Why Air is Drier at Higher Elevations
This article is for you if you’re interested in understanding the science behind natural phenomena.
First, I’ll discuss how higher elevations result in lower atmospheric air pressure, and how lower air pressure means less moisture, or water vapor, in the air.
1a. Higher Elevations = Lower Air Pressure
1b. Less Air Pressure = Less Water Vapor
Then, I’ll talk about how higher elevations also results in lower temperature, which further discourages moisture in the air.
2a. Higher Elevations = Lower Temperature
2b. Lower Temperature = Less Water Vapor
1a. Higher Elevations = Less Air Pressure
If you’ve ever been driven up a really tall mountain for a ski trip, you might have noticed your bag of chips or your disposable water bottle bulging, and that sometimes pasta has separate cooking instructions for at higher elevations.
Well, the reason for this is reduced altitude pressure at higher elevations.
Pressure is the result of force divided by area, so for example one Pascal (a measurement of pressure) is by definition 1 Newton (a measure of force) applied over one square meter (a measure of surface area).
Atmospheric pressure is the result of air particles pushing down on you due to gravity.
At higher elevations, you have less air above you. Less air means less pressure pushing down on your meter by meter square, and as a result less overall air pressure at higher altitudes.
Meanwhile, as you ascend, the amount of air inside your non-rigid closed containers is the same, but because the pressure it exerts inside is now exceeding the pressure being exerted by the outside atmosphere, it bulges outward.
On a related note, water will boil at lower temperatures at higher elevations because the amount of heat energy needed for liquid water molecules to overcome the reduced atmospheric pressure pushing down on it has also been decreased. That’s why cooking times at higher elevations tend to be longer, because if “boiling water” is the indicator for a hot enough temperature (100C or 212F at sea level), the water will be at a lower cooking temperature when it boils sooner.
For comparison, water boils at a lower temperature of around 94.4C or 202F in Denver, Colorado, which is famously known as the mile high city for having an elevation of 5280 feet above sea level. (1 mile = 5280 feet, for those of us lacking in both a science education as well as the literary one necessary to deduce information from context clues).
1b. Less Air Pressure = Less Water Vapor
When on an alpine trip, you also might have noticed that your eyes feel dry, your hands get cut more easily, and that you’re more susceptible to bloody noses. The reason for this is because there is less vaporized water, or moisture, in the air.
But why exactly is there less moisture at higher elevations?
The composition of air is the same from sea level to 25 kilometers (or 15.5 miles) up, which means that even if the pressure changes, the proportion of water vapor remains constant — so long as the pressure isn’t so high that the water vapor condenses into liquid water.
Note: Past our atmosphere, there are basically no more air particles (nitrogen, oxygen, carbon dioxide etc. molecules) being suspended by Earth’s gravity: outer space is a near nothingness that is almost a perfect vacuum.
For those of you who learned the ideal gas law, you can see and also intuit that less pressure (P) also means less air (n) if everything else is constant.
If there’s less air, there must be less of everything that it consists of, including moisture and even oxygen.
That’s also why air at higher elevations is described as “thin” and difficult to breath in. You can get altitude sickness in a place like Cusco, Peru (above 11,000 feet) if you don’t give yourself time to acclimate and exert yourself — your body isn’t able to inspire enough oxygen to fuel yourself since the air is less dense.
2a. Higher Elevation = Lower Temperature
As you go farther from the Earth’s surface into space, you have more and more volumetric “space.” For example, if you took a slice of earth, you would have more volume the further you are from the center because as your radius increases linearly, your volume increases cubically. (V = 4/3 * pi * r³)
Even though heat rises, as air rises, it expands. And as air expands, it cools down, which you can observe via the Joule-Thompson effect whenever you spray an aerosol and notice that the gas being rapidly expelled from high to lower pressure feels cold, and the same when you stick your finger on the valve of a car tire and let some air escape.
This expansive cooling is shown via our ideal gas law PV=nRT:
As Pressure dramatically decreases and so does Volume as gas is rapidly released to the outside, but n (# of molecules) and R (universal gas constant) are constant, then T also dramatically decreases.
2b. Lower Temperature = Less Water Vapor
If your room is ever dry, you can boil some water and increase the temperature of the air as well as the amount of water vapor in the air. Boiling water tends to be more effective than using an electric humidifier — where you might see the moisture condense on the floor below it.
Higher temperatures are able to hold more water vapor in the air because temperature is the measurement of the average kinetic energy of atoms or molecules in a parcel. If each water molecule has enough energy to free itself from the other water molecules really close by that are threatening to entrap it into liquid form with its hydrogen bonds, then it vaporizes into vapor.
On the converse, lower temperatures are able to hold a lower level of moisture content, and as a result the air will be less dry at the same pressure but at lower temperatures.
Combining Everything Together
Less Air Pressure (1a) + Lower Temperature (2a) = Less Water Vapor (1b, 2b)
If both scenarios result in less moisture in the air, and higher elevations result in both reduced air pressure as well as cooler climates, then it makes sense that the air is so dry in alpine biomes!
