Introducing Density Altitude
Density Altitude is a measure that represents the density of the air in terms of the air’s pressure and temperature at a specific altitude.
It’s not about how dense the air feels, but rather a calculated value that helps pilots understand how the density of the air affects aircraft performance.
Higher density altitude means the air is less dense, which can impact aircraft performance such as engine power, lift, and speed
Density Altitude: “Explain this to me like I’m in 3rd grade.”
Example 1: Toy Car
Imagine you have a toy car that you’re racing down a ramp. The ramp is like a mountain, and at the top of the mountain, the air feels thinner and lighter. That’s because there’s less air pressing down on everything up there.
Now, imagine you have another toy car racing down a ramp at the bottom of the mountain where the air feels thicker and heavier. There’s more air pressing down on everything down there.
Density altitude is like imagining how high up the top of the mountain is where the air feels thinner. Even though you’re still on the ground, the air at higher places can feel lighter or heavier depending on how hot or cold it is and how much pressure there is.
Pilots and people who fly drones need to know about density altitude to understand how well their aircraft can fly in different places. If the density altitude is high (like being high up a mountain where the air is thin), planes and drones might not fly as well because there’s less air for them to push against.
Example 2: Running Up A Mountain
Imagine you are running a 100k ultra trail marathon in the Rocky Mountains. Some parts of the trail are flat, but other parts of the trail are go steeply to the top of the mountain. As you are running up the mountain and gaining elevation, it feels harder because there is less oxygen in the air and you’re having to work harder to get the same amount you’d breathe at the bottom with each breath.
The same thing happens with airplanes and drones when they fly at different altitudes.
Density altitude is like imagining how hard it is to trail run the higher up the mountain’s elevation you climb. In the same way, airplanes and drones have a harder time flying at higher density altitudes, where the air is thinner and there’s less support for them to fly well.
Density Altitude and Drone Performance:
Remember, drones perform best in oxygen-dense, dry air, which provides more air molecules for the propellers to generate lift and maneuver effectively.
- High Density Altitude: When the air is less dense (warmer temperatures, higher humidity, and lower pressure), the density altitude is higher, making it harder for drones to perform optimally. It also reduces aircraft takeoff and climb performance, meaning its motor needs to use significantly more power to get up into the air.
- Low Density Altitude: When the air is denser (cooler temperatures, lower humidity, and higher pressure), the density altitude is lower, and drones perform better because there are more air molecules for the propellers to generate lift.
Ambient Temperature + Pressure Altitude Impact on Density Altitude
Density Altitude is influenced by several factors, including:
- Pressure
- Temperature
- Humidity
Under standard conditions with a temperature of 15º C and pressure of 29.92 inHg, density altitude matches pressure altitude due to standard air density.
However, if the outside air temperature exceeds 15º C, the warmer air becomes less dense than cooler air. This is because with higher temperatures, air molecules expand, reducing air density. This lower air density in high temperatures (i.e. thinner air) increases the density altitude.
Relationship of Density Altitude and Pressure Altitude
Density altitude and pressure altitude are two different concepts in aviation, but they are related to atmospheric conditions:
Let’s address the statement: “If the outside air temperature is warmer than standard air temperature, then the density altitude will be higher than pressure altitude.”
- Warmer Air Temperature: Warmer air is less dense than cooler air. As temperature increases, air molecules spread out and become less dense.
- Higher Density Altitude: When the air is less dense (warmer temperature), the density altitude increases. This means the aircraft behaves as if it is flying at a higher altitude where the air is less dense. So, density altitude rises with warmer temperatures.
- Pressure Altitude: Pressure altitude, on the other hand, is affected primarily by atmospheric pressure. It decreases with higher altitudes above sea level where atmospheric pressure is lower.
Part 1. The “Standard Day”
The concept of a “standard day” is a fundamental concept in aviation and meteorology that provides a baseline for comparing and calculating various atmospheric properties.
This is used globally to ensure that there is a common reference for atmospheric conditions.
At the following standard conditions, density altitude and pressure altitude will be equal to each other.
*When air temperature and/or air pressure are higher or lower than standard, the density altitude will change.
The concept of density altitude and pressure altitude being equal under standard conditions is a fundamental principle in aviation.
Let’s break down what this means and why it is important.
- Standard Temperature and Pressure: At sea level, the standard temperature is 15°C and the standard pressure is 29.92 inHg. Under these conditions, the air density is also at its standard value.
- No Corrections Needed: Since both temperature and pressure are at their standard values, there is no need to correct for deviations. Therefore, the pressure altitude (which is based on standard pressure) and the density altitude (which is based on standard temperature and pressure) are the same.
When conditions deviate from the standard (e.g., higher temperatures or lower pressures), the density altitude will differ from the pressure altitude:
Real World Examples
Higher Density Altitude:
- Location: Denver, known as the “Mile-High City,” sits at an elevation of approximately 5,280 feet above sea level.
- Conditions: During summer, temperatures can reach into the 90s°F (30s°C), and the atmospheric pressure is naturally lower due to the high elevation.
- Impact: The combination of high elevation and high temperatures results in a higher density altitude. Pilots and drone operators will notice reduced aircraft performance, requiring more power for takeoff and reduced lift efficiency.
Lower Density Altitude:
San Francisco, California (Lower Elevations, Lower Temperatures):
- Location: San Francisco is located at sea level, with an elevation of approximately 52 feet above sea level.
- Conditions: The city experiences relatively cool temperatures year-round, often in the 50s-60s°F (10-20°C), and benefits from higher atmospheric pressures due to its low elevation.
- Impact: The combination of low elevation and cooler temperatures results in a lower density altitude. The denser air improves aircraft performance, providing better lift and more efficient power usage for drones.
Quick Review
Density Altitude
is defined by?Pressure Altitude & Ambient Temperature
High density altitude equals thinner air, making your propellers less efficient.High Density Altitude
Impact on propeller efficiency?Decreased Efficiency
High density altitude equals thinner air, making your propellers less efficient.Hot and Humid Weather
Impact on aircraft performance?Decreased Efficiency
Hot and humid conditions further increase density altitude, worsening performance.
Standard Day
Standard air temperature and pressure at sea level?15º C 29.92” Hg
At these standard conditions, density altitude and pressure altitude will be equal to each other.
Pressure Altitude
If the outside air temperature is *warmer* than standard, the density altitude is?Higher than Pressure Altitude
Density altitude is lower than the pressure altitude because colder air is denser.
Pressure Altitude
If the outside air temperature is *colder* than standard, the density altitude is?Lower than Pressure Altitude
Density altitude is higher than the pressure altitude because warmer air is less dense.
– Practice Quiz –
Defining Density Altitude
- Density altitude is a measure of how dense the air is at the altitude that the aircraft is flying. This significantly impacts your drone’s performance and propellor efficiency. Your drone’s propellers need air to generate lift and thrust.
