Water Cycle Processes PDF

Water is found everywhere on Earth---in the oceans, glaciers, rivers, lakes, air, soil, and living tissue. The vast majority of the water on or close to Earth\'s surface (over 97 percent) is saltwater found in the oceans. Much of the remaining 3 percent is stored in the ice sheets of Antarctica and...

Water is found everywhere on Earth---in the oceans, glaciers, rivers, lakes, air, soil, and living tissue. The vast majority of the water on or close to Earth\'s surface (over 97 percent) is saltwater found in the oceans. Much of the remaining 3 percent is stored in the ice sheets of Antarctica and Greenland. Only a meager 0.001 percent is found in the atmosphere, and most of this is in the form of water vapor.The continuous exchange of water among the oceans, the atmosphere, and the continents is called the hydrologic cycle (Figure 4.1). Water from the oceans and, to a lesser extent, from land areas evaporates into the atmosphere. Winds transport this moisture-laden air, often over great distances, until the process of cloud formation causes the water vapor to condense into tiny liquid cloud droplets. The process of cloud formation may result in precipita-tion. The precipitation that falls into the ocean has ended its cycle and is ready to begin another. The remaining precipitation falls over the land. A portion of the water that falls on land soaks into the ground (infiltration) to become ground-water. The remainder, which flows along Earth\'s surface, is called runoff.Much of the groundwater and runoff eventually returns Water is found everywhere on Earth---in the oceans, glaciers, rivers, lakes, air, soil, and living tissue. The vast majority of the water on or close to Earth\'s surface (over 97 percent) is saltwater found in the oceans. Much of the remaining 3 percent is stored in the ice sheets of Antarctica and Greenland. Only a meager 0.001 percent is found in the atmosphere, and most of this is in the form of water vapor. to the atmosphere through evaporation. A smaller amount of groundwater is taken up by plants, which release it into the atmosphere through a process called transpiration (or evapotranspiration). The total amount of water vapor in the atmosphere remains fairly constant. Therefore, the average annual precipitation over Earth must be roughly equal to the quantity of water lost through evaporation. However, over the continents, precipitation exceeds evaporation. Evidence for the Movement of Water Through the Atmosphere The continuous exchange of water among the oceans, the atmosphere, and the continents is called the hydrologic cycle (Figure 4.1). Water from the oceans and, to a lesser extent, from land areas evaporates into the atmosphere. Winds transport this moisture-laden air, often over great distances, until the process of cloud formation causes the water vapor to condense into tiny liquid cloud droplets. The process of cloud formation may result in precipita-tion. The precipitation that falls into the ocean has ended its cycle and is ready to begin another. The remaining precipitation falls over the land. A portion of the water that falls on land soaks into the ground (infiltration) to become ground-water. The remainder, which flows along Earth\'s surface, is called runoff. The hydrologic cycle is the roughly balanced hydrologic movement of water between cycle is found in the fact that the oceans, the atmosphere, the level of the world\'s oceans and the continents. is not dropping. This continuous movement of water through the hydrologic cycle holds the key to the distribution of moisture over the surface of our planet and is intricately related to all atmospheric phenomena.Water has unique properties that set it apart from most other substances. For instance, (1) water is the only liquid found at Earth\'s surface in large quantities; (2) water is readily con- verted from one state of matter to another (solid, liquid, gas); (3) water\'s solid phase, ice, is less dense than liquid water; and (4) water has a high heat capacity-meaning changing its temperature requires considerable energy. All these properties influence Earth\'s weather and climate and are favorable to life as we know it. Precipitation These unique properties are largely a result of water\'s ability to form hydrogen bonds. Hydrogen bonds are the attractive forces that exist between hydrogen atoms in one water molecule and oxygen atoms of any other water molecule. To better grasp the nature of hydrogen bonds, let\'s examine a water mol-ecule. Water molecules (HO) consist of two hydrogen atoms that are strongly bonded to an oxygen atom (Figure 4.2A). Because oxygen atoms have a greater affinity for the bonding electrons (negatively charged subatomic par- ticles) than do hydrogen atoms, the oxygen end of a water molecule acquires a partial negative charge. For the same reason, the hydrogen atoms of a water molecule acquire a partial Video Hydrologic Cycle positive charge. Because oppositely charged particles attract, a hydrogen atom on one water molecule is attracted to an oxygen atom on another water moleculeHydrogen bonds hold water molecules together to form the solid we call ice. In ice, hydrogen bonds produce a rigid hexagonal network. The resulting molecular configurationWater is the only substance that naturally exists on Earth as a solid (ice), liquid, and gas (water vapor). Because all forms of water are composed of hydrogen and oxygen atoms that are bonded together to form water molecules (H,), the primary difference among water\'s three phases is the arrangement of these water moleculesis very open (lots of empty spaces). When ice is heated suffi-ciently, it melts. Melting causes some, but not all, of the hydrogen bonds to break. As a result, the water molecules in liquid water display a more compact arrangement and no rigid struc-ture. This explains why water in its liquid phase is denser than it is in the solid phase. Because ice is less dense than the liquid water beneath it, a water body freezes from the top down. This has far-reaching effects, both for our daily weather and aquatic life. When ice forms on a water body, it insulates the underlying liquid and slows the rate of freezing at depth. If water bodies froze from the bottom, many lakes would freeze solid during the winter, killing its aquatic life. Further, deep bodies of water, such as the Arctic Ocean, would never become ice covered. Such changes in freezing patterns would alter Barth\'s energy budget, which in turn would modify atmospheric and oceanic circulation patterns. Water\'s heat capacity is also related to hydrogen bond-ing. When water is heated, some of the energy is used to break hydrogen bonds rather than to increase molecular motion. (Recall that an increase in average molecular motion corresponds to an increase in temperature.) Thus, under similar conditions, water heats up and cools down more slowly than most other common substances. As a result, large water bod- ies tend to moderate tem- Water heats up and cools down peratures by remaining more slowly than most other warmer than adjacent common substances, and as a landmasses in winter and result, large water bodies tend to cooler in summer, as dis-moderate temperatures. cussed in Chapter 3.Ice is composed of water molecules that have low kinetic ener gies (motion) and are held together by mutual molecular attrac tions (hydrogen bonds). These molecules form a tight, orderl network and are not free to move relative to each other; rather they vibrate within their fixed sites. When ice is heated, the molecules oscillate more rapidly. When the rate of molecular move- ment increases sufficiently, the When water changes state, hydrogen bonds between some hydrogen bonds either form of the water molecules are bro- or are broken. ken, resulting in melting. In the liquid state, water molecules are still tightly packed but move fast enough to be able to easily slide past one another. As a result, liquid water is fluid and will take the shape of the container that holds it. When liquid water gains heat from the environment, some of the molecules acquire enough energy to break their hydrogen bonds and escape from the water surface to become water vapor. Water vapor molecules are widely spaced compared to liquid water and exhibit very energetic and random motion. These processes are reversed when water vapor returns to its liquid state and when water freezes. When water changes state, hydrogen bonds either form or are broken.Whenever water changes state, energy is exchanged between water and its surroundings (see Figure 4.3). For example, heat is required to evaporate water. The heat involved when water changes state is measured in units called calories. One calorie is the amount of energy required to raise the temperature of 1 gram of water 1°C (1.8°F). Thus, when 10 calories of heat are Video Global Evaporation Rates absorbed by 1 gram of water, the molecules vibrate faster, and a 10°C (18°F) temperature increase occurs. (In the International System of Units \[SIl, joules \[J\] are used to denote energy; 1 calorie = 4.184J） During a phase change, energy may be added to a substance without an accompanying rise in temperature. For example, when the ice in a glass of ice water melts, the temperature of the mixture remains a constant 0°C (32°F) until all the ice has melted. If the added energy does not raise the temperature of ice water, where does this energy go? In this case, the added energy breaks the hydrogen bonds that once bound the water molecules into a crystalline structure. Because the energy used to melt ice does not produce a temperature change, it is referred to as latent heat. (Latent means hidden, like the fingerprints hidden at a crime scene.) The energy, stored in liquid water, is released to its surroundings when the water freezes. In fact, your refrigerator runs more frequently when making ice cubes to counteract the additional energy released during the freezing process. Melting 1 gram of ice requires 80 calories (334 J), an amount termed the latent heat of melting. The reverse process, freezing, releases these 80 calories (334 J) per gram to the environment as the latent heat of fusion. We will consider the importance of latent heat of fusion in Chapter 5, in the section on frost prevention. Evaporation and Condensation Latent heat is also involved in evaporation, the process of converting a liquid to a gas (vapor). The energy absorbed by water molecules during evaporation oration is used to give them the motion needed to escape the surface of the liquid and become a gas. This energy is referred to as the latent heat of vaporization and varies from about 600 calories (2500 J) per gram for water at 0°C to 540 calories (2260 J) per gram at 100°C. (Notice in Figure 4.3 that it takes much more energy to evaporate 1 gram of water than it does to melt the same amount of ice.) During the evaporation process, the faster-moving molecules escape the surface. As a result, the average molecular motion (temperature) of the remaining water is lowered-hence the expression \"evaporation is a cooling process.\" You have undoubtedly experienced this cooling effect when you step, dripping wet, out of a swimming pool or shower. In this situation, the energy used to evaporate water comes from your skin---hence, you feel cool. Condensation, the reverse process, occurs when water vapor changes to the liquid state. During condensation, water vapor molecules release energy (latent heat of condensation) in an amount equivalent to what was absorbed during evaporation. When condensation occurs in the atmosphere, it results in the formation of fog or clouds (Figure 4.4A). Latent heat plays an important role in many atmospheric processes. In particular, when water vapor condenses to form cloud droplets, latent heat is released, which warms the surrounding air, making it less dense and buoyant. When the moisture content of air is high, this process can spur the growth of towering storm clouds. In addition, the evaporation of water over the tropical oceans and the subsequent condensation at higher latitudes results in significant energy transfer from equa- torial regions to more poleward The six phase changes of locations. On a smaller scale, water are: evaporation, when condensation occurs on the condensation, freezing, outside of a glass filled with ice, melting, deposition, and the condensation heats the glass sublimation. and eventually melts the ice.Sublimation and Deposition You are probably least familiar with the last two processes illustrated in Figure 4.3-sublimation and deposition. Sublimation is the conversion of a solid directly to a gas, without passing through the liquid state. Examples you may have observed include the gradual shrinkage of unused ice cubes in a freezer and the rapid conversion of dry ice (frozen carbon dioxide) to wispy clouds that quickly disappear. Deposition is the reverse process: the conversion of a vapor directly to a solid. An example is water vapor deposited as ice on solid objects such as grass or a window pane (Figure 4.4B). These deposits are called white frost or simply frost. As shown in Figure 4.3, the process of deposition returns the combined energy released by condensation and freezing to the environment. Humidity is the general term used to describe the amount of water vapor in the air (Figure 4.5). Water vapor constitutes only a small fraction of the atmosphere, varying from as little as 0.1 percent up to about 4 percent by volume. But the importance of water in the air is far greater than these small percentages indicate. In fact, water vapor is the primary source of energy (latent heat) for the formation of weather systems-thunderstorms, tornadoes, and hurricanes. How Is Humidity Expressed? Meteorologists employ several methods to express the water vapor content of the air, including (1) absolute humidity, (2) mixing ratio, (3) vapor pressure, (4) relative humidity, and (5) dew point. Two of these methods, absolute humidity and mixing ratio, are similar in that both are expressed as the quantity of water vapor contained in a specific amount of air. Absolute Humidity The mass of water vapor in a given volume of air (usually as grams per cubic meter) is known as absolute humidity: Mass of water vapor (grams) Absolute humidity = Volume of air (cubic meters) As air moves from one place to another, changes in pressure and temperature cause changes in its volume. When volume changes, the absolute humidity also changes, even if no water vapor is added or removed. Consequently, it is difficult to monitor the water vapor content of a moving mass of air when using the absolute humidity index. Therefore, meteorologists generally prefer to use mixing ratio to express the water vapor content of air.The mixing ratio is the mass of water vapor in a unit of air compared to the remaining mass of dry air: Mass of water vapor (grams) Mixing ratio = Mass of dry air (kilograms) Because it is measured in units of mass (usually grams per kilogram), the mixing ratio is not affected by changes in pressure or temperature (Figure 4.6).\* Vapor Pressure and Saturation We can determine the moisture content of the air from the pressure exerted by water vapor. To understand how water vapor exerts pressure, imagine a closed flask containing pure water and overlain by dry air, as shown in Figure 4.7A. Almost immediately some of the water molecules begin to leave the water surface and evaporate into the dry air above. The addition of water vapor into the air can be detected by a small increase inpressure (Figure 4.7B). This increase in pressure is a result of the motion of the water vapor molecules that were added to the air through evaporation. This pressure, called vapor pressure, is defined as that part of the total atmospheric pressure attributable to its water vapor content. Initially, many more molecules will leave the water surface (evaporate) than will return (condense). However, as more and more molecules evaporate from the water surface, the steadily increasing vapor pressure in the air above forces more and more water molecules to return to the liquid. Eventually a balance is reached in which the number of water molecules returning to the surface equals the number leaving. At that point, the air is said to have reached an equilibrium called Pressu gauge saturation (Figure 4.7C). When air is satu- rated, the pres- 20°C Vapor pressure is that part of the total atmospheric pressure attributable to its water vapor content. sure exerted by the motion of the water vapor molecules is called the saturation vapor pressure. If the water in the closed container is heated, the equilibrium between evaporation and condensation will be disrupted, as illustrated in Figure 4.7D. The added energy increases the rate of evaporation, which causes the vapor pressure in the air above to increase, until a new equilibrium is reached. Thus, we can conclude that the saturation vapor pres-When air is saturated, sure is tempera- A. the pressure exerted ture dependent, by the motion of the such that at higher temperatures it water vapor molecules Gauge recor saturation vapor pressu for 20°C is called the saturation takes more water 20°C vapor to saturate vapor pressure. air (Figure 4.8). The amount of water vapor required to saturate 1 kilogram (2.2 pounds) of dry air at various temperatures is shown in Table 4.1. Note that for every 10°C (18°F) increase in temperature, the amount of water vapor needed for saturation almost doubles. Evaporation Thus, roughly four times more water vapor is needed to saturate 30°C (86°F) air than 10°C (50°F) airThe atmosphere behaves in much the same manner as our closed container. In nature, gravity, rather than a lid, prevents water vapor (and other gases) from escapinginto space. Also as with our container, water molecules are constantly evaporating from liquid surfaces (such as lakes or oceans), and other water vapor molecules are condensing (into cloud droplets or dew). However, in nature, a balance is not always achieved. More often than not, more water molecules are leaving the surface of a water puddle than are arriving. By contrast, during the formation of fog, more water molecules are condensing than are evaporating from the tiny fog droplets. What determines whether the rate of evaporation exceeds the rate of condensation or vice versa? One major factor is the tempi of the water which in turn determines how much motion (kenetic energy) the water molecules posses. At higher ten, water molecules have more energy and can more readily escape. Vapor pressure is the other major factor that determines whether evaporation or condensation is the dominant process. Recall from our closed container example that vapor pressure influences the rate at which the water molecules leave (evap-orate) and also the rate at which they return to the surface (condense). When the air is dry (low vapor pressure), the rate at which water molecules escape from a liquid surface is high. As the vapor pressure increases, the rate at which water vapor returns to the liquid phase increases as well.The most familiar and, unfortunately, the most misunderstood term used to describe the moisture content of air is relative humidity. Relative humidity is a ratio of the air\'s actual water vapor content compared with the amount of water vapor required for saturation at that temperature (and pressure). Thus, relative humidity measures how near the air is to saturation rather than the actual quantity of water vapor in the air (Box 4.1). Relative humidity can be expressed as follows:\* Mixing ratio (actual water 100 percent- -often occurs in the middle and upper troposphere, and is addressed in Chapter 5.) You may have experienced saturation conditions while taking a hot shower. The water is composed of very energetic (hot) molecules, which means that the rate of evaporation is high. As long as you run the shower, the process of evaporation continually adds water vapor to the unsaturated air in the bathroom. If that hot water runs for enough time, the air eventually becomes saturated, which makes the air foggy. Relative humidity = vapor in the air, g/kg) Saturation mixing ratio (maximum × 100 % water vapor the air can hold, g/kg) V Figure 4.9 At a constant temperature (in this example, it is 25°C), the relative humidity will increase as water vapor is added to the air The saturation mixing ratio for air at 25°C is 20 g/kg (see Table 4.1). As the How Relative Humidity Changes water-vapor content in the flask increases, the relative humidity rises from 25 percent in A to 100 percent in C. Because relative humidity is based on the air\'s water-vapor content, as well as the amount of moisture required for saturation, it Temperature 25°C 25 C 25°C can change in one of two ways. First, relative humidity changes when water vapor is added to or removed from the atmosphere. Second, because the amount of moisture required for saturation is a function of air temperature, relative humidity varies with temperature. How Changes in Moisture Affect Relative Humidity Notice in Figure 4.9 that when water vapor is added to air through evaporation, the relative humidity of the air increases until saturation occurs (100 percent relative humid-ity). What if even more moisture is added to this parcel of saturated air? Does the relative humidity exceed 100 percent? In the lower atmosphere, this situation is rare. Instead, the excess water vapor condenses to form liquid water. (Note that supersaturation-RH above100 percent- -often occurs in the middle and upper troposphere, and is addressed in Chapter 5.) You may have experienced saturation conditions while taking a hot shower. The water is composed of very energetic (hot) molecules, which means that the rate of evaporation is high. As long as you run the shower, the process of evaporation continually adds water vapor to the unsaturated air in the bathroom. If that hot water runs for enough time, the air eventually becomes saturated, which makes the air foggy.In nature, moisture is added to the air mainly via evaporation from the oceans. However, plants, soil, and smaller bodies of water also make substantial contributions. Unlike with your shower, however, the natural processes that add water vapor to the air generally do not operate at rates fast enough to cause saturation to occur directly. One exception is when you exhale on a cold winter day and \"see your breath\": The warm, moist air from your lungs mixes with the cold outside air. Your breath has enough moisture to saturate a small quan- tity of cold outside air, producing Increasing moisture or a miniature \"cloud.\" Almost as decreasing temperature fast as the \"cloud\" forms, it mixes both result in an increase with the surrounding dry air and in relative humidity. evaporates. How Relative Humidity Changes with Temperature The second condition that affects relative humidity is air temperature. Examine Figure 4.10A carefully, and note that when air at 25°C contains 10 grams of water vapor per kilogram, it has a relative humidity of 50 percent. When the flask in Figure 4.10A is cooled from 25° to 15°C, as shown in Figure 4.10B, the relative humidity increases from 50 to 100 percent. We can conclude that when the water-vapor content remains constant, a decrease in temperature results in an increase in relative humidity. In the shower example, the bathroom gets foggy when the air is saturated, but the mirror becomes foggy more quickly than the air in the bathroom. This is because the mirror is cooler than the moist air in the room and cools the adjacent air sufficiently to cause condensation directly on the mirror. But there is no reason to assume that cooling would cease the moment the air reached saturation. What happens when the air is cooled below the temperature at which saturation occurs? Figure 4.10C illustrates this situation. Notice from Table 4.1 that when the flask is cooled to 5°C, the air is saturated, at 5 grams of water vapor per kilogram of air. Because this flask originally contained 10 grams of water vapor, 5 grams of water vapor will condense to form liquid droplets that collect on the walls of the container. In the meantime, the relative humidity of theair inside remains at 100 percent. This illustrates an important concept: When air aloft is cooled below its saturation level, some of the water vapor condenses to form clouds. Since clouds are made of liquid droplets (or ice crystals), this moisture is no longer part of the water-vapor content of the air. Conversely, an increase in temperature results in a decrease in relative humidity. For example, assume that the flask in Figure 4.10A containing 10 grams of water vapor is heated from 25°C to 40°C. Table 4.1 indicates that at 40°C, saturation occurs at 47 grams of water vapor per kilogram of air. Consequently, when the air is heated from 25° to 40°C, the relative humidity will drop from 10/20 (or 50 percent) to 10/47 (or about 21 percent). In nature there are three major ways that air temperatures change (over relatively short time spans) to cause corresponding changes in relative humidity: Daily changes in temperatures (daylight versus nighttime temperatures) Temperature changes that result when air moves horizontally from one location to another Temperature changes caused when air moves vertically in the atmosphere The effect of the first of these three processes (daily changes) is shown in Figure 4.11. Notice that during midafter-noon, relative humidity reaches its lowest level, whereas the cooler evening hours are associated with higher relative humid-ity. In this example, the actual water-vapor content (mixingratio) of the air remains unchanged; only the relative humidity varies. We will consider the other two processes in more detail in later chapters. Dew-Point Temperature The dew-point temperature, or simply the dew point, is the temperature at which water vapor begins to condense. The term dew point stems from the fact that during nighttime hours, objects near the ground often cool below the dew-point temperature and become coated with dew. You have undoubtedly seen \"dew\"Dew point can also be defined as the temperature at which air reaches saturation and, hence, is directly related to the actual moisture content of a parcel of air. Recall that the saturation vapor pressure is temperature dependent and that for every 10°C (18°F) increase in temperature, the amount of water vapor needed for saturation doubles. Therefore, cold saturated air (0°C \[32°F\]) contains about half the water vapor of saturated air having a temperature of 10°C (50°F) and roughly one-fourth that of saturated air with a temperature of 20°C (68°F). Because the dew point is the temperature at which saturation occurs, we can conclude that high dew-point temperatures equate to moist air and, conversely, low dew-point temperatures indicate dry air (Table 4.2). More precisely, based on what we have learned about vapor presssure and saturation, we can state that for every 10 degrees Celsius (18 degrees Celsius) increases in the dew-point temperature, ar contains about twice as much water vapor. Therefore, we know that when the dew-point temperature is 25°C (77°F), air contains about twice the water vapor as when the dew point is 15°C (59°F) and four times that Dew point is the temperature at of air with a dew point of which air reaches saturation. 5°C (41°F). Because the dew-point temperature is a good measure of the amount of water vapor in the air, it commonly appears on weather maps. When the dew point exceeds 65°F (18°C), most people consider the air to feel humid; air with a dew point of 75°F (24°C) or higher is considered oppressive. Notice on the map in Figure 4.13 that much of the southeastern United States has dew-point temperatures that exceed 65°F (18°C). Also notice in Figure 4.13 that although the Southeast is dominated by humid conditions, most of the remainder of the country is experiencing comparatively drier air. How Is Humidity Measured? Instruments called hygrometers are used to measure the moisture content of the air. In addition to being used in mete-orology, hygrometers are used in greenhouses, humidors, museums, and numerous industrial settings that are sensitive to humidity, such as paint booths where protective coatings are applied to products. Because it is difficult to directly measure absolute humidity and the mixing ratio, most hygrometers measure either relative humidity or dew-point temperature. Once either of these is known, it is relatively easy to convert to any of the other humidity measurements as long as we know the temperature. Psychrometers One of the simplest hygrometers, a psychrometer (called a sling psychrometer when connected to a handle and spun) consists of two identical thermometers mounted side by side (Figure 4.14A). One thermometer, called the dry bulb, measures air temperature, and the other, called the wet bulb, has a thin cloth wick tied at the bottom. This cloth wick is saturated with water, and a continuous current of air is passed over the wick, either by swinging the psychrometer or by using an electric fan to move air past the instrument (Figure 4.14B,C). As a result, water evaporates from the wick, absorbing heat energy from the wet-bulb thermometer, which causes its temperature to drop. The amount of cooling that takes place is directly proportional to the dryness of the air: The drier the air, the greater the cooling. Therefore, the larger the difference between the wet- and dry-bulb tempera-tures, the lower the relative humidity. By contrast, if the air is sat-urated, no evaporation will occur, and the two thermometers will have identical readings. By using a psychrometer and the tables provided in Appendix C, you can easily determine the relative humidity and the dew-point temperature.Hair Hygrometers One of the oldest instruments used for measuring relative humidity, called a hair hygrometer, operates on the principle that hair changes length in proportion to changes in relative humidity. Hair lengthens as relative humidity increases and shrinks as relative humidity drops. People with naturally curly hair experience this phenomenon: In humid weather their hair lengthens and hence becomes curlier. A hair hygrometer uses a bundle of hairs linked mechanically to an indicator that is calibrated between 0 and 100 percent. How- ever, these instruments have Hygrometers are instruments become largely obsolete as that measure the quantity of more accurate tools have moisture in the air. been developed. Electric Hygrometers Today, a variety of electric hygrometers are widely used to measure humidity. One type of electric hygrometer uses a chilled mirror and a mechanism that detects the temperature at which condensation begins to form on the mir-ror. Thus, a chilled mirror hygrometer measures the dew-point temperature of the air. The Automated Weather Observing System (AWOS) operated by the National Weather Service (NWS) employs an electric hygrometer that works on the principle of capacitance---a material\'s ability to store an electrical charge. The sensor consists of a thin hygroscopic (water-absorbent) film that is connected to an electric current. As the film absorbs or releases water the capacitance of the sensor changes at a rate proportional to the relative humidity of the surrounding air. Thus, relative humidity can be measured by monitoring the change in the film\'s capacitance. Higher capacitance equates to higher relative humidity.