1 Water Vapor -
A greenhouse gas
3 Dimensions
Water, moisture, air
5 Measurement -
Determination of humidity
Back to
Overview
2 Absorption -
Absorption of radiation
4 Concentrations -
How humid is air?
6 Forecast -
Water and global warming
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School Page
Section 3
Dimensions

Lilac cows

How heavy is air?

How much water does air contain?

How thick is the atmosphere?

Summary

To the overview

Dimensions
Water, moisture, air

Lilac cows

How heavy is air?
If we would ask this question to our parents at home or to anyone, they would certainly look puzzled.

How thick is the atmosphere?
Eh ... what?

How many water molecules are in one liter?
Oh, eh... yeah, well, dunno...

Of many things surrounding us, we often hardly know their size, weight, and number.  In this chapter, we will show a number of pictures and cartoons about water. Very often, however, even in these pictures, the dimensions of water are not correctly represented.

 

Because of the different sizes of objects, it is often very hard to display them in the right dimensions. Suppose that we would want to draw a map of England.  Now we should draw a car in the map, which is parked at 21 Oxford Street in London. We could just draw a car somewhere in the south of England, but that will be either not visible, or much too big in relation to England.


Wrong dimensions:
Cloud altitude: max 10 km
Earth size: 13,000 km

In the same way, in this picture, the atmosphere with its clouds is drawn much too thick in relation to the size of the Earth.

Once, a large number of children was asked to draw a cow. A considerable part of the group had drawn the cow lilac - obviously they were influenced by the chocolate advertisement campaign. And obviously, these children had never seen a real cow, bad enough as it is! The children who drew a lilac cow, have a false impression of reality, because they have been influenced by a fictive cow in a commercial. This chapter is written in order to prevent us from getting a false image of reality because of the many (necessary!) false pictures we get to see in our lives.

How heavy is air?

Now back to our question: how heavy is air?

Air is weightless, right?!? Oh no!

Air consists of molecules, and molecules do have a weight. They are very small (around 1 nanometer, and a nanometer is 1/1000 of a micrometer, and a micrometer is 1/1000 of a millimeter) but because there are so many of them, together they make up a considerable mass. Otherwise there would not be air pressure, and this air pressure would not decrease with altitude.

Air consists of 21% oxygen, 78% nitrogen, and a myriad of other gases, including water.  If you calculate an approximate density of air, you will see that 1000 liter of air weighs approximately 1 kg.  The density of water is approximately 1 kg per liter, so fluid water is about 1000 times as heavy as air.


Source: MPI Mainz - Brenninkmeijer Group

How much water does air contain?

Source: ESPERE

In the next chapter, about concentrations, we will learn that where the climate is the warmest and the most humid, the air contains the most water.  In the tropics the concentration of water vapour can amount to up to 30 g of water per kg of air.  At the poles it can be 500 times less, or about 0.1 g per kg air.  If we take a look at relatively humid air, say 18 g/kg water.  You can calculate from this that in every 100 air molecules, there must be about 3 water molecules, so 3%.  This is not a lot, but it is still a considerable amount if you consider that all other trace gases with low concentrations, such as helium or carbon dioxide, are not indicated in percent but in parts per million, or 1/1000 of a percent!

 

Parts per million = ppm
Water as a greenhouse gas in warmer areas can have concentrations of 1000 to 40,000 ppm (= 0.1 to 4%).
Some other greenhouse gases:

Carbon dioxide, CO2 370 ppm today
280 ppm pre-industrial
Methane,CH4 1.7 ppm today
0.8 ppm pre-industrial
Nitrous oxide, N2O
(Laughing gas)
0.31 ppm today
0.28 ppm pre-industrial

This means that, of one million molecules in the air, about 370 are carbon dioxide, two are methane, and between 100 (poles) and 40,000 (tropics) are water.


How thick is the atmosphere?

In relation to the Earth, the atmosphere is only a very thin layer: The lower two parts (troposphere and stratosphere, they reach to an altitude of 50 km) and the Earth would correspond to a piece of paper wrapped around an orange.  The troposphere, which is the layer where clouds form and the weather as we notice it takes place, is about 10 km high (somewhat higher in the tropics, 18 km).  The troposphere contains 80% of the total air mass, because air gets thinner with higher altitudes.  The Earth has a radius of 6370 km.  It is important to realize that all pictures where the Earth is surrounded by a thick blue layer are actually not proportional to the real situation. This picture shows the dimensions of the atmosphere in a better way:


Source: MPI Mainz - Brenninkmeijer Group

There are also real photographs that show the atmosphere as a bright blue layer:

Thus the atmosphere is not so thick at all.  How are the molecules of water and air distributed between the atmosphere and the oceans? An approximation can be done on this. If we assume a ocean coverage of the Earth of 71%, an average depth of the ocean of 2000 to 6000 meters, and compare this to the atmosphere, which has a "depth" of 10 to 15 km, the result is that the oceans must contain about 450 times as many molecules as the atmosphere. In the air, we saw before that on average only 1% of the molecules in air are water. We can therefore safely say that practically all water on the Earth is in the oceans. There is some frozen on the poles and in glaciers, some flowing in rivers or present in groundwater, and a very small amount is present in a gaseous form in the atmosphere. This relationship is in the following picture.


Source: ESPERE

In Summary:

Pictures of atmospheres, clouds, the Earth and molecules always should be seen in the proper perspective. To make a picture clearer, though, the dimensions are often deliberately changed.  To get an idea about the relations in the atmosphere around us, some simple calculations can be very helpful.  Water is 1000 times as heavy as air.  There are 450 times as many molecules in the oceans as in the air, however, and of those molecules in the air, on average only 1% is water.  The air/ocean distribution for water is thus 1/45000. Water however, is still the most important greenhouse gas, because its concentration in the atmosphere is on average about 25 times that of carbon dioxide.

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Basic text: Elmar Uherek - MPI for Chemistry Mainz / GER
Translation and modifying: Heleen De Coninck - MPI for Chemistry Mainz / GER
Edited by Stephen Gawtry - University of Virginia / US