As I sat in my kayak in Maunalua Bay in the muggy air and dwindling afternoon light last week, my thoughts focused on humidity.

Humidity is the invisible part of the water cycle that is the transition between surface water and clouds.

Earth’s surface maintains the perfect range of temperatures and pressures to allow water to exist in all three states of matter: solid, liquid and gas. Arguably, the most influential interaction is between the liquid and gaseous phases.

Air can dissolve water molecules, much as water can dissolve salt ions. Like salt in water, the warmer the air, the more water molecules it can dissolve and hold in solution.

Water vapor is an important constituent of the atmosphere although it rarely exceeds 3 percent of the total. Because it is so variable, it is not typically included in accounts of atmospheric composition.

Air at any temperature is saturated when it has all the water molecules it can keep in solution at that temperature.

Cool air can hold fewer water molecules in solution than warm air.

Weather reports use percent relative humidity (RH), which is the amount of water vapor in the air compared with how much it can hold at saturation. RH does not give a true picture of the actual amount of water vapor in the air because it is relative to saturation, which depends on temperature.

Weather reports use RH because it is easy to measure and it relates to how muggy the air feels and how quickly water will evaporate (e.g., dissolve) into air.

Other methods of stating humidity are less than satisfactory for weather reports but more useful in understanding the processes of evaporation and condensation involved in cloud formation and weather.

One such measure is the mixing ratio. It is the actual mass of water vapor compared with the mass of dry air, expressed as grams per kilogram.

Weather reports do not use the mixing ratio because it is difficult to measure and it does not signify whether evaporation or condensation will take place at a given temperature.

On one hand, RH is far more useful, and we can calculate the mixing ratio from RH.

On the other hand, RH can be misleading and lead us to false conclusions.

For example, compare a January day in Seattle to one in Phoenix.

In Seattle the temperature is 40 degrees Fahrenheit with 100 percent RH, which equals a mixing ratio of 5 grams per kilogram. In Phoenix the temperature is 75 degrees with an RH of only 40 percent, which equals a mixing ratio of 8 grams per kilogram.

The “dry” air in Phoenix contains 50 percent more water vapor than the “damp,” saturated air in Seattle.

Using RH gives a better sense of how “muggy” the air feels because water evaporates more readily the lower the RH, being further from saturation.

Another important concept related to RH is dew point, which is the temperature at which the air would be saturated or reach 100 percent RH, at which point water would condense.

The higher the RH, the closer to ambient temperature the dew point will be and the more “sweat” will form on that cold beverage can.

Richard Brill is a professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.