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| {{essay-like|date=September 2012}}
| | I know that all of you have watched the term calorie several instances over by this point, however how numerous of you have actually thought about what a calorie actually is? Probably not too countless. Whenever it certainly comes right down to it, calories are what help us achieve or goals of either losing or gaining weight, thus its important to know what a calorie really is plus how it effects these goals.<br><br>Don't give [http://safedietplansforwomen.com/bmr-calculator bmr calculator] up when you blow your diet for the day. Tomorrow is a hot day for eating healthy foods. The worst thing you can do is beat yourself up with guilt following overeating or bingeing, plus you're most vulnerable whenever anxious or depressed.<br><br>The basal metabolic rate offers a superior baseline for minimum calories. Obviously, the right foods and exercise are crucial to the success. A diet of sugary foods and/or an exercise system consisting of endless walking on a treadmill usually create fat reduction difficult. However should you use a BMR because a beginning point, you will learn not to go below that level plus add food and exercise accordingly to create a calorie deficit.<br><br>Our bodies require 1,800 to 2,000 calories a day inside order to function properly. Some individuals want less or more, yet this might be the average. So don't try to drop a noticeable amount of calories at 1 time. There is not any have to place the body from starvation. If you do, a metabolic rate really slows down, that makes the entire process harder.<br><br>The initially part is the Basic stuff you do. This really is everything except the work outs you'll add to the daily routine. But you don't like to count each time we sneeze, despite that which does burn calories. If you are basically sedentary during the day, you are able to multiply a bmr by .2, plus that is a wise estimate of your Basic calories. Remember, for this step never count any extra exercise we are doing in order to lose weight. Count the little basic aspects you do each day. If you type all day long in an workplace, choose the sedentary level. If you chase small kids around the home all day, choose a higher level. Many of us that are struggling to get rid of fat may be starting at the sedentary level.<br><br>At any given time, 25 % of all men plus 33 % of all females are on several sort of formal diet in the United States. More than 55 percent gain back all of their fat and more than what they started with.1 Unfortunately, many diets are a one-size-fits-all approach. With any diet book you pick off the bookstore shelf, or any aged diets passed down by the perfect aunt, there are the same diet for everyone. Some of those are completely unsound nutritionally while others could be backed by good nutrition principles. Yet, even those with wise nutrition principles don't personalize their approach to fit every person's body makeup. They are sadly a one-size-fits-all dieting approach.<br><br>McArdle, William, Katch, Frank, and Katch, Victor. (1998). Exercise Physiology: Energy, Nutrition, and Human Performance. Baltimore, Maryland: Williams and Wilkins. |
| [[Image:Umidaderelativa.jpg|thumb|A [[hygrometer]] is a device used for measuring the humidity of air.]]
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| '''Relative humidity''' is the ratio of the [[partial pressure]] of water vapor in an air-water mixture to the saturated [[vapor pressure]] of water at a prescribed temperature. The relative humidity of air depends on temperature and the pressure of the system of interest.
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| == Definition ==
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| The relative humidity <math> \left(\phi\right) </math> of an air-water mixture is defined as the ratio of the [[partial pressure]] of water vapor (H<sub>2</sub>O) <math> \left({e_w}\right) </math> in the mixture to the saturated [[vapor pressure]] of water <math> \left({{e^*}_w}\right) </math> at a given temperature.
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| Relative humidity is normally expressed as a percentage and is calculated by using the following equation:<ref name="Perry">Perry, R.H. and Green, D.W, ''[[Perry's Chemical Engineers' Handbook]]'' (7th Edition), [[McGraw-Hill]], ISBN 0-07-049841-5 , Eqn 12-7</ref>
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| :::<math> \phi = {{e_w} \over {{e^*}_w}} \times 100\% </math>
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| == Significance ==
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| ===Climate control===
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| Climate control refers to the control of temperature and relative humidity for human comfort, health, and safety; for the technical requirements of machines and processes; and in buildings, vehicles, and other enclosed spaces.
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| ===Comfort===
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| Humans are sensitive to humid air because the human body uses evaporative cooling (perspiration) as the primary mechanism to rid itself of waste heat. The rate at which perspiration evaporates on the skin under humid conditions is lower than under arid conditions. Because humans perceive a low ''rate of heat transfer'' from the body the same as a higher air temperature,<ref>{{Citation | title = Astro | publisher = Cornell | type = Q&A | contribution = Curious | url = http://curious.astro.cornell.edu/question.php?number=86}}.</ref> the body experiences greater distress of waste heat burden at a lower temperature with high humidity than at a higher temperature at lower humidity.
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| If, for example, the air temperature is 24 °C (75 °F) and the relative humidity is zero percent then the air temperature feels like 21 °C (69 °F).<ref name="hswref">{{Citation | title = Science | url = http://science.howstuffworks.com/question651.htm | contribution = What is relative humidity and how does it affect how I feel outside? | publisher = How stuff works | type = Q&A}}.</ref> If the relative humidity is 100 percent at the same air temperature then it feels like 27 °C (80 °F).<ref name = "hswref" /> If the air is 24 °C (75 °F) and contains saturated water vapor, in other words, then the human body cools itself at the same rate as it would if it were 27 °C (80 °F) and dry.<ref name="hswref" /> The [[heat index]] and the [[humidex]] are indices that reflect the combined effect of temperature and humidity on the cooling effect of the atmosphere on the human body.
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| Humans can be comfortable within a wide range of humidities depending on the temperature — from thirty to seventy percent - <ref>{{cite journal | last= Gilmore| first= CP | title= More Comfort for Your Heating Dollar| journal= Popular Science|date=September 1972 | page = 99}}</ref> but ideally between fifty<ref>{{Citation | url = http://www.infoplease.com/ipa/A0001412.html | publisher = Pearson | title = Information please | type = database | year = 2007 | contribution = Winter Indoor Comfort and Relative Humidity | quote = …by increasing the relative humidity to above 50% within the above temperature range, 80% or more of all average dressed persons would feel comfortable. | accessdate = 2013-05-01}}.</ref> and sixty percent.<ref>{{Citation | url = http://www.engineeringtoolbox.com/relative-humidity-d_895.html | title = The engineering toolbox | contribution = Recommended relative humidity level | quote = Relative humidity above 60% feels uncomfortable wet. Human comfort requires the relative humidity to be in the range 25–60% RH. | accessdate = 2013-05-01}}</ref>
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| ===Buildings===
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| For climate control in buildings using [[HVAC]] systems, the key is to maintain the relative humidity at a comfortable range—low enough to be comfortable but high enough to avoid problems associated with very dry air.
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| When the temperature is high and the relative humidity is low, evaporation of water is rapid; soil dries, wet clothes hung on a line or rack dry quickly, and perspiration readily evaporates from the skin. Wooden furniture can shrink, causing the paint that covers these surfaces to fracture.
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| When the temperature is high and the relative humidity is high, evaporation of water is slow. When relative humidity approaches 100 percent, condensation can occur on surfaces, leading to problems with mold, corrosion, decay, and other moisture-related deterioration.
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| Certain production and technical processes and treatments in factories, laboratories, hospitals, and other facilities require specific relative humidity levels to be maintained using humidifiers, [[dehumidifier]]s and associated control systems.
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| ===Vehicles===
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| The basic principles for buildings, above, also apply to vehicles. In addition, there may be safety considerations. For instance, high humidity inside a [[vehicle]] can lead to problems of condensation, such as misting of [[windshield]]s and [[Short circuit|shorting]] of electrical components. | |
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| In sealed vehicles and [[pressure vessel]]s such as pressurized [[airliner]]s, [[submersible]]s and [[spacecraft]], these considerations may be critical to safety, and complex [[environmental control system]]s including equipment to maintain [[pressure]] are needed. Airliners operate with low internal relative humidity, often under 10%, especially on long flights. The low humidity is a consequence of drawing in the very cold air with a low [[Humidity#Absolute humidity|absolute humidity]], which is found at airliner cruising altitudes. Subsequent warming of this air lowers its relative humidity. This causes discomfort such as sore eyes, dry skin, and drying out of [[mucosa]], but humidifiers are not employed to raise it to comfortable mid-range levels because the volume of water required to be carried on board can be a significant weight penalty. As airliners descend from colder altitudes into warmer air (perhaps even flying through clouds a few thousand feet above the ground), the ambient relative humidity can increase dramatically. Some of this moist air is usually drawn into the pressurized aircraft cabin and into other non-pressurized areas of the aircraft and condenses on the cold aircraft skin. Liquid water can usually be seen running along the aircraft skin, both on the inside and outside of the cabin. Because of the drastic changes in relative humidity inside the vehicle, components must be qualified to operate in those environments. The recommended environmental qualifications for most commercial aircraft components is listed in RTCA DO-160.
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| === Aviation ===
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| Cold humid air can promote the formation of ice, which is a danger to aircraft as it affects the wing profile and increases weight. Carbureted engines have a further danger of ice forming inside the [[carburetor]]. Aviation weather reports ([[METAR]]s) therefore include an indication of relative humidity, usually in the form of the [[dew point]].
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| Density altitude is the altitude relative to the standard atmosphere conditions (International Standard Atmosphere) at which the air density would be equal to the indicated air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground. "Density Altitude" is the pressure altitude adjusted for non-standard temperature.
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| Both an increase in temperature, pressure, and, to a much lesser degree, humidity will cause an increase in density altitude. Thus, in hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.
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| == Measurement ==
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| The humidity of an air-water vapor mixture is determined through the use of psychrometric charts if both the [[Dry-bulb temperature|dry bulb temperature]] (''T'') and the [[Wet-bulb temperature|wet bulb temperature]] (''T<sub>w</sub>'') of the mixture are known. These quantities are readily estimated by using a sling [[hygrometer|psychrometer]].
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| There are several empirical correlations that can be used to estimate the saturated vapor pressure of water vapor as a function of temperature. The [[Antoine equation]] is among the least complex of these formulas, having only three parameters (A, B, and C). Other correlations, such as those presented by [[Goff-Gratch Equation|Goff-Gratch]] and [[Clausius–Clapeyron relation#Meteorology and climatology|Magnus Tetens approximation]], are more complicated but yield better accuracy{{Citation needed|date=January 2010}}. The correlation presented by [[Arden Buck equation|Buck]]<ref>http://www.public.iastate.edu/~bkh/teaching/505/arden_buck_sat.pdf</ref> is commonly encountered in the literature and provides a reasonable balance between complexity and accuracy{{Citation needed|date=January 2013}}:
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| :::<math> {{e^*}_w} = (1.0007 + 3.46 \times 10^{-6} P) \times (6.1121) e^{\left(\frac {17.502 T} {240.97 + T}\right)}</math>
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| where <math> T </math> is the dry bulb temperature expressed in degrees Celsius ('''°C'''), <math> P </math> is the absolute pressure expressed in hectopascals, and <math> {{e^*}_w} </math> is the saturated vapor pressure expressed in hectopascals.
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| Buck has reported that the maximum relative error is less than 0.20% between -20°C and +50°C when this particular form of the generalized formula is used to estimate the saturated vapor pressure of water.
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| == Misconception ==
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| The notion of air holding water vapor is sometimes used to describe the concept of relative humidity. This, however, is a misconception. [[Air]] is a mixture of gases (nitrogen, oxygen, argon, water vapor, and other gases) and as such the constituents of the mixture simply act as a transporter of water vapor but are not a holder of it.
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| ''Relative Humidity'' is defined in the physical properties of water and thus is unrelated to the notion of ''air holding water''.<ref>http://www.atmos.umd.edu/~stevenb/vapor/</ref><ref>http://www.ems.psu.edu/~fraser/Bad/BadFAQ/BadCloudsFAQ.html</ref> An air-less volume, can in fact, contain water vapor and therefore the humidity of this volume can be readily determined.
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| Saturated air is really just air which contains water vapor in equilibrium with a liquid water source. However, the temperature of the air, not the temperature of the liquid source, determines the amount of water vapor that can be held in vapor form in the air. Cold air above boiling water will not hold as much water vapor as warm air over a cold water source.
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| == Pressure Dependence ==
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| The relative humidity of an air-water system is dependent not only on the temperature but also on the absolute pressure of the system of interest. This dependence is demonstrated by considering the air-water system shown below. The system is closed (i.e., no matter enters or leaves the system).
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| [[Image:Changes in Relative Humidity.png]]
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| If the system at State A is isobarically heated (heating with no change in system pressure) then the relative humidity of the system decreases because the saturated vapor pressure of water increases with increasing temperature. This is shown in State B.
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| If the system at State A is isothermally compressed (compressed with no change in system temperature) then the relative humidity of the system increases because the partial pressure of water in the system increases with the volume reduction. This is shown in State C. Above 202.64 kPa, the RH would exceed 100% and water may begin to condense. | |
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| If the pressure of State A was changed by simply adding more dry air, without changing the volume, the relative humidity would not change.
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| Therefore a change in relative humidity can be explained by a change in system temperature, a change in the volume of the system, or change in both of these system properties.
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| === Enhancement factor ===
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| The enhancement factor <math> \left(f_w\right) </math> is defined as the ratio of the saturated vapor pressure of water in moist air <math> \left(e'_w\right) </math> to the saturated vapor pressure of pure water.
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| :::<math> f_W = \frac {e'_w} {e^*_w} </math>
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| The enhancement factor is equal to unity for ideal gas systems. However, in real systems the interaction effects between gas molecules result in a small increase of the saturation vapor pressure of water in air relative to saturated vapor pressure of pure water vapor. Therefore, the enhancement factor is normally slightly greater than unity for real systems.
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| The enhancement factor is commonly used to correct the saturated vapor pressure of water vapor when empirical relationships, such as those developed by Wexler, Goff, and Gratch, are used to estimate the properties of psychrometric systems.
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| Buck has reported that, at sea level, the vapor pressure of water in saturated moist air amounts to an increase of approximately 0.5% over the saturated vapor pressure of pure water.<ref name="Buck">Arden L. Buck, New Equations for Computing Vapor Pressure and Enhancement Factor, Journal of Applied Meteorology, December 1981, Volume 20, Page 1529.</ref>
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| == Related concepts ==
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| The term relative humidity is reserved for systems of water vapor in air. The term ''relative saturation'' is used to describe the analogous property for systems consisting of a condensable phase other than water in a non-condensable phase other than air.<ref>http://blowers.chee.arizona.edu/201project/GLsys.interrelatn.pg1.HTML</ref>
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| == Other important facts ==
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| [[Image:Relative Humidity.png]]
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| A gas in this context is referred to as saturated when the vapor pressure of water in the air is at the equilibrium vapor pressure for water vapor at the temperature of the gas and water vapor mixture; liquid water (and ice, at the appropriate temperature) will fail to lose mass through evaporation when exposed to saturated air. It may also correspond to the possibility of [[dew]] or [[fog]] forming, within a space that lacks temperature differences among its portions, for instance in response to decreasing temperature. Fog consists of very minute droplets of liquid, primarily held aloft by isostatic motion (in other words, the droplets fall through the air at terminal velocity, but as they are very small, this terminal velocity is very small too, so it doesn't look to us like they are falling, and they seem to be held aloft).
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| The statement that relative humidity (RH%) can never be above 100%, while a fairly good guide, is not absolutely accurate, without a more sophisticated definition of humidity than the one given here. An arguable exception is the [[Wilson cloud chamber]], which uses, in nuclear physics experiments, an extremely brief state of "[[supersaturation]]" to accomplish its function.
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| For a given [[dewpoint]] and its corresponding [[absolute humidity]], the relative humidity will change inversely, albeit nonlinearly, with the [[temperature]]. This is because the partial pressure of water increases with temperature – the operative principle behind everything from [[hair dryer]]s to [[dehumidifier]]s.
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| Due to the increasing potential for a higher water vapor partial pressure at higher air temperatures, the water content of air at sea level can get as high as 3% by mass at 30 °C (86 °F) compared to no more than about 0.5% by mass at 0 °C (32 °F). This explains the low levels (in the absence of measures to add moisture) of humidity in heated structures during winter, resulting in dry [[Human skin|skin]], [[itch]]y [[Human eye|eye]]s, and persistence of [[static electricity|static electric]] charges. Even with saturation (100% relative humidity) outdoors, heating of infiltrated outside air that comes indoors raises its moisture capacity, which lowers relative humidity and increases evaporation rates from moist surfaces indoors (including human bodies and household plants.)
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| Similarly, during summer in humid climates a great deal of liquid water condenses from air cooled in air conditioners. Warmer air is cooled below its dewpoint, and the excess water vapor condenses. This phenomenon is the same as that which causes water droplets to form on the outside of a cup containing an ice-cold drink.
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| A useful rule of thumb is that the maximum [[absolute humidity]] doubles for every 20 °F or 10 °C increase in temperature. Thus, the relative humidity will drop by a factor of 2 for each 20 °F or 10 °C increase in temperature, assuming conservation of absolute moisture. For example, in the range of normal temperatures, air at 68 °F or 20 °C and 50% relative humidity will become saturated if cooled to 50°F or 10 °C, its [[dewpoint]], and 41 °F or 5 °C air at 80% relative humidity warmed to 68 °F or 20 °C will have a relative humidity of only 29% and feel dry. By comparison, a relative humidity between 40% and 60% is considered healthy and comfortable in comfort-controlled environments (ASHRAE Standard 55 - see [[thermal comfort]]).
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| Water vapor is a lighter gas than other gaseous components of air at the same temperature, so humid air will tend to rise by natural [[convection]]. This is a mechanism behind [[thunderstorms]] and other [[weather]] phenomena. Relative humidity is often mentioned in [[weather forecasting|weather forecasts]] and reports, as it is an indicator of the likelihood of [[precipitation (meteorology)|precipitation]], dew, or fog. In hot summer [[weather]], it also increases the [[heat index|apparent temperature]] to [[human]]s (and other [[animal]]s) by hindering the [[evaporation]] of [[perspiration]] from the skin as the relative humidity rises. This effect is calculated as the [[heat index]] or [[humidex]].
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| A device used to measure humidity is called a [[hygrometer]]; one used to regulate it is called a [[humidistat]], or sometimes [[hygrostat]]. (These are [[analogous]] to a [[thermometer]] and [[thermostat]] for temperature, respectively.)
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| == See also ==
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| *[[Humidity]]
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| **[[Humidity#Absolute humidity|Absolute Humidity]]
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| **[[Humidity#Specific humidity|Specific Humidity]]
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| *[[Concentration]]
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| *[[Heat index]]
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| *[[Dew point]]
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| **[[Dew point depression]]
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| *[[Humidity buffering]]
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| *[[Humidity indicator]]
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| *[[Humidity indicator card]]
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| *[[Hygrometer]]
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| **[[Psychrometrics]]
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| *[[Saturation vapor density]]
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| *[[Water activity]]
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| ==References==
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| {{Reflist}}
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| * {{Cite book
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| | last = Himmelblau
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| | first = David M.
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| | authorlink =
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| | title = Basic Principles And Calculations In Chemical Engineering
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| | publisher = [[Prentice Hall]]
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| | year = 1985454545
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| | doi =
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| | isbn = 0-13-066572-X }}
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| * {{Cite book
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| | last = Perry, R.H. and Green, D.W
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| | first =
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| | authorlink =
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| | title = [[Perry's Chemical Engineers' Handbook]] (7th Edition)
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| | publisher = [[McGraw-Hill]]
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| | year = 1997
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| | doi =
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| | isbn = 0-07-049841-5 }}
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| ==External links==
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| *[http://nsidc.org/arcticmet/glossary/psychrometric_tables.html Glossary definition of psychrometric tables] - National Snow and Ice Data Center
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| *[http://www.ems.psu.edu/~fraser/Bad/BadFAQ/BadCloudsFAQ.html Bad Clouds FAQ, PSU.edu]
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| *[http://itg1.meteor.wisc.edu/wxwise/relhum/rh.html Simulation of the indoors/outdoors change relationship]
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| {{DEFAULTSORT:Relative Humidity}}
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| [[Category:Psychrometrics]]
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| [[Category:Atmospheric thermodynamics]]
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| [[Category:Physical quantities|Humidity]]
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| [[cs:Vlhkost vzduchu#Relativní (poměrná) vlhkost vzduchu]]
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| [[de:Luftfeuchtigkeit#Relative Luftfeuchtigkeit]]
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| [[es:Humedad ambiental]]
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I know that all of you have watched the term calorie several instances over by this point, however how numerous of you have actually thought about what a calorie actually is? Probably not too countless. Whenever it certainly comes right down to it, calories are what help us achieve or goals of either losing or gaining weight, thus its important to know what a calorie really is plus how it effects these goals.
Don't give bmr calculator up when you blow your diet for the day. Tomorrow is a hot day for eating healthy foods. The worst thing you can do is beat yourself up with guilt following overeating or bingeing, plus you're most vulnerable whenever anxious or depressed.
The basal metabolic rate offers a superior baseline for minimum calories. Obviously, the right foods and exercise are crucial to the success. A diet of sugary foods and/or an exercise system consisting of endless walking on a treadmill usually create fat reduction difficult. However should you use a BMR because a beginning point, you will learn not to go below that level plus add food and exercise accordingly to create a calorie deficit.
Our bodies require 1,800 to 2,000 calories a day inside order to function properly. Some individuals want less or more, yet this might be the average. So don't try to drop a noticeable amount of calories at 1 time. There is not any have to place the body from starvation. If you do, a metabolic rate really slows down, that makes the entire process harder.
The initially part is the Basic stuff you do. This really is everything except the work outs you'll add to the daily routine. But you don't like to count each time we sneeze, despite that which does burn calories. If you are basically sedentary during the day, you are able to multiply a bmr by .2, plus that is a wise estimate of your Basic calories. Remember, for this step never count any extra exercise we are doing in order to lose weight. Count the little basic aspects you do each day. If you type all day long in an workplace, choose the sedentary level. If you chase small kids around the home all day, choose a higher level. Many of us that are struggling to get rid of fat may be starting at the sedentary level.
At any given time, 25 % of all men plus 33 % of all females are on several sort of formal diet in the United States. More than 55 percent gain back all of their fat and more than what they started with.1 Unfortunately, many diets are a one-size-fits-all approach. With any diet book you pick off the bookstore shelf, or any aged diets passed down by the perfect aunt, there are the same diet for everyone. Some of those are completely unsound nutritionally while others could be backed by good nutrition principles. Yet, even those with wise nutrition principles don't personalize their approach to fit every person's body makeup. They are sadly a one-size-fits-all dieting approach.
McArdle, William, Katch, Frank, and Katch, Victor. (1998). Exercise Physiology: Energy, Nutrition, and Human Performance. Baltimore, Maryland: Williams and Wilkins.