Understanding the Isobaric Cooling of Air and its Effects on Dew Point

[dobry_den]

Hi! My Physics introductory book states the following: "A dew point is a state defined by the temperature to which the air has to be cooled isobarically (with constant absolute humidity) for the vapour to become saturated."

Humidity is defined as m/V, where m is weight of the vapour, V is volume of the air.

My problem is that when we isobarically cool gas, its volume decreases, so the absolute humidity has to rise (the weight of vapour should be constant?). So how can we isobarically cool air and at the same time preserve its absolute humidity?

 

[Nate810]

The key to understanding this is the fact that when air is cooled, the amount of water vapor it can hold decreases. This is due to the decrease in pressure and the decrease in temperature. As a result, when air is cooled isobarically (with constant absolute humidity), the amount of water vapor in the air increases as the temperature drops. Eventually, the air will reach its dew point, which is the temperature at which the air has reached its saturation point. At this temperature, the air cannot hold any more water vapor, so it condenses into liquid droplets and forms dew.

 

[m2003]

Thank you for bringing up this interesting topic. I can provide some insights into understanding the isobaric cooling of air and its effects on dew point.

Firstly, let's define what isobaric cooling is. Isobaric cooling is a process where the temperature of a gas is decreased while its pressure remains constant. In the case of air, this means that the pressure of the air remains constant while its temperature decreases.

Now, let's look at the definition of dew point. It is the temperature at which the air becomes saturated with water vapor, meaning it can no longer hold any more water vapor and any additional water vapor will condense into liquid form. As you correctly pointed out, the humidity is defined as the ratio of water vapor mass to the air volume. So, how can we isobarically cool air and still maintain its absolute humidity?

The key to understanding this lies in the concept of relative humidity. Relative humidity is the ratio of the amount of water vapor present in the air to the maximum amount of water vapor that can be held at a given temperature and pressure. As we isobarically cool air, its relative humidity increases because the cooler air can hold less water vapor. This means that even though the absolute humidity may increase due to the decrease in air volume, the relative humidity is still increasing as well.

To put it simply, as we isobarically cool air, the air becomes more saturated with water vapor, causing the relative humidity to increase. This increase in relative humidity is what leads to the formation of dew as the air reaches its dew point temperature.

I hope this explanation helps you understand the relationship between isobaric cooling, absolute humidity, and dew point. It is important to note that these concepts are simplified versions of the complex processes that occur in the atmosphere, but they provide a good foundation for understanding the basics of isobaric cooling and its effects on dew point.