points.

That means there is not much moisture in the air.

(Photo Credit: Ray Petelin)

A simple way to show this is with glasses of water. This little glass will represent the cold air, and how much moisture it can hold.

Since colder air has a lower dew point, or less moisture, you can fill that small glass all the way to the top and it does not take much water to do that.

If you were to think of the relative humidity represented by this small glass, you would think it was 100% humidity.

It is saturated, and cannot hold onto any more moisture.

(Photo Credit: Ray Petelin)

Now, when we bring this “air” into our homes, and warm it up we don’t add more water.

This bigger glass represents the warmer air, or higher temperatures.

With no moisture added, you can see that the bigger glass, while representing warmer air has a lot of empty space, meaning it is no longer close to saturation and has a very low relative humidity.

To you, you sense that as drier air, so the less moisture content in colder air means even drier conditions, relatively speaking, when you warm it up inside.