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 is the ratio of absolute humidity to maximum humidity, 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 a relative humidity value.
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 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 The 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.
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.
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).
Units of measurementabsolute 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: .
Units of measurementrelative 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 will consider condensinghygrometer(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, intense 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, instruments 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 allows you 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.
References
- 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
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 at the 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 precisely 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 increased 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.
In conclusion...
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.
The weight, or more precisely the mass, of water vapor contained in 1 m3 of air is called absolute air humidity. In other words, this water vapor density in the air. At the same temperature, air can absorb a very certain amount of water vapor and reach a state of complete saturation. in the state of its saturation is called moisture capacity.
The moisture capacity of air increases sharply with increasing temperature. Magnitude ratio absolute air humidity at a given temperature to the value of its moisture capacity at the same temperature is called relative air humidity.
To determine temperature and relative air humidity They use a special device - a psychrometer. The psychrometer consists of two thermometers. The ball of one of them is moistened with a gauze cover, the end of which is lowered into a vessel with water. The other thermometer remains dry and shows the ambient temperature. A wet thermometer shows a lower temperature than a dry thermometer, since the moisture from the gauze requires a certain amount of heat. The wet bulb temperature is called cooling limit. The difference between the readings of dry and wet bulb thermometers is called psychrometric difference.
There is a certain relationship between the magnitude of the psychrometric difference and the relative one. The greater the psychrometric difference at a given air temperature, the lower the relative humidity of the air and the more moisture the air can absorb. When the difference is equal to zero, the air is saturated and further evaporation of moisture in such air doesn't happen.
Absolute humidity
(f)- this is the amount of water vapor actually contained in 1m3 of air:f= m (mass of water vapor contained in the air)/ V (volume)
Commonly used unit of absolute humidity: (f)= g/m 3
Relative humidity
Relative humidity: 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
What is dew point
Air humidity is an important environmental characteristic. But not everyone fully understands what is meant by the weather reports. and absolute humidity is related concepts. It is not possible to understand the essence of one without understanding the other.
Air and moisture
Air contains a mixture of substances that are in a gaseous state. Primarily it is nitrogen and oxygen. Them in general composition(100%) contains approximately 75% and 23% by weight, respectively. About 1.3% is argon, less than 0.05% is carbon dioxide. The remainder (the missing amount is about 0.005% in total) is made up of xenon, hydrogen, krypton, helium, methane and neon.
There is also some amount of moisture in the air at all times. It enters the atmosphere after the evaporation of water molecules from the world's oceans and from moistened soil. In a confined space, its content may differ from external environment and depends on the availability of additional sources of income and consumption.
For more precise definition physical characteristics and quantitative indicators, two concepts are used: relative humidity and absolute humidity. In everyday life, excess is formed when drying clothes and during cooking. People and animals excrete it through breathing, plants as a result of gas exchange. In production, changes in the water vapor ratio can be associated with condensation due to temperature changes.
Absolute and features of the use of the term
How important is knowledge? exact quantity water vapor in the atmosphere? Based on these parameters, weather forecasts, the possibility of precipitation and its volume, and the paths of movement of fronts are calculated. Based on this, the risks of cyclones and especially hurricanes, which can pose a serious danger to the region, are determined.
What is the difference between the two concepts? What they have in common is that both relative humidity and absolute humidity measure the amount of water vapor in the air. But the first indicator is determined by calculation. The second one can be measured by physical methods with the result in g/m3.
However, with changes in ambient temperature, these indicators change. It is known that the air can contain a maximum certain amount of water vapor - absolute humidity. But for +1°C and +10°C modes these values will be different.
The dependence of the quantitative content of water vapor in the air on temperature is displayed in the relative humidity indicator. It is calculated using the formula. The result is expressed as a percentage (an objective indicator of the maximum possible value).
Influence of environmental conditions
How will the absolute and relative air humidity change with an increase in temperature, for example, from +15°C to +25°C? As it increases, the water vapor pressure increases. This means that more water molecules will fit in a unit volume (1 cubic meter). Consequently, absolute humidity also increases. The relative value will decrease. This is explained by the fact that the actual water vapor content remained at the same level, and the maximum possible meaning increased. According to the formula (dividing one by the other and multiplying the result by 100%), the result will be a decrease in the indicator.
How will absolute and relative humidity change as the temperature decreases? What happens when you decrease from +15°C to +5°C? The absolute humidity will decrease. Accordingly, in 1 cubic meter. The maximum amount of air mixture of water vapor that can fit is smaller. Calculation using the formula will show an increase in the final indicator - the percentage of relative humidity will increase.
Meaning for humans
If there is an excess amount of water vapor, you feel stuffy; if there is too little, you feel dry skin and thirst. Obviously, the humidity of damp air is higher. In case of excess excess water is not retained in a gaseous state and passes into a liquid or solid medium. In the atmosphere it rushes down, this is manifested by precipitation (fog, frost). Indoors, a layer of condensation forms on interior items, and there is dew on the surface of the grass in the morning.
An increase in temperature is easier to tolerate in a dry room. However, the same regime, but at a relative humidity above 90%, causes rapid overheating of the body. The body fights this phenomenon in the same way - heat is released through sweat. But in dry air it quickly evaporates (dries out) from the surface of the body. In a humid environment this practically does not happen. The most suitable (comfortable) mode for a person is 40-60%.
Why is this necessary? In bulk materials in wet weather, the dry matter content per unit volume decreases. This difference is not so significant, but with large volumes it can “result” into a really detectable amount.
Products (grain, flour, cement) have an acceptable humidity threshold at which they can be stored without loss of quality or technological properties. Therefore, monitoring indicators and maintaining them at an optimal level is mandatory for storage facilities. By reducing humidity in the air, it is achieved in reducing it in products.
Devices
In practice, actual humidity is measured by hygrometers. Previously, there were two approaches. One is based on changes in the elongability of hair (human or animal). The other is based on the difference in thermometer readings in a dry and humid environment (psychrometric).
In a hair hygrometer, the pointer of the mechanism is connected to a hair stretched on a frame. It changes depending on the humidity of the surrounding air physical properties. The needle deviates from the reference value. Its movements are tracked on a scale.
Relative humidity and absolute air humidity are known to depend on ambient temperature. This feature is used in a psychrometer. When determining, readings are taken from two adjacent thermometers. The flask of one (dry) is under normal conditions. In the other (wet) it is shrouded in a wick, which is connected to a reservoir of water.
In such conditions, the thermometer measures the environment taking into account the evaporating moisture. And this indicator depends on the amount of water vapor in the air. The difference in readings is determined. The relative humidity value is determined using special tables.
IN lately Sensors that use changes in the electrical characteristics of certain materials are more widely used. To confirm the results and verify the instruments, there are reference settings.