To characterize air humidity, the following values are used: absolute, maximum and relative humidity, saturation deficit, dew point.
Absolute humidity is the amount of water vapor in grams currently contained in 1 m³ of air.
Maximum humidity - this is the amount of water vapor in grams contained in 1 m³ of air at the moment of complete saturation.
Relative humidity called relation absolute humidity to the maximum, expressed as a percentage.
Deficiency of saturation is the difference between maximum and absolute humidity.
Dew point - temperature at which absolute humidity is equal to maximum.
When assessing air humidity highest value has the value relative humidity.
Relative humidity can be measured with a hygrometer or psychrometer. The basis hygrometer consists of defatted human hair connected through a block with an arrow that moves along a scale. Hair lengthens when air humidity increases and becomes shorter when it decreases.
Psychrometers consist of two identical thermometers (mercury or alcohol), the reservoir of one of them is covered with a cloth, which is pre-moistened with distilled water. As the water evaporates, the tank cools. The temperature difference is used to judge air humidity, since the intensity of evaporation depends on the degree of saturation of the surrounding air with water vapor. Two types of psychrometers are used: stationary (Augusta) and aspiration (Assmann).
Psychrometer Augusta used in stationary conditions (on weather stations, in hospitals), placing it in places where the device is not exposed to heat and wind.
Absolute humidity is calculated using the Regnault formula:
K = f - a (t c - t c) x B,
Where TO- absolute humidity, mm Hg;
f- maximum air humidity at wet-bulb temperature (determined from Table 1.6);
a- psychometric coefficient equal to 0.0001;
t s - dry bulb temperature;
t in - wet bulb temperature;
B- atmospheric pressure at the time of observation, mm Hg.
In the Assmann psychrometer Thermometer reservoirs are protected by double metal screens from exposure to radiant heat. There are ventilation channels around the tanks through which air is sucked in at a constant speed (4 m/s). To measure humidity, a thermometer wrapped in a cloth is moistened with distilled water, then the fan spring is wound up and the device is placed at the desired point. The readings of dry and wet thermometers are recorded 4 - 5 minutes after the fan starts.
One of the very important indicators in our atmosphere. It can be either absolute or relative. How is absolute humidity measured and what formula should be used for this? You can find out about this by reading our article.
Air humidity - what is it?
What is humidity? This is the amount of water contained in any physical body or environment. This indicator directly depends on the very nature of the medium or substance, as well as on the degree of porosity (if we're talking about about solids). In this article we will talk about a specific type of humidity - air humidity.
From a chemistry course, we all know very well that atmospheric air consists of nitrogen, oxygen, carbon dioxide and some other gases, which make up no more than 1% of the total mass. But in addition to these gases, the air also contains water vapor and other impurities.
Air humidity refers to the amount of water vapor that this moment(and in this place) is contained in the air mass. At the same time, meteorologists distinguish two of its values: absolute and relative humidity.
Air humidity is one of the most important characteristics of the Earth's atmosphere, which affects the nature of local weather. It is worth noting that the amount of humidity in the atmospheric air is not the same - both in the vertical section and in the horizontal (latitudinal) section. So, if in subpolar latitudes the relative air humidity (in the lower layer of the atmosphere) is about 0.2-0.5%, then in tropical latitudes it is up to 2.5%. Next, we will find out what absolute and relative air humidity is. We will also consider what difference exists between these two indicators.
Absolute humidity: definition and formula
Translated from Latin, the word absolutus means “full”. Based on this, the essence of the concept of “absolute air humidity” becomes obvious. This is a value that shows how many grams of water vapor are actually contained in one cubic meter of a particular air mass. As a rule, this indicator is denoted by the Latin letter F.
G/m 3 is a unit of measurement in which absolute humidity is calculated. The formula for calculating it is as follows:
In this formula, the letter m denotes the mass of water vapor, and the letter V denotes the volume of a specific air mass.
The value of absolute humidity depends on several factors. First of all, these are air temperature and the nature of advection processes.
Relative humidity
Now let's look at what relative humidity is. This is a relative value that shows how much moisture is contained in the air in relation to the maximum possible amount of water vapor in that air mass at a particular temperature. Relative air humidity is measured as a percentage (%). And it is this percentage that we can often find out in weather forecasts and weather reports.
It is also worth mentioning such an important concept as dew point. This is the phenomenon of the maximum possible saturation of the air mass with water vapor (the relative humidity at this moment is 100%). In this case, excess moisture condenses and forms precipitation, fog or clouds.
Methods for measuring air humidity
Women know that they can detect the increase in humidity in the atmosphere with the help of their voluminous hairstyle. However, there are other, more accurate methods and technical devices. These are a hygrometer and a psychrometer.
The first hygrometer was created back in the 17th century. One of the types of this device is precisely based on the properties of hair to change its length with changes in environmental humidity. However, today there are also electronic hygrometers. A psychrometer is a special device that contains a wet and dry thermometer. Based on the difference in their indicators, air humidity is determined at a specific point in time.
Air humidity as an important environmental indicator
It is considered optimal for human body is the relative air humidity of 40-60%. Humidity indicators also greatly influence a person’s perception of air temperature. So, with low humidity, it seems to us that the air is much colder than in reality (and vice versa). That is why in the tropical and equatorial latitudes of our planet, travelers experience the heat and heat so hard.
Today there are special humidifiers and dehumidifiers that help a person regulate air humidity in enclosed spaces.
Finally...
Thus, absolute air humidity is the most important indicator that gives us an idea of the condition and characteristics of air masses. In this case, you need to be able to distinguish this value from relative humidity. And if the latter shows the proportion of water vapor (in percent) that is present in the air, then absolute humidity is the actual amount of water vapor in grams in one cubic meter of air.
In this lesson, the concept of absolute and relative air humidity will be introduced, terms and quantities associated with these concepts will be discussed: saturated steam, dew point, instruments for measuring humidity. During the lesson we will get acquainted with the tables of density and saturated vapor pressure and the psychrometric table.
For humans, humidity is a very important parameter. environment, because our body reacts very actively to its changes. For example, a mechanism for regulating the functioning of the body, such as sweating, is directly related to the temperature and humidity of the environment. At high humidity, the processes of evaporation of moisture from the surface of the skin are practically compensated by the processes of its condensation and the removal of heat from the body is disrupted, which leads to disturbances in thermoregulation. At low humidity, moisture evaporation processes prevail over condensation processes and the body loses too much fluid, which can lead to dehydration.
The amount of humidity is important not only for humans and other living organisms, but also for the flow of technological processes. For example, due to the known property of water to conduct electric current, its content in the air can seriously affect the correct operation of most electrical appliances.
In addition, the concept of humidity is the most important evaluation criterion weather conditions, which everyone knows from weather forecasts. It is worth noting that if we compare humidity at different times of the year in our usual climatic conditions, then it is higher in summer and lower in winter, which is associated, in particular, with the intensity of evaporation processes at different temperatures.
Main characteristics humid air are:
- density of water vapor in the air;
- relative humidity.
Air is a composite gas and contains many different gases, including water vapor. To estimate its amount in the air, it is necessary to determine what mass water vapor has in a certain allocated volume - this value is characterized by density. The density of water vapor in the air is called absolute humidity.
Definition.Absolute air humidity- the amount of moisture contained in one cubic meter of air.
Designationabsolute humidity: (as is the usual designation for density).
Unitsabsolute humidity: (in SI) or (for the convenience of measuring small amounts of water vapor in the air).
Formula calculations absolute humidity:
Designations:
Mass of steam (water) in air, kg (in SI) or g;
The volume of air containing the indicated mass of steam is .
On the one hand, absolute air humidity is an understandable and convenient value, since it gives an idea of the specific water content in the air by mass; on the other hand, this value is inconvenient from the point of view of the susceptibility of humidity by living organisms. It turns out that, for example, a person does not feel the mass content of water in the air, but rather its content relative to the maximum possible value.
To describe such perception, the following quantity was introduced: relative humidity.
Definition.Relative humidity– a value indicating how far the steam is from saturation.
That is, the value of relative humidity, in simple words, shows the following: if the steam is far from saturation, then the humidity is low, if it is close, it is high.
Designationrelative humidity: .
Unitsrelative humidity: %.
Formula calculations relative humidity:
Designations:
Water vapor density (absolute humidity), (in SI) or ;
Density of saturated water vapor at a given temperature, (in SI) or .
As can be seen from the formula, it includes absolute humidity, with which we are already familiar, and saturated vapor density at the same temperature. The question arises: how to determine the latter value? There are special devices for this. We'll consider condensationhygrometer(Fig. 4) - a device that is used to determine the dew point.
Definition.Dew point- the temperature at which steam becomes saturated.
Rice. 4. Condensation hygrometer ()
An easily evaporating liquid, for example, ether, is poured into the container of the device, a thermometer (6) is inserted, and air is pumped through the container using a bulb (5). As a result of increased air circulation, intensive evaporation of ether begins, the temperature of the container decreases because of this and dew (droplets of condensed steam) appears on the mirror (4). At the moment dew appears on the mirror, the temperature is measured using a thermometer; this temperature is the dew point.
What to do with the obtained temperature value (dew point)? There is a special table in which data is entered - what density of saturated water vapor corresponds to each specific dew point. It is worth noting a useful fact that as the dew point increases, the value of the corresponding saturated vapor density also increases. In other words, the warmer the air, the greater the amount of moisture it can contain, and vice versa, the colder the air, the lower the maximum vapor content in it.
Let us now consider the principle of operation of other types of hygrometers, devices for measuring humidity characteristics (from the Greek hygros - “wet” and metreo - “I measure”).
Hair hygrometer(Fig. 5) - a device for measuring relative humidity, in which hair, for example human hair, acts as an active element.
The action of a hair hygrometer is based on the property of defatted hair to change its length when air humidity changes (with increasing humidity, the length of the hair increases, with decreasing it decreases), which makes it possible to measure relative humidity. The hair is stretched over a metal frame. The change in hair length is transmitted to the arrow moving along the scale. It should be remembered that a hair hygrometer does not give exact values relative humidity, and is used primarily for domestic purposes.
A more convenient and accurate device for measuring relative humidity is a psychrometer (from the ancient Greek ψυχρός - “cold”) (Fig. 6).
A psychrometer consists of two thermometers, which are fixed on a common scale. One of the thermometers is called a wet thermometer because it is wrapped in cambric fabric, which is immersed in a reservoir of water located on the back of the device. Water evaporates from the wet fabric, which leads to cooling of the thermometer, the process of reducing its temperature continues until the stage is reached until the steam near the wet fabric reaches saturation and the thermometer begins to show the dew point temperature. Thus, the wet bulb thermometer shows a temperature less than or equal to the actual ambient temperature. The second thermometer is called a dry thermometer and shows the real temperature.
On the body of the device, as a rule, there is also a so-called psychrometric table (Table 2). Using this table, you can determine the relative humidity of the surrounding air from the temperature value shown by the dry bulb thermometer and from the temperature difference between the dry and wet bulb bulbs.
However, even without such a table at hand, you can approximately determine the humidity value using the following principle. If the readings of both thermometers are close to each other, then the evaporation of water from the humid one is almost completely compensated by condensation, i.e., the air humidity is high. If, on the contrary, the difference in thermometer readings is large, then evaporation from the wet fabric prevails over condensation and the air is dry and humidity is low.
Let us turn to the tables that allow us to determine the characteristics of air humidity.
|
Temperature, |
Pressure, mm. rt. Art. |
Vapor density |
Table 1. Density and pressure of saturated water vapor
Let us note once again that, as stated earlier, the value of the density of saturated steam increases with its temperature, the same applies to the pressure of saturated steam.
Table 2. Psychometric table
Let us recall that relative humidity is determined by the value of the dry bulb readings (first column) and the difference between the dry and wet readings (first row).
In today's lesson we learned about an important characteristic of air - its humidity. As we have already said, humidity decreases in the cold season (winter) and increases in the warm season (summer). It is important to be able to regulate these phenomena, for example, if it is necessary to increase humidity, place the room in winter time several reservoirs of water to enhance evaporation processes, however, this method will only be effective at the appropriate temperature, which is higher than outside.
In the next lesson we will look at what gas work is and the principle of operation of an internal combustion engine.
Bibliography
- Gendenshtein L.E., Kaidalov A.B., Kozhevnikov V.B. / Ed. Orlova V.A., Roizena I.I. Physics 8. - M.: Mnemosyne.
- Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
- Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.
- Internet portal “dic.academic.ru” ()
- Internet portal “baroma.ru” ()
- Internet portal “femto.com.ua” ()
- Internet portal “youtube.com” ()
Homework
What is this article about?
Definition
In addition to relative air humidity, there is also such a value as absolute humidity. The amount of water vapor per unit volume of air is called absolute humidity. Since mass is taken as a unit of measurement of quantity, and its values for steam in a cubic meter of air are small, it was customary to measure absolute humidity in g/m³. These indicators vary from parts of a unit of measurement to more than 30 g/m³, depending on the time of year and geographical location surface above which humidity is measured.
Absolute humidity is the main indicator characterizing the state of the air, and great importance To determine its properties, it is necessary to compare humidity with ambient temperature, since these parameters are interrelated. For example, when the temperature drops, water vapor reaches a state of saturation, after which the condensation process begins. The temperature at which this occurs is called the dew point.
Instruments for determining absolute humidity
Determination of the absolute humidity value is based on its calculations based on thermometer readings. In particular, according to the readings of the Augustus psychrometer, consisting of two mercury thermometers - one of which is dry and the other wet (picture A in the picture). The evaporation of water from the surface indirectly in contact with the tip of the thermometer leads to a decrease in its readings. The difference between the readings of both thermometers is the basis of August's formula, which determines absolute humidity. Air flows and thermal radiation can influence the error in such measurements.
The aspiration psychrometer proposed by Assmann is more accurate (picture B). It is designed with a protective tube that limits the effects of thermal radiation and an aspiration fan that creates a stable air flow. Absolute humidity is determined by a formula that reflects its dependence on thermometer readings and barometric pressure during this period of time.
Absolute Humidity Measurement Value
Monitoring absolute humidity values is necessary in meteorology, since these readings play a large role in predicting possible precipitation. Psychrometers are also used in mines. The need for constant monitoring of absolute humidity in many automation systems is a prerequisite for the development of more modern meters. These are electronic sensors that take the necessary measurements, analyze the readings and display the already calculated value of absolute humidity.
General information
Humidity depends on the nature of the substance, and in solids, in addition, on the degree of fineness or porosity. The content of chemically bound, so-called constitutional water, for example, hydroxides, which are released only during chemical decomposition, as well as crystalline hydrate water is not included in the concept of humidity.
Units of measurement and features of the definition of humidity
- Humidity is usually characterized by the amount of water in a substance, expressed as a percentage (%) of the original mass of the wet substance ( mass humidity) or its volume ( volumetric humidity).
- Humidity can also be characterized by moisture content, or absolute humidity- the amount of water per unit mass of the dry part of the material. This determination of moisture content is widely used to assess the quality of wood.
This value cannot always be measured accurately, because in some cases it is not possible to remove all the unconstitutional water and weigh the item before and after this operation.
- Relative humidity characterizes the moisture content relative to maximum quantity moisture that can be contained in a substance in a state of thermodynamic equilibrium. Relative humidity is usually measured as a percentage of maximum.
Determination methods
Karl Fischer titrator.
Establishing the degree of humidity of many products, materials, etc. has important. Only at a certain humidity are many bodies (grain, cement, etc.) suitable for the purpose for which they are intended. Animal life and plant organisms possible only within certain limits of humidity and relative air humidity. Humidity can introduce a significant error in the weight of an item. Kilograms of sugar or grain with moisture content of 5% and 10% will contain different quantities dry sugar or grain.
Humidity measurement is determined by drying the moisture and Karl Fischer titration of the moisture. These methods are primary. In addition to them, many others have been developed, which are calibrated based on the results of moisture measurements using primary methods and standard humidity samples.
Air humidity
Air humidity is a value characterizing the content of water vapor in various parts Earth's atmosphere.
Humidity - the content of water vapor in the air; one of the most significant characteristics of weather and climate.
Air humidity in the earth's atmosphere varies widely. Yes, y earth's surface The water vapor content in the air averages from 0.2% by volume in high latitudes up to 2.5% in the tropics. Vapor pressure in polar latitudes in winter is less than 1 mb (sometimes only hundredths of mb) and in summer below 5 mb; in the tropics it increases to 30 mb, and sometimes more. On Saturday tropical deserts vapor pressure is reduced to 5-10 mb.
Absolute air humidity (f) is the amount of water vapor actually contained in 1m³ of air:
f = (mass of water vapor in the air)/(volume of moist air)
Commonly used unit of absolute humidity: (f) = g/m³
Relative air humidity (φ) is the ratio of its current absolute humidity to the maximum absolute humidity at a given temperature (see table)
| t(°C) | -30 | -20 | -10 | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 70 | 80 | 90 | 100 |
| f max (g/m³) | 0,29 | 0,81 | 2,1 | 4,8 | 9,4 | 17,3 | 30,4 | 51,1 | 83,0 | 130 | 198 | 293 | 423 | 598 |
φ = (absolute humidity)/(maximum humidity)
Relative humidity is usually expressed as a percentage. These quantities are related to each other by the following relationship:
φ = (f×100)/fmax
Relative humidity is very high in equatorial zone(annual average up to 85% or more), as well as in polar latitudes and in winter inside continents of mid-latitudes. In summer, high relative humidity is characteristic of monsoon regions. Low relative humidity values are observed in subtropical and tropical deserts and in winter in monsoon areas(up to 50% and below).
Humidity decreases quickly with altitude. At an altitude of 1.5-2 km, vapor pressure is on average half that of the earth's surface. The troposphere accounts for 99% of the atmospheric water vapor. On average over each square meter The air on the earth's surface contains about 28.5 kg of water vapor.
Literature
Usoltsev V. A. Measuring air humidity, L., 1959.
Gas moisture measurement values
The following quantities are used to indicate the moisture content in the air:
Absolute air humidity is the mass of water vapor contained in a unit volume of air, i.e. density of water vapor contained in the air, [g/m³]; in the atmosphere ranges from 0.1-1.0 g/m³ (in winter over the continents) to 30 g/m³ or more (in the equatorial zone); maximum air humidity (saturation limit) the amount of water vapor that can be contained in the air at a certain temperature in thermodynamic equilibrium (the maximum value of air humidity at a given temperature), [g/m³]. As air temperature rises, its maximum humidity increases; vapor pressure the pressure exerted by the water vapor contained in the air (water vapor pressure as part of atmospheric pressure), [Pa]; humidity deficit is the difference between saturated vapor pressure and vapor pressure [Pa], that is, between maximum and absolute air humidity [g/m³]; relative air humidity is the ratio of vapor pressure to saturated vapor pressure, that is, absolute air humidity to maximum [% relative humidity]; dew point temperature at which the gas is saturated with water vapor °C. The relative humidity of the gas is 100%. With a further influx of water vapor or when air (gas) is cooled, condensation appears. Thus, although dew does not fall at temperatures of −10 or −50°C, it does