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Hey Ray: Cool science for hot nights

Hey Ray: Bernoulli Principle
Hey Ray: Bernoulli Principle 02:34

PITTSBURGH (KDKA) - Now that we are in the hotter months of the year, it is time we use some cool science to cool us off.  Sometimes, that can be a difficult task if you do not have air conditioning. 

However, if you use a fan to cool a room on a hot summer night, there is a way to more efficiently blow hot air out of a room.

An oscillating fan Ray Petelin

As we have shown you many times in this segment, air molecules pretty much fill everything.  Including your home.  When some air molecules are pushed out of your room, new air molecules rush in to fill those spaces.  Knowing that we can use science to cool your room more efficiently.

No, this is not a segment about putting a fan in the window.  Quite the contrary.  This is about why you shouldn't put a fan in the window!

The idea of this has to do with a principle discovered by a scientist named Bernoulli.  It states that as the speed of air increases, pressure decreases.

What is the Bernoulli Principle? Ray Petelin

First, let's talk about the air that a fan blows. 

You will notice that the fan's most efficient air current comes from the middle of the fan. 

A fan blowing outward Ray Petelin

Toward the fan's edges, the air seems to be sucked into the fan.

Air appearing to be sucked into the fan Ray Petelin

If you were to try blowing hot air out of a room with the fan in the window, the middle of the fan would blow out air, while the edges pull outside air in and blow it right back out. 

That is not the most efficient way to blow the hot air out of a room.

When you set your fan away from the window, to blow the warm air out, the breeze made by the fan creates an area of low pressure.  Air molecules try to rush into that area of low pressure to essentially fill it back up. 

Air flow 1 Ray Petelin

That rush ends up pushing more of the room's hot air outside, bringing the temperature down.

Air flow 2 Ray Petelin

You can see how this works with a simple experiment.

Take a garbage bag and blow it up like a balloon.  While you are doing that, think of the opening as the window.  

This will take you a while...if you don't pass out first. 

Now, let's use Bernoulli's principle to our advantage. 

Hold that bag away from your face. Take a big breath and blow air toward the bag. 

The fast-moving air creates an area of low pressure.  That causes more air molecules to rush into that area of low pressure, and that pulls more air molecules into the bag.  You can fill that bag with a breath or two.

A fan blowing outside. Ray Petelin

Placing the fan away from the window works the same way! 

Just make sure you do this trick during the coolest part of the day or night. 

If it is hot outside, this trick will just replace the hot air in your room with the hot air outside. 

If it is still hot outside, it would be best to probably just blow the fan toward yourself.

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