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Science Corner: Humidity is relative

RAIN FORESTS, like Cloud Forest on Mt. Kinabalu in Borneo, often have relatively high humidity.

Wikimedia Commons
RAIN FORESTS, like Cloud Forest on Mt. Kinabalu in Borneo, often have relatively high humidity.

When our hot summer temperatures arrive, I think back to time spent in the Midwest. Sun or shade, breeze or not, high temperatures just felt more uncomfortable there than here in the gorge.

We’ve all heard that it’s not the heat, it’s the humidity. That simple statement hides a lot of complex information about water in the atmosphere and its many effects.

Our planet’s atmosphere consists mostly of nitrogen, oxygen and small amounts of other gases. Depending on where you happen to be, it also contains anywhere from a trace to about 4 percent water vapor, the gas phase of H2O.

Water occurs in nature as a gas, liquid and solid. As a gas, water vapor is invisible, odorless and a critical part of our weather and climate. Clouds, fog, rain and other precipitation don’t count as water vapor since they’re all forms of liquid water.

The atmosphere contains only a tiny fraction of the planet’s moisture, but that tiny fraction adds up to over 3,000 cubic miles of water. That sounds like a lot, but if it all rained down at one time only about an inch of water would cover the Earth’s surface.

That single inch represents a key part of the planet’s water cycle. Water constantly enters the atmosphere by evaporation of surface water and soil moisture, as well as from the leaves of plants (transpiration). It stays in the air, moving in air masses around the world, until falling back down as rain or snow to start the cycle again.

Under the right conditions, water vapor condenses into liquid to form clouds and precipitation. And whenever water changes between a gas, liquid and solid, it releases or absorbs energy that helps power weather and storm systems. Water vapor is also the major natural greenhouse gas, accounting for almost 90 percent of the warming effect that makes our planet habitable.

The amount of water vapor in the atmosphere varies around the world, depending on the amount of moisture available (think desert or swamp). Geography, the season, weather patterns, air pressure and especially temperature all affect the air’s water content, better known as the humidity. Weather forecasts and complaints about humidity typically refer to the relative humidity.

Relative humidity is the measure of how close air is to being saturated, or containing all the water vapor it can hold. The laws of physics rule that air at a given pressure and temperature can only hold so much water vapor before it starts condensing out as a liquid like dew (100 percent saturated). For example, if the measured amount of water vapor present is half the amount the air could hold at total saturation, the relative humidity is 50 percent.

Earlier this summer when the temperature hit 100° Fahrenheit in The Dalles, the relative humidity was just 12 percent. That morning, when the temperature was 60 degrees F, the relative humidity was nearly 65 percent even though the actual amount of water vapor in the air hadn’t significantly changed. The relative humidity changes because hot air can hold more water vapor than cool air (laws of physics again!), so the hot air was much less saturated than the cool air.

Roughly speaking, relative humidity is cut in half for every 20-degree F rise in temperature, and doubles for every 20-degree drop. This explains why humidity is usually highest first thing in the morning and drops steadily as the day heats up. This matters for a couple of reasons.

The closer air gets to becoming saturated, or 100 percent humidity, the slower water evaporates into the atmosphere. Since our sweat cools us by evaporating, higher humidity means less effective cooling. We feel even hotter than the temperature would indicate.

Low humidity and faster evaporation makes high temperatures feel more comfortable, but there is a trade-off. Forest fuels like grass and wood also lose more moisture into the air and dry out faster, resulting in lower ignition temperatures and hotter-burning wild fires.

Obviously, the water cycle, humidity and weather are much more complicated than this. But when you can sit in the shade and be comfortable on a summer day in Eastern Oregon, be thankful knowing that it’s more the heat and less the humidity you have to deal with.

Lifelong Oregonian Fred Schubert, a The Dalles biologist, has a lifelong interest in general science and science writing. Feel free to submit any comments on this article or suggestions for new topics to fcscience


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