Gases, including Water Vapor

 

More than 99% of the lower atmosphere is made up of three gases, not counting water vapor (Table 1.1 of text).  The three major gases are:

  •         Nitrogen - 78%

  •         Oxygen - 21%.

  •         Argon - 1%

Some of the gases that occur in small amounts and that vary from place to place are of greatest interest. Carbon Dioxide, Carbon Monoxide, Methane, and Ozone are gases we hear about in terms of air pollution, global warming and the green house effect. All of these gases account for a small fraction of the atmosphere gases.

The atmosphere also contains solid particles as well as liquids. Dust and haze are good example of the solids. Clouds and fog are examples of liquids in the atmosphere. We will look at these pollutants, particles and droplets later.

Water vapor is found throughout our lower atmosphere. It is a gas just like nitrogen and oxygen. But, unlike those gases water vapor varies greatly in the composition of our atmosphere. Under very warm conditions water vapor may account for almost 4% of the gases of the atmosphere. Under very cold conditions water vapor will be very low, accounting for only a small fraction of 1%. As the amount of water vapor goes up, the relative proportions of all other gases go down.

The fact that water vapor varies so greatly in the atmosphere is a major factor in weather and climate. In the winter it can be very dry. When it is very dry your skin may itch, your nose and lips crack and you create static electricity walking across the rug. Many people use humidifiers to add moisture into the air in winter to make it more comfortable.

In the heat of summer if it is very humid cold drinks sweat and leave a pool of water on the table, you sweat and have trouble getting comfortable. Have you ever heard ‘Its not the heat, it’s the humidity’? Air conditioners take moisture out of the air in summer.

The absence or presence of water vapor in the atmosphere is a major difference between winter and summer

The Capacity of the Air to Hold Moisture

The amount of moisture the air can hold varies with temperature. This relationship is shown below with a few simple values.

 

Temp ° C

  maximum possible    water vapor in g/km

-10° C 1.79
0° C 3.84
10° C 7.76
20° C 14.95
30° C 27.69
 
 

Note that as the air becomes warmer, it can hold much more moisture. This has led to the development of the Rule of Thumb that ‘For Every 10° C Increase in Temperature, Air Can Hold Twice As Much Moisture.’

How close does the Rule of Thumb fit reality? Here is the table above with doubled values in the third column. It is based on the 10° C value as the standard.

 

Temp ° C

g/km

Doubling

-10° C 1.79  1.94
0° C 3.84 3.88
10° C 7.76 7.76
20° C 14.95 15.52
30° C 27.69 31.05
 
 

The Rule of Thumb is not perfect, but it serves very well to work through problems to gain an understanding of how measures of moisture vary throughout the day as temperatures rise and fall. It is much easier to use the Rule of Thumb than to consult a table of values to see basic relationships.

So, use the Rule of Thumb to get a foundation in the relationship of the various measures of humidity, but do not adopt the Rule of Thumb as fact, or as reality. If and when you need precise numbers, look for better values.

The Rule-of-Thumb is that the amount of moisture the air can hold doubles with every 10°C increase in temperature. Remember, a 10°C change in temperature is equivalent to an 18°F change in temperature.  So, the Rule can be stated as  the amount of moisture the air can hold doubles with every 18°F increase in temperature.

In summary form

 
 

Air at

 

can hold

-10° C

or 14° F

1 unit of humidity

0° C

or 32° F

2 units of humidity

10° C

or 50° F

4 units of humidity

20° C

or 68° F

8 units of humidity

30° C

or 86° F

16 units of humidity

 
 

This range of temperature is common in the middle latitudes. Warm air can hold far more moisture than cold air. Of course the air is seldom filled to capacity. But, even if the air is at only 50% of capacity, the warm air will contain far more moisture than the cold air.

There are many measures of humidity. See chapter 5, pages 109-121 for a good discussion of these measures.

One measure is vapor pressure, expressed in pressure terms such as millibars, or mb. This is a measure of the atmospheric pressure attributable to the presence of the water vapor.

Another measure is the mixing ratio expressed in grams / kilograms. Very similar is specific humidity expressed in the same units. The difference is the mixing ratio is the weight of water vapor to the weight of dry air, while specific humidity is the weight of water vapor to the weight of the air (with the moisture in it).

We find it useful to compare how much moisture is in the air compared to how much moisture the air can hold. Remember, the amount of moisture the air can hold is a function of the air temperature.

So, we go to a table or a graph and determine how much moisture the air can hold at a particular temperature. We express the capacity of the air to hold moisture in the same units as above but express the terms as: saturation vapor pressure, saturation mixing ratio, and saturation specific humidity.  When the air is found to be at 100% of its capacity, we say the air is saturated.  Thus, the terms saturation vapor pressure, etc.

Then we compare the moisture present to the capacity of the air to hold moisture and get the measure relative humidity, expressed in %.  We multiply by 100 to get a percentage figure.  Thus, (15/30) = 0.5, but (15/30) X 100 = 50%.

Relative Humidity = (moisture present / capacity of air to hold moisture) X 100

        = (vapor pressure / sat vapor pressure) X 100

        = (mixing ratio / sat mixing ratio) X 100

        = (spec humidity / sat spec humidity) X 100

When the Relative Humidity is 100% the air is saturated--the moisture present is at the capacity of the air to hold moisture at that temperature.  

Now, let’s put these pieces together to see how the atmosphere behaves.

We observe that at sunrise, the air is at 0°C and the relative humidity is 60%. As the day warms the temperature rises to 10°C. Nothing adds or takes moisture from the air. So the amount of moisture present stays the same. But, the capacity of the air to hold moisture will change as the temperature rises. If it goes up 10°C the capacity of the air to hold moisture doubles.

        RH = present / capacity

so if capacity doubles the RH is cut in half. Thus, when the air gets to 10°C the RH falls to 30%. If during the afternoon the temperature goes up another 10 degrees to 20°C the capacity doubles again and the RH falls to 15%

Assuming no moisture is added or subtracted from the air, what happens to RH when the temperature falls in the afternoon and into evening? At 20°C the RH was 15%. When the temperature falls to 10°C, the RH rises to 30%. If and when it falls to 0°C the RH would be 60%. If it continues to decline at some temperature the RH would be 100% and the air would be saturated.

Now, go to the self tests and work through some humidity problems.