Know your atmosphere
How thick is the atmosphere?
Earth's atmosphere gets thinner as you go higher.
About 3/4 of Earth's atmosphere lies within the lowest 11 km. (1/580 of Earth's radius).
University Consortium for Atmospheric Research (UCAR)
What would you guess is the gas that is most common in Earth's atmosphere?
$CO_2$ 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%]
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 red line shows the "saturation curve" for water vapor: the amount of water in the air at 100% relative humidity.
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?
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?
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
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). 
An apple skin is much thicker relative to the apple (1/120).
Domiriel
An onion skin is more like it (1/560).
Susan Murtaugh
Melanie Windridge, Oxygen problems on Everest
What do you already know?
Earth's atmosphere
Is mostly nitrogen
This chart shows the composition of *dry* air.
Humidity
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
Hyperphysics @ gsu
Study questions
a. 24 grams
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.
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.
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