Sensible and Latent Heat

I am sure you have heard about sensible and latent heat before, but what do they mean?

Sensible heat

As the name Sensible suggests we can sense (feel) this type of heat, but how can you sense heat. Well you can’t really sense heat by itself, but you can sense its effect, the temperature stored in matter.

Let’s look at an example to explain this idea. In the picture below we boil water on the top a stove and as you can see the temperature rises to 212o F or 100o C.

Temperature of boiling water

Latent Heat

Now let’s look at latent heat in this same example. You may notice that once the water reaches the boiling point 212o F or 100o C., you really can’t raise the temperature any higher, yet we continue to add heat. So where does the heat go? I am adding heat, I am sure, the stove is still on adding heat, and I may remember from the heat pump thermal theory section, that “Heat can’t be created from nothing nor can it vanish. We can only transform it from one form to another”. Yea I remember the 1st Law of Thermodynamics.  So where did it go?

Latent-heat for water evaporation

Well it is simple; there is an energy that attracts water molecules together, keeping it in a liquid form. Now what you are really doing is breaking those bonds and as a result the water is transforming to a vapor form and you need energy for that. In simple terms you are evaporating the water and you need to add energy in the form of heat for that.

So latent heat or the heat of evaporation is the heat that the matter uses to transform or reject from one form to another without raising or reducing its temperature.

Hold on a second. So I do understand that we need heat to evaporate any matter, but what dose reject mean? Excellent question. When we condense any matter such as water from vapor, that water rejects the heat that it used it before to evaporate, yet without dropping its temperature.

 So is latent heat, the heat used for evaporation and condensation only? No, any time matter needs to change from one form to anther it either uses or rejects heat we called that heat latent …

From Solid to liquid (melting) from liquid to vapor (evaporation) from vapor to liquid (condensation) from liquid to solid (freezing)

latent heat of freezing and melting

latent heat of condensation and evaporation

Heat is Heat

Before we go any further, there is a very important fact I want you to understand “heat is heat”. It is the way the matter uses it that defines whether it is sensible or latent.  To explain this more

latent heat of melting ice

clearly let’s do this fun experiment. Hold a small cube of ice in your as shown in this image. Did you notice what happened? The cube of ice starts to melt, changing it form from solid to liquid. That requires heat, or more precisely latent heat. Where does it get the heat? It must get it from the nearest surface to it, which is your hand, so the cube of ice is absorbing heat from your hand and using it to melt the cube of ice. This is an example of latent heat. So far so good.

But your hand didn't melt (I hope J) from the heat that was absorbed by the ice cube. Rather your hand’s temperature dropped, so the same heat that melts the ice cube (latent heat) was sensible to your hand because its temperature dropped.

Your very own personal Air Conditioner

Sensible and Latent heat interchange play a very important role in the Air Conditioning Industry. In fact it is the very core of its cycle, as we will discuss in the Refrigeration Cycle section, but nature is full of examples for Sensible and Latent heat interchange.

Did you ever think that every one of us has a very personal Air Conditioner? Well, when you sweat, this is a very clear example for Sensible and Latent heat interchange. When you do any effort you sweat, which is basically water on the surface of your skin. When air hits your skin’s surface, the sweat will evaporate and that’s need heat (latent heat) and yes you are correct it will absorb that heat from the nearest surface to it, which is your skin. Then your skin will drop in temperature (sensible heat). That’s why in humid climates when sweat doesn't evaporate that fast because the climate is Saturated with water you feel hotter. Sweat evaporation process is a very clear example for Sensible and Latent heat interchange.

human natural cooling by sweat evaporation

Nature’s Air conditioner 

A cool ocean breeze is another clear example for the sensible and latent heat interchange. When the sun hits the water’s surface, the water will evaporate absorbing heat from the rest of the water, which in turn cools down. Then when the wind hits that waters surface, the water will cool down as well. That’s why you feel that cool breeze coming when you sit in front of the ocean or a lake.

Ocean breeze cooling by water evaporation

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