Humidity
Humidity is the measurement of water vapor present in the air, significantly influencing how individuals perceive weather conditions, regardless of actual temperature. High humidity levels often lead to discomfort, as the body relies on the evaporation of sweat for cooling. Locations near large bodies of water tend to experience more muggy conditions compared to inland areas, even at the same temperature. Humidity is typically classified into three types: absolute humidity, specific humidity, and relative humidity, with the latter being the most common in weather reports. Relative humidity indicates the current water vapor in the air relative to the maximum it can hold at a specific temperature, measured as a percentage. Health-wise, elevated humidity can pose risks, particularly for vulnerable groups such as the elderly, young children, and those with pre-existing respiratory conditions. Additionally, humidity is a vital factor in climate science, influencing weather forecasting and models of global climate change due to its role as a greenhouse gas. The relationship between humidity and temperature is complex, as increases in temperature can lead to higher specific humidity, potentially exacerbating the greenhouse effect. Understanding humidity's effects is crucial for both daily weather and broader environmental discussions.
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Humidity
Humidity is a measure of how much water vapor is in the air. Because the body reacts physically to the presence of water vapor, humidity plays a critical role in how people actually experience the weather, whether the air temperature itself is high or low. It is why locations surrounded by large bodies of water feel warmer and muggier on hot days than inland locations or sites that border desert areas, even if the air temperature is the same.
High humidity can impact the health of both young and old, healthy and infirm. In recent years, humidity has also become a hot-button issue, a critical component in the ongoing debate over global warming and its true magnitude.
Background
According to the United States Geological Survey, Earth has an estimated 332.5 million cubic miles (1.386 billion cubic kilometers) of water, not just in surface bodies of water but also in the form of groundwater, atmospheric water, and the water in living organisms. The atmosphere contains an average of about 3,100 cubic miles (12,900 cubic kilometers) of water at any one time, mostly in the form of water vapor. Without atmospheric water, Earth would be barren of life and absent any sort of weather, much like the planet Mars is now.
Humidity is typically measured in one of three ways: as absolute humidity, specific humidity, or relative humidity. Absolute humidity is a measure of the density of water vapor in the air, calculated in terms of the total mass of water vapor that is present in a given volume of air. Specific humidity is the ratio of the mass of water vapor to the total mass of the moist air that contains it. Because they do not take temperature into account, absolute humidity and specific humidity are more academic measures and are seldom used in everyday weather forecasts.
The measure of humidity most people are familiar with is relative humidity, which is the ratio of the amount of water vapor currently in the air to the amount of water vapor that would be required to saturate the air at the current temperature. This ratio is typically expressed as a percentage; a relative humidity of 80 percent means that the air contains 80 percent of the water vapor that it would contain if it were saturated. When air is saturated, it cannot absorb any more water vapor. This is why people feel hotter and sweatier when the relative humidity is high. Sweating is how the human body regulates its temperature, as the evaporation of perspiration from the skin creates a cooling effect. However, when the air is already heavy with moisture, sweat does not evaporate but instead stays on the skin.
A familiar metric related to humidity and air saturation is the dew point. Dew forms on the ground when the relative humidity is 100 percent—that is, when the air is completely saturated—causing water vapor to condense (transition from gas to liquid) at the same rate that liquid water evaporates (transitions from liquid to gas). The dew point is the temperature at which this will occur, assuming that barometric pressure and absolute humidity remain constant.
When humidity is high, precautions are encouraged. Elderly people and those with breathing conditions should stay inside with a fan or, ideally, an air conditioner, which will siphon off the water vapor. Children under five should also remain indoors as much as possible, as developing respiratory systems can be taxed by high humidity. People between these age extremes may also be vulnerable, especially those who work outdoors or who regularly exercise outside, if they do not compensate for the increased humidity. Without aggressively increasing fluid intake and taking breaks to allow their bodies to cool, people who exert themselves in high humidity are susceptible to a variety of health hazards, including fainting, heat exhaustion or heatstroke, and in extreme cases even death.
Humidity Today
Humidity is a standard element of both weather forecasting and public health management. It is also an important factor in scientific models of global climate change, although the relationship between humidity and temperature on a global scale—specifically, how long-term, large-scale changes in temperature might affect humidity, and vice versa—is not completely understood. Water vapor is a greenhouse gas, meaning that it absorbs thermal infrared radiation emitted from Earth’s surface; in fact, it is the most abundant greenhouse gas in Earth’s atmosphere. Scientists generally agree that an increase in the amount of water vapor in the upper troposphere (the lowest layer of Earth’s atmosphere) would result in a significant warming effect. However, this scenario is complicated by the fact that water vapor has much higher boiling and freezing points than other greenhouse gases, and the temperature of Earth’s atmosphere is always below its boiling point and often below its freezing point as well. Thus, water vapor does not remain in the atmosphere but rather is constantly precipitating as a liquid (rain) or solid (snow, ice) and then evaporating again, which makes modeling its effects in a nonequilibrium system such as the atmosphere even more difficult.
A common assumption of scientists modeling climate change has been that as global temperatures increase, relative humidity in the upper troposphere will remain more or less constant, which by extension means that specific humidity will rise. This is because as air temperature increases, so too does the amount of water vapor it can hold. If two locations have the same relative humidity, but one has an air temperature of 60 degrees and the other is 90 degrees, the hotter location will have a higher specific humidity, because the same mass of air will contain more water vapor than in the cooler location. According to models based on constant relative humidity, as the air becomes hotter, more water will evaporate from Earth’s surface and remain in the upper troposphere, where it will increase the greenhouse effect and trap more heat, which will in turn cause more water to evaporate, and so on. This mechanism is known as water-vapor feedback. When it leads to increased humidity, the feedback is said to be positive.
A study published in 2004 by Ken Minschwaner and Andrew Dessler, both researchers at the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center, called into question the assumption that relative humidity will stay the same as Earth warms. The study was the first to be based on direct observations of water vapor in the upper troposphere; the resulting model predicted that relative humidity in this region of the atmosphere will actually decrease. Some who argue against the existence of anthropogenic (human-caused) climate change have cited this as evidence that the water-vapor feedback mechanism is negative rather than positive—that is, that it may cause the amount of water vapor in the upper troposphere to decrease rather than increase. However, such an interpretation is based on a misunderstanding of relative humidity. Minschwaner and Dessler’s model predicts that the specific humidity will still rise, just not enough to maintain constant relative humidity. While this would still result in a global temperature increase, it does mean that the increase may take longer than previous models predicted.
Bibliography
Cullen, Heidi. The Weather of the Future: Heat Waves, Extreme Storms, and Other Scenes from a Climate-Changed Planet. New York: Harper, 2010. Print.
Flannery, Tim. The Weather Makers: How Man Is Changing the Climate and What It Means for Life on Earth. New York: Grove, 2005. Print.
"How Much Water Is There on, in, and above the Earth?" The USGS Water Science School. US Geological Survey, 2 May 2016. Web. 14 Aug. 2016.
"Humidity." National Geographic, 19 Oct. 2023, education.nationalgeographic.org/resource/humidity/. Accessed 21 Nov. 2024.
Kusaka, Hiroyuki. "Humidity." Encyclopedia of Geography. Ed. Barney Warf. Vol. 3. Thousand Oaks: Sage, 2010. 1485. Print.
McKibben, Bill, ed. The Global Warming Reader: A Century of Writing about Climate Change. 2011. New York: Penguin, 2012. Print.
Minschwaner, Ken, and Andrew E. Dessler. "Water Vapor Feedback in the Tropical Upper Troposphere: Model Results and Observations." Journal of Climate 17.6 (2004): 1272–82. Web. 8 Sept. 2016.
Riebeek, Holli. "Will Runaway Water Warm the World?" Earth Observatory. NASA, 15 Mar. 2004. Web. 8 Sept. 2016.
"Understanding Humidity." USA Today. Gannett, 2011. Web. 14 Aug. 2016.
Woods, Catherine. "8 Things You Didn’t Know about Humidity." PBS NewsHour. NewsHour Productions, 31 July 2015. Web. 14 Aug. 2016.