The U.S. Geological Survey estimates that the Earth has about 3,100 cubic miles of water in the air, mostly as water vapor, but also as clouds or precipitation, at any one time.

Humidity is the condition of the atmosphere in relation to the water vapor it contains and is a fairly complicated subject to deal with fully, but a few brief notes will help you to understand enough of the subject for our purpose.

Water vapor is always present in the air in varying amounts, the amount that the air can hold depending on its temperature, the higher the temperature the more water vapor it can hold. The dew-point is the temperature at which air containing a certain amount of water vapor becomes saturated; any further reduction in temperature would result in condensation.

What relative humidity means

The warmer air is, the more water vapor it can "hold." Dew point is a measure of how much water vapor is actually in the air. Relative humidity is a measure of the amount of water in the air compared with the amount of water the air can hold at the temperature it happens to be when you measure it. To see how this works, let's use the chart below, which is adapted from Meteorology Today by C. Donald Ahrens, published by West Publishing.

Air temperature in degrees C   Water vapor air can hold
                               at this temperature.
30 degrees                     30 grams per cubic meter of air 
20 degrees                     17 grams per cubic meter of air 
10 degrees                      9 grams per cubic meter of air

These numbers, which apply to air at sea level pressure, are based on measurements over the years. They are basic physical facts.

Now, let's see how dew point and relative humidity work. Imagine, that at 3 p.m. you measure the air's temperature at 30 degrees and you measure its humidity at 9 grams per cubic meter of air. What would happen if this air cooled to 10 degrees with no water vapor being added or taken away? As it cools to 10 degrees the air becomes saturated; that is, it can't hold any more water vapor than 9 grams per cubic meter. Cool the air even a tiny bit more and its water vapor will begin condensing to form clouds, fog or dew - depending on whether the air is high above the ground, just above the ground, or right at the ground. Back at 3 p.m., when we made the measurements, we could say that the air's dew point is 10 degrees C. That is, if this particular air were cooled to 10 degrees at ground level, its humidity would begin condensing to form dew.

How about relative humidity? At 3 p.m. the air has 9 grams of water vapor per cubic meter of air. We divide 9 by 30 and multiply by 100 to get a relative humidity of 30% In other words, the air actually has 30% of the water vapor it could hold at its current temperature. Cool the air to 20 degrees. Now we divide 9, the vapor actually in the air, by 17, the vapor it could hold at its new temperature, and multiply by 100 to get a relative humidity of 53% (rounded off). Finally, when the air cools to 10 degrees, we divide 9 by 9 and multiply by 100 to get a relative humidity of 100% - the air now has all the vapor it can hold at its new temperature.

Why humidity can be less than 100% when it's raining

Humidity is a measure of the amount of water vapor in the air, not the total amount of vapor and liquid. For clouds to form, and rain to start, the air does have to reach 100% relative humidity, but only where the clouds are forming or where the rain is coming from. This normally happens when the air rises and cools. Often, rain will be falling from clouds where the humidity is 100% into air with a lower humidity. Some water from the rain evaporates into the air it's falling through, increasing the humidity, but usually not enough to bring the humidity up to 100%.

Humidity and air density

Most people who haven't studied physics or chemistry find it hard to believe that humid air is lighter, or less dense, than dry air. How can the air become lighter if we add water vapor to it?

Scientists have known this for a long time. The first was Isaac Newton, who stated that humid air is less dense than dry air in 1717 in his book, Optics. But, other scientists didn't generally understand this until later in that century.

To see why humid air is less dense than dry air, we need to turn to one of the laws of nature the Italian physicist Amadeo Avogadro discovered in the early 1800s. In simple terms, he found that a fixed volume of gas, say one cubic meter, at the same temperature and pressure, would always have the same number of molecules no matter what gas is in the container. Most beginning chemistry books explain how this works.

Imagine a cubic foot of perfectly dry air. It contains about 78% nitrogen molecules, which each have an atomic weight of 28. Another 21% of the air is oxygen, with each molecule having an atomic weight of 32. The final one percent is a mixture of other gases, which we won't worry about. Molecules are free to move in and out of our cubic foot of air. What Avogadro discovered leads us to conclude that if we added water vapor molecules to our cubic foot of air, some of the nitrogen and oxygen molecules would leave — remember, the total number of molecules in our cubic foot of air stays the same. The water molecules that replace nitrogen or oxygen have an atomic weight of 18. This is lighter than both nitrogen and oxygen. In other words, replacing nitrogen and oxygen with water vapor decreases the weight of the air in the cubic foot; that is, it's density decreases.

Wait a minute, you might say, "I know water's heavier than air." True, liquid water is heavier, or more dense, than air. But, the water that makes the air humid isn't liquid. It's water vapor, which is a gas that is lighter than nitrogen or oxygen.

Compared to the differences made by temperature and air pressure, humidity has a small effect on the air's density. But, humid air is lighter than dry air at the same temperature and pressure.

The Role of Humidity

Because of the temperature gradient, humidity decreases rapidly with altitude. The extremely low humidity at 12,000m is responsible for the uncomfortably low humidity in commercial aircraft. The ground-level outdoor humidity is constantly changing. During the day, water vaporizes from forests, fields and lawns at about 1mm/day, about the same rate as from lakes. As moist air rises to cooler air levels, clouds are formed. Also, there is always a humidity gradient between sunny and shadowy spots on the ground. Even a slight wind is effective in transferring moisture to a cooler spot, where condensation can occur. Traces of atmospheric dust or other matter, such as the leaves of some plant species, are capable of inducing from saturated air condensation that then drips to the ground and waters the roots.

During the diurnal cycle, the water content of air increases while the sun shines. During the night the temperature drops and dew forms and recycles moisture to the soil. In coastal areas, the humidity can approach 100% at night on a regular basis.

The indoor climate differs in several fundamental ways from the outdoor climate, because air inside of buildings is confined in a comparatively small volume. In fact, it is often insufficient to maintain human moisture below a noticeable level. As a result, the quality of indoor air undergoes tremendous variations in a short time. This is reflected in indoor humidity trends. The moisture content of a closed room increases rapidly because each occupant continuously adds moisture to the air in the form of perspiration and with every breath day and night at a rate of several liters of water per day. This water readily condenses on cold walls and windows, but since there is no soil or surface to absorb it. Thus, if buildings are not carefully ventilated, the indoor habitat becomes saturated with moisture, causing not only air quality problems, but eventually structural damage.

Relative Humidity

The amount of water vapor in the air at any given time is usually less than that required to saturate the air. The relative humidity is the percent of saturation humidity, generally calculated in relation to saturated vapor density.

Saturated Vapor Pressure, Density for Water

-10	14	2.15	2.36	37	98.6	47.07	44
0	32	4.58	4.85	40	104	55.3	51.1
5	41	6.54	6.8	60	140	149.4	130.5
10	50	9.21	9.4	80	176	355.1	293.8
11	51.8	9.84	10.01	95	203	634	505
12	53.6	10.52	10.66	96	205	658	523
13	55.4	11.23	11.35	97	207	682	541
14	57.2	11.99	12.07	98	208	707	560
15	59	12.79	12.83	99	210	733	579
20	68	17.54	17.3	100	212	760	598
25	77	23.76	23	101	214	788	618
30	86	31.8	30.4	200	392	11659	7840

Saturated Vapor Pressure for Water

Saturated Vapor Density for Water

Boiling Point

The boiling point is defined as the temperature at which the saturated vapor pressure of a liquid is equal to the surrounding atmospheric pressure. For water, the vapor pressure reaches the standard sea level atmospheric pressure of 760 mmHg at 100C. Since the vapor pressure increases with temperature, it follows that for pressure greater than 760 mmHg (e.g., in a pressure cooker), the boiling point is above 100C and for pressure less than 760 mmHg (e.g., at altitudes above sea level), the boiling point will be lower than 100C. As long as a vessel of water is boiling at 760 mmHg, it will remain at 100C until the phase change is complete. Rapidly boiling water is not at a higher temperature than slowly boiling water. The stability of the boiling point makes it a convenient calibration temperature for temperature scales.

At the boiling point, saturated vapor pressure equals atmospheric pressure.

Dewpoint

Definitions:

Absolute humidity: The mass of water vapor in a given volume of air( i.e., density of water vapor in a given parcel, usually expressed in grams per cubic meter

Actual vapor pressure: The partial pressure exerted by the water vapor present in a parcel. Water in a gaseous state (i.e. water vapor) exerts a pressure just like the atmospheric air. Vapor pressure is also measured in millibars.

Condensation: The phase change of a gas to a liquid. In the atmosphere, the change of water vapor to liquid water.

Dewpoint: the temperature air would have to be cooled to in order for saturation to occur. The dewpoint temperature assumes there is no change in air pressure or moisture content of the air.

Dry bulb temperature: The actual air temperature. See wet bulb temperature below.

Freezing: The phase change of liquid water into ice.

Evaporation: The phase change of liquid water into water vapor.

Melting: The phase change of ice into liquid water.

Mixing ratio: The mass of water vapor in a parcel divided by the mass of the dry air in the parcel (not including water vapor)

Relative humidity: The amount of water vapor actually in the air divided by the amount of water vapor the air can hold. Relative humidity is expressed as a percentage and can be computed in a variety of ways. One way is to divide the actual vapor pressure by the saturation vapor pressure and then multiply by 100 to convert to a percent.

Saturation of air: The condition under which the amount of water vapor in the air is the maximum possible at the existing temperature and pressure. Condensation or sublimation will begin if the temperature falls or water vapor is added to the air.

Saturation vapor pressure: The maximum partial pressure that water vapor molecules would exert if the air were saturated with vapor at a given temperature. Saturation vapor pressure is directly proportional to the temperature.

Specific humidity: The mass of water vapor in a parcel divided by the total mass of the air in the parcel (including water vapor)

Sublimation: In U.S. meteorology, the phase change of water vapor in the air directly into ice or the chance of ice directly into water vapor. Chemists, and sometimes meteorologists, refer to the vapor to solid phase change as "deposition."

Wet bulb temperature: The lowest temperature that can be obtained by evaporating water into the air at constant pressure. The name comes from the technique of putting a wet cloth over the bulb of a mercury thermometer and then blowing air over the cloth until the water evaporates. Since evaporation takes up heat, the thermometer will cool to a lower temperature than a thermometer with a dry bulb at the same time and place. Wet bulb temperatures can be used along with the dry bulb temperature to calculate dew point or relative humidity.

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