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NAS 125: Meteorology. Humidity, Saturation, and Stability. Cloud forests, part 1. Some tropical and subtropical forests are perpetually enshrouded in clouds and mist; these are called cloud forests.
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NAS 125: Meteorology Humidity, Saturation, and Stability
Cloud forests, part 1 • Some tropical and subtropical forests are perpetually enshrouded in clouds and mist; these are called cloud forests. • Cloud forests play an important role in terms of biodiversity and water supply, but they are among the most threatened ecosystems in the world as a result of development pressures as well as possible climate change. • There are 605 cloud forests in 41 nations, primarily in Central and South America. Humidity, Saturation, and Stability
Cloud forests, part 2 • Cloud forests are typically found at elevations ranging from 2,000 m to 3,500 m, but may be found at lower elevation on humid islands. • Onshore, upslope flow of warm, moist air helps fuels the low clouds, fog, and mist characteristic of cloud forests. • Air becomes saturated with moisture as it rises and cools. • Forest canopy traps the moisture, encouraging the formation of water droplets that drip to the forest floor. • Trapped moisture is equivalent to 20 percent to 60 percent of the local precipitation totals. Humidity, Saturation, and Stability
Cloud forests, part 3 • Persistent cloud cover reinforces the cool, moist conditions by reducing solar radiation reaching the ground and in turn suppressing the evapotranspiration of water from the surface. • Climate change could reduce the extent of cloud forests by increasing the amount of cooling necessary to produce conditions favorable for condensation of water, but could conversely increase the amount of water vapor in the air that feeds the cloud forest ecosystems. Humidity, Saturation, and Stability
Water • Water occurs in all three states of matter: • Solid (snow, sleet, hail, ice); • Liquid (rain, water droplets); • and Gas (water vapor). • The gaseous state is the most important driver of the dynamics of the atmosphere. • Changes of state of water serve an important role in transferring energy through the Earth-atmosphere system. Humidity, Saturation, and Stability
Reservoirs of water • The primary reservoir of water is the ocean basins (97 percent). • Two percent of water is in ice sheets and glaciers. • About 0.6 percent is in groundwater. • Very little water is stored in the atmosphere. • But the atmosphere is the primary conduit for bringing water from the oceans to the land surfaces. Humidity, Saturation, and Stability
Hydrologic cycle, part 1 • Water is distributed very unevenly around Earth. • Less than 1% of Earth’s total moisture is involved in the hydrologic cycle. • The hydrologic cycle is a series of storage areas interconnected by various transfer processes, in which there is a ceaseless interchange of moisture in terms of its geographical location and its physical state. Humidity, Saturation, and Stability
Hydrologic cycle, part 2 • Surface-to-air water movement • Evaporation is responsible for most of the moisture that enters the atmosphere from Earth’s surface. • Of the moisture evaporated, more than 84% comes from ocean surfaces. • The water evaporated becomes water vapor, and though it stays in atmosphere only briefly (hours to days), it can travel a considerable distance, either vertically or horizontally. Humidity, Saturation, and Stability
Hydrologic cycle, part 3 • Air-to-surface water movement • Water vapor will either condense to liquid water or sublimate to ice to form cloud particles. • Clouds drop precipitation (rain, snow, sleet, hail). • Precipitation and evaporation/transpiration balance in time. • They do not balance in place. • Evaporation exceeds precipitation over oceans. • Precipitation exceeds evaporation over lands. Humidity, Saturation, and Stability
Hydrologic cycle, part 4 • Movement on and beneath Earth’s surface • Runoff is the flow of water from land to oceans by overland flow, streamflow, and groundwater flow. • Runoff is why the oceans do not dry up and continents become flooded despite the imbalance of evaporation and precipitation through space (oceans and continents). • Runoff water amounts to 8% of all moisture circulating in global hydrologic cycle. Humidity, Saturation, and Stability
Hydrologic cycle, part 5 • Residence times • At any given movement, the atmosphere contains only a few days’ potential precipitation. • The residence time of a molecule of water can be hundreds of thousands of years to only a few minutes. Humidity, Saturation, and Stability
Evaporation, part 1 • Evaporation is the process by which liquid water is converted to gaseous water vapor. • Molecules of water escape the liquid surface into the surrounding air. • Temperature is a key factor in evaporation, both in water and in the air around it. • Molecules become more agitated the higher the temperature, and this agitation leads to evaporation. Humidity, Saturation, and Stability
Evaporation, part 2 • Temperature works in conjunction with pressure. • Vapor pressure is the pressure exerted by water vapor in the air. • At any given temperature, there is a maximum vapor pressure that water vapor molecules can exert. • Saturated air: Air becomes saturated when it reaches the point at which some water vapor molecules must become liquid because maximum vapor pressure is exceeded. • The warmer the air, the more water vapor it can hold before becoming saturated. Humidity, Saturation, and Stability
Evaporation, part 3 • Still Versus Moving Air • Movement in air through windiness and/or turbulence helps promote evaporation by removing saturated air. • Disperses vapor molecules and thus makes air above water surface less saturated, so rate of evaporation can increase. Humidity, Saturation, and Stability
Evapotranspiration, part 1 • Evapotranspiration is the process of water vapor entering the air from land sources. • Evapotranspiration occurs through two ways: • Transpiration is the process by which plant leaves give up their moisture to the atmosphere; • Evaporation from soil and plants. • Most evapotranspiration occurs through plants. Humidity, Saturation, and Stability
Evapotranspiration, part 2 • Potential evapotranspiration is the maximum amount of moisture that could be lost from soil and vegetation if the ground were sopping wet all the time. • Potential evapotranspiration rate and actual rate of precipitation play a key role in determining a region’s groundwater supply (or lack of it). Humidity, Saturation, and Stability
Condensation, part 1 • Condensation is the process by which water vapor is converted to liquid water; it is the opposite of evaporation. • For condensation to take place, air must be saturated. • Condensation cannot occur, however, even if the air is saturated, if there is not a surface on which it can take place. • Air becomes supersaturated if surface is not available. Humidity, Saturation, and Stability
Condensation, part 2 • Air must be saturated (continued): • In upper atmosphere, surfaces are available through hygroscopic particles or condensation nuclei—tiny atmospheric particles of dust, smoke, and salt that serve as collection centers for water molecules. • Most common are bacteria blown off plants or thrown into air by ocean waves. Humidity, Saturation, and Stability
Other state changes • Freezing is the process by which liquid water is converted to ice, thus giving off heat. • Melting is the process by which ice is converted to liquid water, thus absorbing heat. • Sublimation is the process by which water vapor is converted directly to ice, or vice versa. Humidity, Saturation, and Stability
Global water budget • The amount of water falling on land surfaces exceeds the amount lost from the land surfaces to the air via evapotranspiration or sublimation. • The amount of water evaporating from oceans exceeds that falling over the oceans. • Excess water from land returns to oceans via runoff. Humidity, Saturation, and Stability
Measures of humidity, part 1 • Humidity is the amount of water vapor in the air. • Dalton’s law: The total pressure of a mixture of gases equals the sum of the partial pressures contributed by each gas. • The partial pressure contributed by water vapor is called the vapor pressure. • Air is said to be saturated if it is at its maximum humidity – when the amount of water molecules leaving the liquid state to become a gas equals the number leaving the gaseous state to become liquid. Humidity, Saturation, and Stability
Measures of humidity, part 2 • Mixing ratio is the ratio of the mass of water vapor per mass of the remaining dry air. • Absolute humidity is a direct measure of the water vapor content of air. • It is expressed as the weight of water vapor in a given volume of air, usually as grams of water per cubic meter of air. • The amount is a function of how much volume is being considered. • If the volume of air doubles, the absolute humidity halves. • Absolute humidity is limited according to temperature. • The colder the air, the less vapor it can hold. Humidity, Saturation, and Stability
Measures of humidity, part 3 • Specific humidity is a direct measure of water-vapor content expressed as the mass of water vapor in a given mass of air (grams of vapor/kilograms of air). • The saturation vapor pressure of air is a function of the temperature of air – the amount of water air can hold increases with increasing temperature. • Related measures: • Saturation mixing ratio • Saturation absolute humidity • Saturation specific humidity Humidity, Saturation, and Stability
Measures of humidity, part 4 • Relative humidity is an expression of the amount of water vapor in the air in comparison with the total amount that could be there if the air were saturated. This is a ratio that is expressed as a percentage. • Relative Humidity = Actual Water Vapor in Air/Capacity x 100 • Relative humidity changes if either the water vapor content or the water vapor capacity of the air changes. • Calculation of relative humidity: • RH = (vapor pressure/saturation vapor pressure) * 100 • RH = (mixing ratio/saturation mixing ratio) * 100 Humidity, Saturation, and Stability
Measures of humidity, part 5 • Relative humidity (continued): • Relative humidity also changes if temperature changes. • The relationship between temperature and relative humidity is one of most important in all meteorology. • Relative humidity and temperature have an inverse relationship – as one increases, the other decreases. • Relative humidity can be determined through the use of a psychrometer. Humidity, Saturation, and Stability
Measures of humidity, part 6 • Cooling is the most common way that air is brought to the point of saturation and condensation. • Dew point is the temperature to which air must be cooled to achieve saturation. • Dew refers to water droplets that form when water vapor condenses on a cold surface. • Frost point is the air temperature at which frost forms. • Frost refers to ice crystals that form when air becomes saturated at a temperature below freezing. Humidity, Saturation, and Stability
Measures of humidity, part 7 • Precipitable water is the depth of liquid water that would be formed if all water vapor in a column of air condenses. • All water in the atmosphere would reach 2.5 cm • Precipitable water ranges from more than 4.0 cm in the humid tropics to 0.5 cm in polar regions. Humidity, Saturation, and Stability
Measurement instruments, part 1 • A hygrometer measures the water vapor concentration of air. • A dewpoint hygrometer air passes over the surface of a metallic mirror that is cooled electronically; the temperature at which a condensation film forms on the mirror is recorded as the dewpoint temperature. • A hair hygrometer uses human hair, which lengthens as it absorbs water and shortens as it dries out; hair is connected to a dial calibrated to read in percent relative humidity. • A hydrograph uses a hair hydrograph and it traces the trend in relative humidity on paper pulled across a clock-driven drum. Humidity, Saturation, and Stability
Measurement instruments, part 2 • Hygrometer (continued): • Electronic hygrometers measure the change in electrical resistance of certain materials as they respond to changing relative humidity. • Psychrometers measures temperature with two identical liquid-in-glass thermometers. • One has a dry bulb; it measures actual air temperature (dry-bulb temperature). • One has a bulb wrapped in muslin, that is in turn moistened; it measures the wet-bulb temperature. Humidity, Saturation, and Stability
Measurement instruments, part 3 • Psychrometers (continued): • The psychrometer can be either whirled around by hand (a sling psychrometer) or it can be ventilated by a small fan (an aspirated psychrometer). • The drier the air, the greater the evaporation from the wet bulb thermometer, thus the cooler the wet-bulb temperature. • The difference between the air temperature and the wet-bulb temperature is called the wet-bulb depression. • The relative humidity is looked up on a chart using the dry-bulb temperature and wet-bulb depression. Humidity, Saturation, and Stability
Measurement instruments, part 4 • Psychrometers (continued): • It is difficult to get accurate wet-bulb temperatures under cold or subfreezing conditions as well as under very dry conditions. • The dry-bulb temperature and wet-bulb temperature can be used to look up the dewpoint temperature. Humidity, Saturation, and Stability
Water vapor satellites • Water vapor satellite images use special infrared sensors to measure water vapor in the atmosphere. • Water vapor does not appear on visible or on conventional infrared sensors. • Such images allow meteorologists to track movement of plumes of moisture through the atmosphere. • Current platforms measure clouds and water vapor concentrations above 3,000 m. • A gray scale is used, so that little or no water vapor appears black, while high concentrations appear milky white. Upper-level clouds appear as bright white blotches. Humidity, Saturation, and Stability
