Lesson 17: Good weather, altitude and air thickness

So, we talked about what your perfect day looks like. Did you know that weather scientists actually have a pretend perfect day?

They call it a “Standard Day”.

Why do they have this concept?

Standard Day represents “perfect” conditions as a reference point so scientists can figure out how aircraft will work. It’s used for calculations and comparisons.

The weather and conditions can be super different day to day, so it helps to have a reference point that scientists everywhere can agree on.

So, what does this perfect day look like? To a scientist, of course!

At sea level, the standard air temperature is 15 degrees Celsius (59 degrees Fahrenheit) and the standard air pressure is 29.92” Hg, or inches of Mercury (1013 millibars).

We measure atmospheric pressure in Mercury (that shiny silver liquid metal!) because it has a high density but flows easily. Changes in Mercury levels are easy to detect.

At these standard conditions, density altitude and pressure altitude are equal to each other.

We’ll talk a lot about what density altitude and pressure altitude means later today.

But for now, let’s just talk about what the Standard Day is.

A perfect day for weather scientists. At sea level, the standard air temperature is 15 degrees Celsius (59 degrees F) and the standard air pressure is 29.92” Hg, or inches of Mercury (1013 millibars).

Today’s Key Word!

Air Density

Awesome! Now, let’s put this idea into action.

Quiz

Pilots should know how _____ works.

Earth Fire Water Air

Pilots should know how air works!

Air is like the canvas that pilots use. It can affect how well a plane flies! So let’s learn about it.

The air is complex! And pilots need to know how it works, the science behind it, and how air changes.

Let’s see if you can guess the answer on this really important concept about air!

Air has weight. True False

Take a Guess!

Air has weight! And gravity pushes it down onto everything.

Exactly! Air’s like this invisible weight that presses on us from all sides.

Here’s a fun fact about air, too. It changes in weight depending on how high up you are.

Air can change in thickness. True False

Great! So we know air has weight, and air can be thick or thin. Let’s learn about what that means.

Air Density

The air is made up of tiny air molecules that are all around us!

Air molecules move around. When there’s more of them, the air is thick – and more dense.

When there are less air molecules, the air is thinner, and less dense. Makes sense, right?

More Dense, Thicker Air Less Dense, Thinner Air

Air density is a measure of how much air (or air molecules) is packed into a given space. When there are more air molecules, the air is more dense! When there are less air molecules, the air is less dense!

When the air is more dense, there are more air molecules, which makes it way easier for aircraft and drones to fly, because they have more stuff to hold onto.

When the air is less dense, there are fewer air molecules, so it’s harder to fly, because they have less stuff to hold onto, and even waste more fuel.

Air density can change depending on temperature, altitude, and humidity. So there can be more or less molecules in the air depending on these things.

Because so many things can affect the density of the air, and the density of air can affect how pilots fly, that’s why they care about air and perfect weather!

Which kind of air is better for drones?

More Dense (Thicker) Air Less Dense (Thinner) Air

Awesome! Now, let’s jump to learning about the four ways we can understand altitude. We already know about two!

The Four Ways to Understand Altitude

There are four different ways we can think about altitude. We already know the first two, so let’s review!

Do you remember what AGL vs. MSL is?

Right! They’re two types of altitude that are used to measure how high something is.

AGL is the height above the ground, whereas MSL is the height above sea level.

Did you know that MSL and AGL have other cool names used by pilots?

The other name for AGL and MSL pilots use is called Absolute Altitude and True Altitude.

Absolute vs. True Altitude

Absolute Altitude True Altitude The height above the ground level, or AGL. The height above the mean sea level, or MSL.

You might see this on the test! Just remember: Absolute Altitude starts with an A, like AGL.

Absolute Altitude is the height above the ground, or AGL. True Altitude is the height above mean sea level, or MSL.

Today’s Key Word!

Civil Twilight

Great! Now that we know about Absolute and True Altitude, let’s cover the last two ways we can think about altitude – they’re a little different.

Let’s explore how drones move through the air, and how air pressure and air density can make the altitude feel different!

Did you know that height isn’t the only way that pilots can think about altitude?

When pilots are high up in the air, they can measure how high up they are – and not by looking down or guessing how far the ground or sea is.

Pilots use special tools called altimeters to read the air pressure, which lets them know how high, or the altitude, they’re flying!

Pretty cool, right? Knowing how air pressure works helps pilots know how high they are, instead of having to guess.

This neat trick is called Pressure Altitude. Let’s look at the science behind it!

Pressure Altitude

Pressure altitude is a way of measuring height above sea level. Air pressure changes (and gets less) the higher you go up!

Think about pressure as a stack of books. At the bottom of the stack, there are many books above it, so the pressure (weight) is high.

Near the top of the stack, there are fewer books above, so the pressure is lower.

Pressure altitude is a cool concept in aviation that helps pilots understand how high they are in the sky based on air pressure. 

Fun Fact: There is less air pressure the higher you go up, so there is less air. Your lungs have to work harder up there because there are fewer oxygen molecules! 

Pressure altitude is a way to measure height above sea level.

It uses a standard air pressure (29.92 inches of mercury) as a reference point, regardless of actual weather conditions.

Pressure altitude gives pilots a reliable way to measure their height by using a fixed air pressure level as a baseline, even when the weather changes the actual air pressure. 

This ensures they have a consistent and accurate measurement, and helps them fly safely and efficiently. 

Formal Definition: 

Pressure Altitude is the indicated altitude when an altimeter is set to 29.92 in Hg. It is primarily used in aircraft performance calculations and in high-altitude flight.

Density Altitude 

Remember air density? It’s the idea that air can be thick or thin, depending on how many particles are in the air. 

Let’s talk about what makes air more dense. 

These factors can change how dense the air is: 

• Altitude 

• Air Pressure 

• Humidity 

• Temperature

Altitude 

The higher you go up, the air gets thinner and less dense. The lower you go, the air is thicker.  

Air Pressure 

When air pressure is low (like on a mountain), the air is also thinner and less dense. 

Humidity

When it’s humid and muggy outside, the air is thinner and less dense. 

Temperature

When it’s really hot outside, the air is also thinner and less dense. 

Let’s focus on altitude, and learn about how air density changes the higher you go up. 

The higher you go up, the air gets less dense, so there are less air particles up there! 

The lower you go down, the air gets more dense, so there are more air particles down where we are! 

When you’re closer to the ground, the air is more thick and dense, so there are more air molecules. 

High Elevation 

• Less air molecules

• Lower air density

• Thinner air  

Low elevation 

• More air molecules

• More air density

• Thicker air  

To recap, the density of the air decreases with altitude. 

If the density altitude is higher, it means the air is thinner! 

And if the density altitude is lower, it means the air is thicker! 

Nice! Did you know that density altitude affects how well an aircraft flies?

When there are less molecules in the air, drones don’t have as many air particles to hold onto! 

Great! We know air can have different densities and why. Now, let’s learn about the final way of thinking about altitude: density altitude. 

Density altitude is the altitude at which your aircraft “feels” like it’s flying.

The density of the air is how “thick” or “thin” that air is, and depends on atmospheric conditions.

Air Density is how pilots measure the density of air, or how thick or thin it is. 

When the air has more molecules, it’s more dense. When it has less, it’s less dense!  

When air is thin, it has high density altitude

When air is dense, it has a low density altitude.

Density Altitude is like the “feeling” altitude of the airplane. 

It tells how high the airplane feels like it’s flying, based on how thick or thin the air is. 

Thin air → high density altitude 

Thick air → low density altitude

Formally, density altitude is defined as “pressure altitude corrected for nonstandard temperature variations.”

If the air is thin (high density altitude), it feels like the plane is flying higher than it actually is, and it won’t perform as well. 

If the air is dense (low density altitude), it feels like the plane is flying lower, and it will perform better.

Density altitude significantly influences aircraft and engine performance, so every pilot needs to understand it. 

Hot, high, and humid weather can cause a regular takeoff or landing to become an accident at the blink of an eye. 

Did you know that when the density altitude is high – when you’re higher up, and the air is thin– your drone will fly worse?

Your drone will fly worse in hot and humid weather, because it makes air molecules spread out.

When air is less dense, your drone propellor has fewer particles to hold onto because the air is too thin! Your drone has to work harder. 

This leads to reduced lift and makes it harder for a drone to maintain its height, maneuver effectively, and perform at its best.

Because it has to use more energy or fuel, flying at high density altitudes is not very efficient! 

Aircraft performance depends on altitude  density! The propeller is more effective when it has more air molecules to grab onto! 

Fun Fact: Did you know that air is thickest and has the strongest pressure at sea level? 

Just like your lungs have to work harder to breathe at higher elevations, it’s harder for your drone to build up energy to move, too! 

Tip: If you’re flying in the mountains or other high locations, your drone won’t fly as well and your battery will drain quickly!

Awesome! And now for a micro-quiz :) 

Which type of air is best for drones to fly in? 

(The more dense the air is, the more your drone has to grab onto!)

High Density Altitude → Worse Performance 😔 

Low Density Altitude → Better Performance 😄 

High Density Altitude →  Lower air density, Less air molecules, Thinner air  

Low Density Altitude → More air density, More air molecules, Thicker air  

Let’s go over what conditions lead to higher density altitude vs. lower density altitude. 

High Density Altitude →

• Higher elevations ⛰️  

• Higher air pressure ⛰️  

• Humid, muggy weather 🌨 

• Higher temperatures ☀️

Low Density Altitude → 

• Lower elevations 🌊

• Lower air pressure 🌊

• Dry weather 🌵 

• Lower temperatures 🌤  

Standard Day

The higher the altitude, the air gets:

Thicker and more dense

Thinner and less dense

High Density Altitude means there is: 

More Air Density 

Less Air Density 

(The higher you go up, the less dense air gets!) 

Drones fly best in:

High Density Altitudes 

Low Density Altitudes 

If it’s hot and humid, your drone will fly:

Better

Worse 

How confident are you in density altitude?

Pretty Confident 

Somewhat Confident 

Not Confident..

You’re doing great! You’ll get the hang of this in no time. 

Pilots need to know density altitude because it affects how well your drone flies, takes off, lifts, and saves energy, and moves safely, especially in high or hot conditions.

Air Recap 

Higher density altitude → Thinner Air

Lower density altitude → Thicker Air 

Drones fly best in lower density altitude  

Humid and hot weather and high elevation → high density altitude 

Drones don’t fly well in these conditions  

Phew! You’ve done a great job so far! 

Let’s take a much-deserved break. 

Excellent work! Let’s wrap up with an activity!    

Great work! I see you’re all a few steps closer to becoming a certified drone pilot.