Why is the Air Dry at Altitude?

So, here's the thing. When I was a kid, I got to go skiing in the Rockies a few times. Sounds great, right? Well, yeah, the skiing was fantastic, but the dry air at those higher elevations? the dryness and the lack of oxygen combined felt like breathing vapor beef jerky. Cut to me getting off the plane back in good old humid Florida, and it was like my lungs were suddenly wrapped in a warm, thick, slightly sweaty blanket of comfort.

You'd think that since water vapor is lighter than dry air (yes, it is, don't argue), higher altitudes should be more humid. It makes sense, right? To help us mere mortals understand why that isn't how it works, let's look at the world of atmospheric pressure, vapor pressure, and adiabatic cooling – HVACR-style.

Atmospheric Pressure and Vapor Pressure: It's Not a Walk in the Park

Picture this: atmospheric pressure and vapor pressure are like two peoples trying to take a walk in the park. Only, this park is way up in the mountains, and the path they're on is getting narrower and more treacherous the higher they go. Basically, it's the worst park ever.

At higher elevations, the atmospheric pressure (our first brave park-goer) decreases, making it harder for our buddy vapor pressure to keep up. It's like trying to walk on a balance beam wearing roller skates – not easy, my friend. The result? Less humidity in the air, which explains one reason why it feels like a desert up in the mountains.

The Law of Partial Pressure and Its Role in Mountain Air Dryness

An important concept that plays a role in the dryness of mountain air is the law of partial pressure, also known as Dalton's Law. This law states that the total pressure exerted by a mixture of gases is equal to the sum of the pressures that each gas would exert if it were alone in the container. In the context of air at high altitudes, the atmospheric pressure is lower, which means that the partial pressure of water vapor is also lower (as well as Oxygen and Nitrogen and everything else). As a result, the air can't hold as much moisture as it can at sea level, where the atmospheric pressure is higher. This reduction in the partial pressure of water vapor contributes to the drier conditions experienced at high elevations

Rarified Air: A Fancy Word for “Thin and Dry” (and a Frank Sinatra Reference)

First things first, let's talk about “rarified air.” As Frank Sinatra croons in the classic song “Fly with Me,” we're “up there where the air is rarified.” Rarified simply means thin or less dense, usually due to reduced pressure or less oxygen. In the context of our mountain adventure, rarified air refers to the drier, cooler, less dense air at higher altitudes caused by lower atmospheric pressure and adiabatic cooling.

Speaking of adiabatic cooling, let's dive a little deeper into that concept. Adiabatic cooling is a process where the temperature of a gas decreases as it expands without any heat exchange with its surroundings, so decreases in temperature but keeps the same BTU/lb. This phenomenon is especially relevant when air rises to higher elevations, where the atmospheric pressure is lower. As a result, the air expands and cools down, ultimately reducing its ability to hold moisture and leading to drier conditions.

The Math: A Sea Level vs. Denver, Colorado Comparison

Now, let's dive into some numbers and compare sea level air to the air in Denver, Colorado, which is nicknamed the “Mile High City” because it sits at an elevation of 5,280 feet above sea level.

At sea level:

  • Atmospheric pressure: 14.7 PSI (pounds per square inch)
  • Temperature: Assume 68°F (20°C)
  • Relative humidity: Assume 60%

Using these values, we can calculate the actual vapor pressure (the pressure exerted by the water vapor in the air) and then determine the grains of moisture per cubic foot of air.

For sea level conditions:

  • Saturation vapor pressure = 0.62198 PSI
  • Actual vapor pressure = Relative humidity * Saturation vapor pressure = 0.6 * 0.62198 ≈ 0.37319 PSI

Now, let's consider Denver's conditions at the same temperature and relative humidity:

At Denver (5,280 feet elevation):

  • Atmospheric pressure: Approximately 12.2 PSI (based on the barometric formula)
  • Temperature: Assume 68°F (20°C)
  • Relative humidity: Assume 60%

For Denver's conditions:

  • Saturation vapor pressure = 0.62198 PSI
  • Actual vapor pressure = Relative humidity * Saturation vapor pressure = 0.6 * 0.62198 ≈ 0.37319 PSI

However, because of the lower atmospheric pressure in Denver, the actual vapor pressure must be adjusted to account for the altitude. We can adjust the actual vapor pressure using the ratio of atmospheric pressures:

Adjusted actual vapor pressure = Actual vapor pressure * (Denver atmospheric pressure / Sea level atmospheric pressure) = 0.37319 * (12.2 / 14.7) ≈ 0.30818 PSI

Now, to find the grains of moisture per cubic foot of air:

1 PSI ≈ 7,000 grains per cubic foot

  • Sea level: Actual vapor pressure = 0.37319 PSI ≈ 2,613 grains per cubic foot
  • Denver: Adjusted actual vapor pressure = 0.30818 PSI ≈ 2,157 grains per cubic foot

As you can see, the air in Denver contains fewer grains of moisture per cubic foot than the air at sea level, even when the temperature and relative humidity are the same… which they also usually aren't because it is usually colder and lower RH in addtion to lower total moisture capacity due to decreased atmospheric pressure.

To Sum It All Up (Because Who Doesn't Love a Good Summary?)

At higher elevations, like in the mountains, the air is rarified – meaning it's thinner and drier – because of lower atmospheric pressure and adiabatic cooling. These factors cause the air to hold less moisture, resulting in drier conditions compared to lower elevations.

Using our sea level vs. Denver comparison, we showed that even when the temperature and relative humidity are the same, the air at higher altitudes contains less moisture due to the reduced atmospheric pressure. So, the next time you're in the mountains and your lungs feel like they're on a one-way trip to mummification, just remember these big words and my nerd tech tip about it.

— Bryan

References, sources and disclaimer

Water vapor is lighter than dry air. This is due to the molecular weight of water vapor (H2O, molecular weight = 18) being less than the average molecular weight of dry air (approximately 29, as dry air consists primarily of nitrogen and oxygen).
Reference: Bohren, C. F., & Albrecht, B. A. (1998). Atmospheric Thermodynamics. Oxford University Press.

The relationship between atmospheric pressure, vapor pressure, and humidity is explained. At higher elevations, atmospheric pressure is lower, and consequently, the vapor pressure of water is also lower. This results in less humidity in the air.
Reference: Wallace, J. M., & Hobbs, P. V. (2006). Atmospheric Science: An Introductory Survey. Elsevier.

Dalton's Law of Partial Pressure is described, and its application to explain the reduced moisture-carrying capacity of the air at higher altitudes.
Reference: Jacob, D. J. (1999). Introduction to Atmospheric Chemistry. Princeton University Press.

Adiabatic cooling is accurately explained as a process where the temperature of a gas decreases as it expands without any heat exchange with its surroundings.
Reference: Ahrens, C. D., & Henson, R. (2015). Meteorology Today: An Introduction to Weather, Climate, and the Environment. Cengage Learning.

The example of sea level vs. Denver, Colorado is a valid comparison for explaining the differences in moisture-carrying capacity due to altitude. Denver, situated at an elevation of 5,280 feet, experiences lower atmospheric pressure compared to sea level.
Reference: National Oceanic and Atmospheric Administration (NOAA). “Air Pressure and Altitude.” Retrieved from https://www.weather.gov/jetstream/pressure_vs_altitude

Please note that the mathematical calculations and specific numbers provided in the example may not be entirely accurate, as they are based on simplifications and assumptions. However, the general idea behind the comparison remains valid.

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