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Here is a term paper on ‘Relative Humidity’. Find paragraphs, long and short term papers on ‘Relative Humidity’ especially written for school and college students.
Term Paper on Relative Humidity
Term Paper Contents:
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- Term Paper on the Introduction to Relative Humidity
- Term Paper on the Factors Influencing Variations in Relative Humidity
- Term Paper on the Effects of Relative Humidity
- Term Paper on Measuring Relative Humidity
Term Paper # 1. Introduction to Relative Humidity:
Saturation of surface air is a condition of favourable fire weather; that is, conducive to low fire danger. Less favourable are conditions of unsaturation, which permit evaporation from forest fuels, increasing their flammability and the fire danger. Therefore, a very useful measure of atmospheric moisture is the relative humidity.
It is the ratio, in per cent, of the amount of moisture in a volume of air to the total amount which that volume can hold at the given temperature and atmospheric pressure. Relative humidity is also the ratio of actual vapour pressure to saturation vapour pressure, times 100. It ranges from 100 per cent at saturation to near zero for very dry air. Relative humidity depends on the actual moisture content of the air, the temperature, and the pressure.
The dependence of relative humidity on temperature must be kept in mind. Suppose that we have air at 80°F. and 24 per cent relative humidity. Using Table: Dew Point, Vapour Pressure, and Absolute Humidity, we find that the saturation vapour pressure for 80° is 1.032 inches of mercury. We can compute the actual vapour pressure by multiplying 1.082 by 0.24. The actual vapour pressure is 0.248 rounded off. The dew point for this vapour pressure is 40°.
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We now know that if the air was cooled from 80°F. to 40°, with no other change, the humidity would increase from 24 per cent to 100 per cent and the air would be saturated. At that temperature the actual vapour pressure would equal the saturation vapour pressure. The absolute humidity in the table could be used in a similar manner. Thus; the relative humidity may change considerably with no addition of moisture, just by cooling alone.
Term Paper # 2.
Factors Influencing Variations in Relative Humidity:
Relative humidity is much more variable than absolute humidity. It often changes rapidly and in significant amounts from one hour to the next and from place to place. Relative humidity is much more variable because it depends not only on absolute humidity but also on air temperature.
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It varies directly with moisture content and inversely with temperature. Because of these relationships, it is often not possible to make general statements about relative humidity variations, particularly vertical variations within short distances above the ground.
i. Variations in Relative Humidity Near the Ground:
Relative humidity is most important as a fire-weather factor in the layer near the ground, where it influences both fuels and fire behaviour. Near the ground, air moisture content, season, time of day, slope, aspect, elevation, clouds, and vegetation all cause important variations in relative humidity.
ii. Diurnal Changes in Relative Humidity:
Since hourly and daily changes of relative humidity are normally measured in a standard instrument shelter, we will consider variations at that level and infer from our knowledge of surface temperatures what the conditions are near the surface around forest fuels.
A typical fair-weather pattern of relative humidity, as shown on a hygrothermograph exposed in a shelter at a valley station or one in flat terrain, is nearly a mirror image of the temperature pattern. Maximum humidity generally occurs about daybreak, at the time of minimum temperature. After sunrise, humidity drops rapidly and reaches a minimum at about the time of maximum temperature.
It rises more gradually from late afternoon through the night. The daily range of humidity is usually greatest when the daily range of temperature is greatest. Variations in the humidity traces within an air mass from one day to the next are usually small, reflecting mostly differences in temperatures. But over several days, there may be noticeable cumulative differences in humidity as the air mass gradually picks up or loses moisture.
iii. Seasonal Changes in Relative Humidity:
Seasonal changes in relative humidity patterns are also apparent. In western fire-weather seasons that begin following a moist spring and continue through the summer and early fall, a seasonal change is particularly noticeable. Daily temperature ranges are greatest early in the fire season when the sun is nearly overhead and night skies are clear.
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Strong nighttime cooling, in combination with ample moisture in the soil and vegetation to contribute moisture to the atmosphere, often boosts night humidities to or near 100%. Intensive day-time surface heating and convective transport of moisture upward combine to drop the relative humidity to low levels in the afternoon.
As the season progresses, soil and vegetation dry out and solar heating diminishes as the Sun tracks farther south. Daytime humidities become even lower late in the season, but, with a greater reduction in night humidities, the daily range is reduced.
In areas that have separate spring and fall fire seasons, the daily temperature extremes are generally not so striking. Also, the cumulative drying of soil and vegetation is not so consistent, except during unusual drought. Because periodic rains generally occur during the seasons, the humidity changes tend to be somewhat variable. In some areas, seasonal increases in relative humidity decrease fire danger during the summer.
In the Great Lakes region, particularly, where the many small lakes become quite warm during the summer and transpiration from vegetation is at its peak, daytime relative humidities do not reach as low values in the same air mass types as they do in spring and fall.
Humidity may vary considerably from one spot to another, depending greatly on the topography. In relatively flat to rolling terrain, the humidity measured at a well-exposed station may be quite representative of a fairly large area. There will be local exceptions along streams, irrigated fields, in shaded woods, or in barren areas. In the daytime particularly, circulation and mixing are usually sufficient to smooth out local effects over relatively short distances.
In mountainous topography, the effects of elevation and aspect become important, and humidities vary more than over gentle terrain. Low elevations warm up and dry out earlier in the spring than do high elevations. South slopes also are more advanced seasonally than north slopes. As the season progresses, cumulative drying tends to even out these differences since stored moisture in the surface is depleted, but the differences do not disappear.
The decrease with height of both temperature and dew point produces higher relative humidities at higher elevations on slopes. The pattern is complicated, however, because of heating of the air next to the slopes, the transport of moisture with upslope winds, and the frequent stratification of moisture into layers, so generalizations are difficult to make.
When nighttime cooling begins, the temperature change with height is usually reversed. Cold air flowing down the slopes accumulates at the bottom. As the night progresses, additional cooling occurs, and by morning, if the air becomes saturated, fog or dew forms. Relative humidity may decrease from 100% at the foot of the slope to a minimum value at the top of the temperature inversion and then may increase slightly farther up the slope above the inversion.
Just as south slopes dry out faster because of their higher day temperatures, they also have somewhat lower day relative humidities than north slopes throughout the summer. At upper elevations, though, the difference between north and south slopes becomes negligible because of the good air mixing at these more exposed sites. At night, humidity differences on north and south slopes become slight.
In most mountainous country, the daily range of relative humidity is greatest in valley bottoms and least at higher elevations. Thus, while fires on lower slopes may burn better during the day, they often quiet down considerably at night when humidity increases. But at higher elevations, particularly in and above the thermal belt, fires may continue to burn aggressively through the night as humidities remain low, temperatures stay higher, and wind speed is greater.
Again, we should be cautious of generalizations. For example, in the summer in the Pacific Coast Ranges, higher humidities are usually found on ridge-tops during the day than during the night. This anomaly results from slope winds carrying moisture upward from the moist marine air layer during the day. Moist air that is not carried away aloft settles back down at night.
iv. Variations in Relative Humidity with Height:
During the day near the surface, particularly with clear skies, both the temperature and absolute humidity usually decrease with height. These two variables have opposite effects on the relative humidity. Which effect is dominant depends upon the dryness of the surface.
The relative humidity usually increases with height over normal surfaces because the effect of the decrease in temperature is greater than that of the decrease in absolute humidity. Over a moist surface, however, the effect of the decrease in absolute humidity may overbalance that of temperature decrease, and the relative humidity in the surface layer will decrease with height.
At night, the change of temperature with height usually predominates, and the relative humidity will decrease with height through the lowest layers. Above the lowest layers, the relative humidity generally increases with height in the day through much of the lower troposphere. Convection alone would account for this increase. As air is lifted, the temperature decreases 5.5°F per 1,000 feet, and the dew point decreases at about 1°F per 1,000 feet. Therefore, the dew point and the temperature become 4.5°F closer per 1,000 feet, and the relative humidity increases until saturation is reached.
A subsiding layer of air in the troposphere warms by the adiabatic process and forms a subsidence inversion. The relative humidity will decrease upward through the temperature inversion at the base of the subsiding layer. The marine inversion along the west coast, for example, is a subsidence inversion. The marine air below has low temperatures and high humidities, and the adiabatically heated subsiding air mass above has higher temperatures and lower humidities. This pronounced change in temperature and humidity is evident along the slopes of coastal mountains when the marine inversion is present.
Term Paper # 3.
Effects of Relative Humidity:
i. Effect of Wind on Humidity:
Wind mixes evaporating water vapour with surrounding air and evens out temperature extremes by moving air away from hot and cold surfaces. Thus, diurnal ranges of relative humidity are less during windy periods than during calm periods. Winds also reduce place-to-place differences by mixing air of different moisture contents and different temperatures. Patches of fog on a calm night indicate poor ventilation.
ii. Effect of Clouds on Humidity:
Clouds strongly affect heating and cooling and therefore influence the relative humidity. The humidity will be higher on cloudy days and lower on cloudy nights. Thus, clouds reduce the daily range considerably. Precipitation in any form raises relative humidities by cooling the air and by supplying moisture for evaporation into the air.
iii. Effect of Vegetation on Humidity:
Vegetation moderates surface temperatures and contributes to air moisture through transpiration and evaporation—both factors that affect local relative humidity. A continuous forest canopy has the added effect of decreasing surface wind speeds and the mixing that takes place with air movement.
The differences in humidity between forest stands and open areas generally vary with the density of the crown canopy. Under a closed canopy, humidity is normally higher than outside during the day and lower at night. The higher daytime humidities are even more pronounced when there is a green understory. Deciduous forests have only slight effects on humidity during their leafless period.
Two factors lessen the humidity difference between forest stands and forest openings. Overcast skies limit both heating and cooling, and drought conditions decrease the amount of moisture available for evaporation and transpiration.
Openings of up to about 20 yards in diameter do not have daytime relative humidities much different from under the canopy—except at the heated ground surface. These openings serve as chimneys for convective airflow, and surface air is drawn into them from the surrounding forest. At night in small openings, the stagnation coupled with strong radiation can cause locally high humidities.
The daytime humidities in larger clearings are much like those in open country. If the airflow is restricted, however, temperatures may rise slightly above those at exposed stations, and humidities will be correspondingly lower. In the afternoon, these may range from 5 to 20% lower in the clearing than within a well-shaded forest. Night humidities are generally similar to those at exposed sites, usually somewhat higher than in the woods.
Open forest stands have humidity characteristics somewhere between those of exposed sites and closed stands, depending on crown density. During dry weather, especially after prolonged dry spells, the differences in relative humidity between forested and open lands become progressively less.
iv. Effect of Air Masses on Humidity:
The amount of moisture in the air is one of the air-mass characteristics. Air masses originating over water bodies will have higher moisture contents than those originating over continents. When a front passes, and a different air mass arrives, a change in absolute humidity can be expected. The change in relative humidity, however, will depend greatly on the air-mass temperature. A warm, dry air mass replacing a cool, moist one, or vice versa, may cause a large change in relative humidity. A cool, dry air mass replacing a warm, moist one, however, may actually have a higher relative humidity if its temperature is appreciably lower.
Along the west coast, when a lower marine layer is topped by a warm, dry, subsiding air mass, the inversion layer is actually the boundary between two very different air masses. Inland, where the inversion intersects the Coast Ranges, very abnormal relative humidity patterns are found. In these inland areas, the inversion is usually higher in the day and lower at night; however, along the coastal low-lands, the reverse is usually true.
Along the slopes of the adjacent mountains, some areas will be in the marine air during the day and in the dry, subsiding air at night. The relative humidity may begin to rise during the late afternoon and early evening and then suddenly drop to low values as dry air from aloft moves down the slopes. Abrupt humidity drops of up to 70% in the early evening have been observed.
Term Paper # 4.
Measuring Relative Humidity:
The most widely used device for accurately measuring atmospheric moisture near the surface is the psychrometer. It consists of two identical mercurial thermometers. One thermometer is used for measuring the air temperature; the other measures the temperature of evaporating water contained in a muslin wicking surrounding the thermometer bulb. The amount that the evaporating surface will cool is determined by the difference between the vapour pressure and the saturation vapour pressure.
The first reading is commonly referred to as the dry-bulb temperature and the second as the wet-bulb temperature. The wet-bulb temperature is the steady value reached during a period of brisk ventilation of the thermometer bulbs. If the air is saturated, the wet-bulb and dry-bulb temperatures are the same.
From the wet-and dry-bulb measurements, computed values of dew- point temperature, absolute humidity, and relative humidity may be read from tables or slide rules.These moisture relations vary with changes in pressure. The daily pressure changes as shown by the barometer are not large enough to be important, but those due to differences in elevation are significant. They have been considered in the construction of the tables or slide rules. The ones labeled with the correct pressure must be used.
Other instruments used to measure relative humidity contain fibers of various materials that swell or shrink with changing relative humidity. One instrument of this type that records a continuous trace of relative humidity is called a hygrograph. A more common form in use at fire- weather stations is the hygrothermograph, which records both relative humidity and temperature. Other devices, such as those commonly used for upper-air soundings, employ moisture-sensitive elements that change in electrical or chemical characteristics with changing humidity.
Standard surface measurements of relative humidity, like those of temperature, are made in an instrument shelter 4-1/2 feet above the ground. A properly operated sling psychrometer, however, will indicate dry-and wet-bulb readings that agree well with those obtained in the shelter. The only necessary precautions are to select a well-ventilated shady spot, and to whirl the instrument rapidly for a sufficient time to get the true (lowest) wet-bulb temperature. Care must be taken not to allow the wicking to dry out, and not to break the thermometer by striking any object while whirling the psychrometer.
Measuring Relative Humidity near the Ground:
The relative humidity that affects fuels on the forest floor is often quite different from that in the instrument shelter, particularly in un-shaded areas where soil and surface fuels exposed to the sun are heated intensely, and warm the air surrounding them. This very warm air may have a dew point nearly the same or slightly higher than the air in the instrument shelter, but because it is much warmer, it has a much lower relative humidity.
With similar exposure at night, humidities are likely to be higher near the ground than in the shelter because of radiative cooling of the surface. Often, dew will form on the surface, indicating 100 per cent relative humidity, when the humidity at shelter height may be considerably below the saturation level. These conditions are typical for relatively still air, clear skies, and open exposure.
When wind speeds reach about 8 miles per hour, the increased mixing diminishes the difference between surface and shelter-height humidities Also, under heavy cloud cover or shade, the humidity differences between the two levels tend to disappear because the principal radiating surface is above both levels.
It is impractical to measure humidity close to the ground with field instruments, but with the aid of tables, the humidity can be estimated from psychrometric readings at the standard height and a dry-bulb temperature reading at the surface. We must assume that the dew point is the same at both levels. Although we know that this may not be exact, it will give a reasonable estimation.