What Is Dew Point And How Does It Affect The Weather, Especially Rainfall?

What Is Dew Point heading

Chances are good you already heard about the term, dew point. You may even already have a good idea what it is. There are, however, quite a bit of confusion and misconceptions doing the rounds about its formation and how it affects other processes like rain.

In this article, we address this issue and take a closer look at exactly what dew point is, how it is formed, as well as how it affects different processes. We also clear up some of the confusion and debunk a few misconceptions surrounding this subject.

So let's start with the most obvious question. What exactly is dew point?

dew on grass

Dew point is the atmospheric temperature below which condensation takes place and water vapour is turned into water droplets. This means the air has become fully saturated and the water can no longer stay in its gaseous form if the temperature continues to drop below this point.

This point is reached close to the surface, where dew is formed on plants and objects as a result. Hence the term, dew point. The same process happens in the atmosphere where the formation of clouds is the result of the temperature dropping to below dew point level.

Basically then, the clouds we see in the sky, as well as the dew we see on the ground in the mornings, are essentially one and the same thing. Especially when you consider the way in which they are formed.

(Clouds are nothing more than water vapour that reached dew point, and forms small water droplets as a result. After all, it is these small water droplets that make clouds visible in the first place.)

How And When Dew Point Is Reached

Knowing what dew point is may be simple enough. All the variables and conditions that need to be in place however, is not that simple and need some explanation.

Let's first have a look at how dew point is reached. In order to do this, first understand that dew point is very closely linked to relative humidity. (To find out more about humidity, you can read all about it in this article.)

For the sake of this argument, let's assume the barometric pressure and volume of air is constant and do not change in this scenario. Now let's say the relative humidity is 50% at 30 Degrees Celsius (86 degrees Fahrenheit). 

As the temperature starts dropping, the relative humidity starts to rise. (As you will discover in the linked article above, air with a warmer temperature can hold more water vapour than the same air at a lower temperature. This simply means the lower the temperature, the higher the relative humidity.)

Once the temperature drops low enough for relative humidity to reach 100%, Dew Point is reached. This is the point where the maximum amount of water vapour can be held without condensation taking place. (Also, at 100% humidity, the actual temperature and dew point temperature is also exactly the same.)

If the temperature continues to drop below this level, condensation will take and water droplets will start forming.

Important Note: The scenario above is just a hypothetical example. The relative humidity DO NOT need to reach 100% in order for condensation and rain to take place. As long as the actual temperature drops to below the dew point temperature with enough water vapour in the air, cloud formation and rain can occur.

Relative humidity is just an indicator of the amount of moisture in the air relative to the actual temperature. It is not uncommon for relative humidity to be below 70% when rainfall occurs.

The exact calculation of how dew point is calculated is beyond the scope and not the purpose of this article, as it is too complex to explain in words. (You are more than welcome to Google the exact calculation, or use any of the online calculators available to calculate a specific dew point.)

The Curious Case Of Dew Point, Relative Humidity And Comfort Level

We all heard and used the expression, "It's not the heat, it's the humidity." (Yep, I am guilty of it myself). The feeling we normally we feel when we get hot and sweaty, yet the thermometer itself does not indicate an abnormally high temperature.

While it is partially true and humidity does play a big part, the best way to measure the discomfort level we experience is actually best measured by the Dew Point.

Relative humidity can actually be 100%, yet it may still not be nearly as uncomfortable as a different situation where the relative humidity is around 70%. For this reason, relative humidity is a fairly poor indicator of comfort levels, and dew point is the chosen standard used by meteorologists to describe comfort/discomfort levels.

Relative Humidity And Dew Point

The illustration above will be used to try and best explain why this is the case. 

It is important that you keep in mind that the amount of discomfort or "humidity" you experience is a direct result of the actual amount of moisture in the air. And this is where relative humidity becomes a problem.

Relative humidity is the result of a calculation of the amount of moisture relative to the temperature in the air, NOT the specific amount of moisture actually present in the air. And this is what makes the dew point temperature a much more accurate and calculated indicator of the discomfort level you are experiencing. 

In the illustrator above, let's first take a look at Figure 1 to illustrate this. Both containers measure a relative humidity of 50%. Yet it is clear that Container B contains much more water vapour than Container A.

Since the temperature in Container B is much higher, allowing the air to hold more moisture, the discomfort level is substantially higher than that in Container A. This is clearly indicated by the much higher dew point temperature of 26° Celsius (compared to the much lower dew point of 10 ° Celsius in Container A).

This point is reinforced by Figure 2. Even if the relative humidity is raised to 100% in Container A, and the air is fully saturated at a dew point of 21° Celsius, it is still below that of Container B where nothing has changed and the dew point remains at 26° Celsius as a result. 

This means in both cases, the level of discomfort in Container B is higher than that of Container A. This is evident as the relative humidity of Container A, even at a 100% in Figure 2, is still below the higher the dew point level of Container B. 

This simply shows that the higher temperature in Container B, allows the air to contain a greater amount of water vapour, which is clearly reflected by the higher dew point temperatures in the illustration above. 

The change from 50% relative humidity to 100% however, did not reflect the reality that the discomfort level (even at a 100% humidity), may not be as uncomfortable as the figures may imply. (As illustrated in the diagram above.)   

I realize this is a bit of mind bender and takes a while to wrap your head around. You may need to reread this part a couple of times to make sense of it all. 

Just know that humidity definitely plays a big part in the discomfort levels we sometimes feel, but the actual discomfort level is much better reflected by the dew point temperature than the relative humidity.

Conclusion

By now you will have a much clearer picture of what exactly dew point is, how it is formed, as well as its effect on the environment.

We also delved into the more complex relationship between relative humidity and dew point, and the role it plays in the level of discomfort we feel. 

Feel free to leave me any comments, questions or suggestions, and I will get back to you as soon as possible.

Remember to join my  Mailing List  to be informed whenever a new article is released, and share new developments and helpful hints & tips.

Until next time, keep your eye on the weather!

Wessel

Wessel Wessels
 

Lifelong weather enthusiast. Interest in all things weather-related, and how global climate and local weather interact. Home weather station owner for almost 2 decades, but still learning and expanding my knowledge every day. Keen on sharing my knowledge and get more people involved and interest in both their local weather and how it interacts with climate on a global scale. Love sharing my knowledge on home weather stations, how they work and the many ways you can use them to your advantage. All in all, just a bit of weather nerd.

Click Here to Leave a Comment Below 2 comments
Mir - May 30, 2019

Why dew point can also happen at higher temperatures, lower temperatures is understandable?
I just remembered Chicago, Chicago is humid in summer in July. May be, Chicago is humid out in July due to water vapors from the Lake Michigan. This means dew point happens in Chicago at higher temperatures in July, right. Now I got it.

It feels better after it rains, then temperature also should drop & why? Is it because the atmosphere gives out its latent heat to evaporate rain water, though the rain water phase changes from liquid to vapor without change in temperature of water, but the atmosphere still loses its heat thus dropping the temperature of the air. & that is the reason the temperature always drops after it rains.

Thanks

Reply
    Wessel Wessels - August 13, 2019

    You’re spot on

    Reply

Leave a Reply: