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Facts of the Matter
Richard Brill
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Measuring humidity is oftentimes relative
WHILE SITTING in my kayak in Kaneohe Bay in the dwindling afternoon light last week, I became aware once again of the importance of water on planet Earth.
Toward the cliffs of the Koolaus, the flat ocean and the flat bottoms of the clouds appeared to converge. The island seemed so small compared to the clouds and ocean as if to stress the minuscule size of the land.
We usually think only of the ocean when we think of Earth as the water planet, but temperature, humidity and clouds are equally important in the water cycle.
Humidity is the invisible part of the water cycle that is the transition between water and clouds.
Earth's surface maintains the perfect combination of temperature and pressure to allow water to exist in all three states of matter: solid, liquid, and gaseous phases, but by far the most influential interaction is between the liquid and gaseous phases.
Earth is in a nearly circular orbit that keeps the amount of solar radiation constant throughout the year, allowing it to maintain a constant average temperature.
Although atmospheric pressure fluctuates slightly, the change amounts to only about 2 percent, even in the most intense storm.
STAR-BULLETIN
The bottoms of the clouds represent the level where the rising air reaches its dewpoint.
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The nearly constant one atmosphere of pressure keeps the balance between evaporation and condensation of water at a reasonable balance.
Often overlooked is the amount of water vapor in the air, which affects the evaporation rate of water, which in turn depends upon the temperature and pressure of the air.
There are several different ways to visualize humidity. Weather reports use relative humidity, which is the amount of water vapor in the air compared to how much it can hold at saturation.
Relative humidity does not give a good picture of the amount of water vapor because it is relative to saturation and does not give a true picture of the amount of water vapor in the air.
A better picture is to compare the actual amount of water vapor at a given temperature compared to the temperature at which the air becomes saturated with water vapor.
This requires that we assume that all molecules are in a constant state of motion and move faster when they are warmer. Some molecules are moving faster than others in a Rayleigh distribution, which is somewhat like the bell curve of a standard distribution but skewed toward the lower end.
Visualize a closed container that is half full of water. The other half contains completely dry air.
At any temperature some water molecules will be moving fast enough to escape from the water's surface to become free molecules of water vapor.
As water evaporates into the air, the pressure inside the container increases slightly due to the extra water molecules that were added to the air through evaporation.
The increase in pressure is called the vapor pressure of water.
The more water molecules escape from the surface of the water the increasing water vapor pressure forces more water molecules back into the liquid.
Eventually the number of molecules returning to the surface of the water will equal the number of molecules that are leaving the surface.
At that point the air is said to be saturated and the pressure exerted by the water vapor is the saturation vapor pressure.
If we increase the temperature of the water in the container, more water would evaporate before reaching a balance between water leaving and returning to the surface.
At the higher temperature more water molecules have enough energy to escape the surface and so the saturation vapor pressure is higher.
The actual vapor pressure of water compared to saturation vapor pressure at any given temperature is the relative humidity, the amount of water vapor in the air compared to the amount that the air can hold at a given temperature.
Relative humidity is used in weather reports because it is easy to measure. Other methods of stating the humidity are less than satisfactory for weather reports but more useful in understanding the processes of evaporation and condensation involved in cloud formation.
One such measure is the mixing ratio. It is the mass of water vapor compared to the mass of dry air, expressed as grams per kilogram.
The mixing ratio is not used in weather reports because it is difficult to measure the absolute amount of water vapor in a given mass of air, and it does not directly signify whether evaporation or condensation will take place at a given temperature.
A more useful way to report humidity is relative humidity, which measures how near the water is to saturation at any given temperature. From the relative humidity we can calculate the mixing ratio and vice-versa.
It is a combination of the two measures, and laboratory data that allow us to understand and to calculate effects due to evaporation and condensation.
Relative humidity can be misleading, however, and lead us to false conclusions. For example compare a typical January day in Winnipeg, Canada, to one in the desert in Arizona.
In Winnipeg, the temperate is 15 degrees Fahrenheit with 100 percent relative humidity, corresponding to a mixing ratio of 2 grams per kilogram.
In Phoenix on this same day the temperature is 75 degrees with a relative humidity of just 20 percent. At that temperature the saturation mixing ratio is 20 grams per kilogram.
So at 20 relative humidity the air in Phoenix contains 4 grams of water vapor per kilogram (0.2 x 20).
At 20 percent relative humidity, the air in Phoenix contains twice as much water vapor as the saturated air in Winnipeg, yet we would say that the Arizona air is dry and the Winnipeg air is damp.
Using relative humidity gives a better sense of how "muggy" the air feels because the lower the RH the more easily water evaporates.
Another important concept related to relative humidity is dew point, which is the temperature at which a given parcel of air would reach saturation if it were cooled.
The higher the relative humidity, the closer to ambient temperature the dew point will be.
For example, say the temperature in Honolulu is 89 degrees with 70 percent RH, corresponding to a mixing ratio of 19 grams of water vapor per kilogram of dry air, making the dew point about 77 degrees.
Under those conditions air would have to be cooled to 77 degrees to reach saturation, so why would clouds form with flat bottoms?
The atmosphere gets colder as elevation increases. As wind lifts the air up and over the Koolaus it cools about 5.5 degrees for every 1,000 feet of uplift.
For air to reach saturation and clouds to form it would have to be cooled from 89 degrees to 77 degrees, which corresponds to an uplift of about 2,200 feet.
So the tops of the Koolaus are covered with clouds and the flat bottoms of the clouds represent that level of around 2,200 feet where the rising air reached its dew point.
Richard Brill picks up where your high school science teacher left off. He is a professor of science at Honolulu Community College, where he teaches earth and physical science and investigates life and the universe. He can be reached by e-mail at
rickb@hcc.hawaii.edu.