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Lesson 2, part 2: Density and Latent Heat

By: Shaun Tanner , 1:18 PM GMT on September 25, 2011

Do you know what is more freaky than a smoke detector going off, waking you up in the middle of the night? A carbon monoxide detector going off and waking you up in the middle of the night. This is what happened to me tonight, and is the reason I am sitting here writing a blog at 4:30 a.m. Pacific Time. The carbon monoxide detector nearest to the garage woke us up but turned itself off within 30 seconds. So, rather than going back to bed, I have opened the windows of my house, shut off the furnace at the switch, ordered a carbon monoxide meter (very expense, by the way), and am now writing this blog to you. I am in the same room as the active carbon monoxide detector and have placed a second one (from upstairs) in the room with me just in case. Why is an active carbon monoxide alarm more freaky than an active smoke detector? You see, when a smoke dectector goes off, you can get up and check on the house fairly quickly. No smoke, no fire. But, when a carbon monoxide alarm goes off, there is really no way to check on the house because carbon monoxide is an odorless, colorless, tasteless gas that you don't know you are inhaling until you are suffering from carbon monoxide poisoning. The gas comes from combustion, including your car's engine and the furnace in your house. A properly working furnace will not output much carbon monoxide, but the only way to check on this is use a meter to check levels. Hence, I bought a meter. I will also be going to buy a couple more detectors today. Just some food for thought before I start the blog.


Density is defined as mass over volume. In meteorology, the units we use most with density is kg/m3. That is kilograms of mass per cubic meter of a substance (most likely air). Picture a balloon filled with air. There are millions of molecules floating around in that balloon totally a certain mass (kg). Also, that balloon has a measurable volume (this is the cubic meter part of the equation). There are two ways to change the density of that balloon. The is to change the top number (kg). By putting more air into that balloon, we increase the mass, and thus the top number in our density equation will go up, as will the overall density as long as we keep the volume of the balloon the same. The reverse is true if we take air out of the balloon. The second way to change the density of the balloon is to change the bottom number. If we squeeze the balloon to half
its original size while keeping the same amount of air in it, its overall density will increase as well. If we expand the balloon by pulling its sides while keeping the same amount of air inside of it, the density will go down.

How does this relate to meteorology? Well, inside that balloon, again, are gas molecules that have a temperature. Remember, temperature is the average speed of the molecules in a substance. If you apply heat to the balloon, thus causing its molecules to go faster, the temperature of the air inside the balloon will go up. These molecules, energized with heat and moving fast, will impact the sides of the balloon at a faster speed and cause the balloon to expand. Think of a car hitting a barrier at 5 mph and another hitting the same barrier at 50 mph. The faster moving car will be able to push the barrier farther. Thus, is the sides of the balloon are expanding, what is happening to the volume of the balloon? It goes up! What is happening to its density? It goes down!

The opposite is true when you cool the air in the balloon. The molecules in the balloon move slower and are unable to keep the balloon as pumped up as it was before. Thus, the volume goes down and the density of the air in the balloon goes up!

Now, we can come to a very important conclusion. Cold air is more dense than warm air.. Let's say you have two balloons at the surface. One of these balloons is filled with air colder than the surrounding air and the other is filled with air warmer than the surrounding air. Based on our statement about density and air temperature, what will happen to those two balloons. Because nature tries to keep things with similar densities together, the warm air balloon will rise until it reaches a place that is its same density, and the cold air balloon will sink until it reaches air that is closer in density. For cold air, this is probably close to the ground. Now, this isn't quite the whole story, but that is going to have to wait until another lesson.

My analog to this density problem is a bottle of Italian salad dressing. The ingredients of Italian salad dressing and be put into two categories; oil, and seasonings. Left unmixed, these two ingredients will separate naturally with the oil on top and the seasonings are bottom. The seasonings are heavier. If you shake that bottom up, sure the ingredients will mix briefly, but then will separate quickly and go back to their original order once the shaking has stopped. The oil is less dense than the seasonings, thus wants to be on top of the heavier seasonings. The atmospheric analogy is cold is the seasonings, and warm air is the oil. Cold air is "heavier" than warm air.

Now that you know about density, let's move on to the most important part of any of the lessons yet.

Latent Heat

Latent heat can be explained if you can grasp the following idea. For warm liquid water to become cold ice, it has to lose heat. That lost heat does not just vanish, never to be seen again. Rather, that heat is put into the atmosphere. That is called latent heat.

Okay, now let's take a step back. When a substance changes from liquid to solid, that is called a phase change or a change of state. There are several types of phase changes.

Figure 1. Phase change diagram showing all types of phase changes for the atmosphere.

While several of these you are familiar with, some of them you might be seeing for the first time. Two interesting phase changes are sublimination and deposition. When something subliminates, it changes from solid to gas, skipping the liquid phase. Snow can do this, turning from an ice crystal to water vapor when a fairly strong Sun hits it. Deposition is the opposite: turning vapor to ice while skipping the liquid phase.

So, pretend you are stepping out of a shower or swimming pool and head to a towel to dry off. Have you ever noticed that you get cold rather quickly in this situation? If you were to zoom in to your skin, you would see several water droplets resting on it. Inside those droplets are millions of liquid water molecules floating around at various speeds. If you took the average speed of all the molecules, you would get a temperature. Some of these molecules are moving faster than others and will be more likely to jump from the liquid phase to the vapor phase. This is called evaporation. When the fast liquid molecules leave the liquid droplet, what happens to the temperature of the water droplet on your skin? Since the fast liquid molecules have left, the average speed of the molecules in the droplet has decreased, and thus the temperature of the droplet has gone down! Thus, you have a droplet on your skin that is colder than it was just a few seconds ago, and you then get cold. What basically happened was the energy that was in the water droplet was taken out of the droplet and put into the air, thus cooling the droplet. Evaporation is a cooling process.

When we say a warming or a cooling process, keep in mind we are speaking of the environment. In the example above, the environment around the water droplet got colder...your skin. This is an important distinction. So, you can do this exercise with all of the phase changes and your conclusions should be something exactly like Figure 2.

"HEAT ENERGY TAKEN FROM ENVIRONMENT" means the phase change is cooling process and "HEAT ENERGY RELEASED TO ENVIRONMENT" means the phase change is a warming process.

Figure 2. Latent heat diagram for the various phase changes.

There are a couple items that should confuse you in the diagram. The first is freezing of the liquid water. What the diagram is telling you is that the act of freezing is a warming process. How does this makes sense? Remember, when we are talking about latent heat, we are talking about the environment around. Thus, when liquid water wants to become solid, or ice, it has to lose warmth. Where does that warmth go? It goes into the environment, warming it. The opposite is true for melting. If an ice cube wants to melt into liquid water, it has to warm. Where does it get the energy to warm? The environment. It takes heat energy from the environment, leaving the surrounding environment cooler. Thus, it is a cooling process.

I think I have given you enough to chew on for now. I am going to wait for my wife and children to wake up so I can go stock up on batteries and carbon monoxide detectors.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

Reader Comments

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3. cyclonebuster
10:41 PM GMT on September 25, 2011
Quoting shauntanner:
It's not. This latent heat transaction is done in the very lower part of the atmosphere, the troposphere. It is want powers a good chunk of many atmospheric features like clouds, hurricanes, tornadoes, thunderstorms, etc.

You mean none?
Member Since: December 31, 1969 Posts: Comments:
2. Shaun Tanner , Senior Meteorologist (Admin)
1:43 PM GMT on September 25, 2011
It's not. This latent heat transaction is done in the very lower part of the atmosphere, the troposphere. It is want powers a good chunk of many atmospheric features like clouds, hurricanes, tornadoes, thunderstorms, etc.
1. cyclonebuster
1:35 PM GMT on September 25, 2011
How much of that latent heat or radiative heat is released to space?
Member Since: December 31, 1969 Posts: Comments:

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Wunderground Meteorologist Shaun Tanner

About shauntanner

Shaun Tanner has been a meteorologist at Weather Underground since 2004.

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