- Ecuador -- Site of Goshen College's Spanish-speaking SST unit -- The capital, Quito, has an elevation 2.85 kilometers (about twice as high as Denver, the "mile high" city). Air pressure is 71% of sea-level.
- Mt Everest: 8.8 km - 29,000 feet - Air pressure is 33% of sea-level: 2/3 less air and 2/3 less oxygen!
Melanie Windridge, Oxygen problems on Everest
- On Everest there is less than 1/3 of the oxygen we enjoy at sea-level. Even so, bar-headed geese migrate yearly over the Himalayas.
What do you already know?
- What is the gas that we breathe in that we need in order to exercise, and live?
- What is in the gas that we exhale?
- What is the gas that is linked to global warming?
- What other gases do you know that exist in the atmosphere?
- What do plants "inhale"? "exhale"?
What would you guess is the gas that is most common in Earth's atmosphere?
Earth's atmosphere
Is mostly nitrogen
This chart shows the composition of *dry* air.
$CO_2$ is currently 421 ppm $\approx 400$ parts per million. That's a fraction of
$$\frac{\text{400 }CO_2\text{ molecules}}{\text{1,000,000 molecules of air}}\cdot \frac {\text{100%}}{1}=\frac{4\cancel{00}\cdot 1\cancel{00}}{100\cancel{0000}}\%=0.04\%.$$
[using a conversion factor, that a fraction of 1 = 100%]
Humidity
When the
weather report says "100% humidity" does that mean that all the
molecules in the air are water? Can you think of any consequences for humans of having 100% of the molecules in air be water molecules?
Actually there is a maximum density of water molecules in
the gas state that depends on temperature. 100% relative
humidity means that the water density has reached this maximum
concentration.
At 68 F and "100% humidity", there are about
18 grams of water in each cubic meter of air in a gaseous state.
About 2% of all molecules in the air are water molecules.
At 77 F and "100% humidity" there are about 22 grams in a cubic meter of air. What do you think happens if you cool the air to 68 F? What happens to those extra 4 grams of water?
On any given day,
there is more or less humidity (water) in the air as well.
The worldwide average concentration of water in the atmosphere is about 0.4% with typical concentrations close to the surface of 1-4%.
Saturation curve
The red line shows the "saturation curve" for water vapor: the amount of water in the air at 100% relative humidity.
Hyperphysics @ gsu
Study questions
1.) At 77${}^o$ F and 100% relative humidity, there are 22 g of water in each cubic meter of air. At 87${}^o$ F and 100% relative humidity, the amount of water in each cubic meter of air is closest to:
- 24 grams
- 20 grams
- unchanged, stays at 22 g
- Not enough information to tell
show / hide
You don't actually need to do any calculation to answer this question. You just need to know that
as the temperature increases, the maximum amount of water vapor in air increases as well. The only answer which is larger than 22 g is:
2.) At room temperature (22 C), 100% relative humidity means there are approximately 20 g of water in every cubic meter of air. For air at 22 C, how many grams of water per cubic meter would there be at 50% relative humidity?
show / hide
100% relative humidity (see the saturation curve) at 22 C means that there is about 20 grams of water vapor in each cubic meter of air.
50% relative humidity means that there is 50% (one half) as much water as at 100% humidity. That is, there is just
10 g / cubic meter of air.
3.) Take that air in the question above at 50% humidity and 22 C... If you cool it down, at what temperature would water start to condense out of the air?
show / hide
The line for 10 g/cubic meter crosses the red line for 100% humidity at about 11 C.
So, the "dewpoint" of air containing 10 g of water for each cubic meter of air is about 11 C.
4.) On a warm, humid day in July, in Indiana put these molecules in the air in order from most abundant to least abundant: Oxygen, Nitrogen, $CO_2$, water vapor
show / hide
Nitrogen is the most common gas, at about 78% of dry air atmosphere, about 4 times as abundant as Oxygen at 20%. There is very little $CO_2$, less than 0.04% (=400 ppm).
What about water vapor?
You might remember that 100% relative humidity at 68 F means about 2% of the molecules in air are $H_2O$. 'Humid' probably means more than 50% relative humidity. Even at 68 F, 50% humidity means 1% water vapor. But in July in Indiana it's probably hotter than 68F, wo even 50% humidity means more than 1% water molecules in the air. And in Indiana, the evening temperature the humidity is only 50% that would still mean 1-2% water: A good deal more than $CO_2$, so the abundances from most to least are:
most: $N_2 \gt O_2 \gt H_2O \gt CO_2$ least.
Image credits
Henri Grissino-Mayer
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