What does relative humidity characterize? Relative air humidity, amount of heat
Air humidity is an important characteristic environment. But not everyone fully understands what is meant by the weather reports. And absolute humidity- these are 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. Their total 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.
To more accurately determine 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 it to know the exact amount of 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 because the actual water vapor content remained the same, but the maximum possible value 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. If there is an excess, the excess water is not retained in a gaseous state and turns 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.
Recently, sensors that use changes in the electrical characteristics of certain materials have become increasingly used. To confirm the results and verify the instruments, there are reference settings.
When it comes to our health, knowledge of relative air humidity and the formula for determining it comes first. However, it is not necessary to know the exact formula, but it is good to at least have a general idea of what it is, why measure humidity in the house, and in what ways this can be done.
What should be the optimal humidity?
Humidity in a room where a person works, spends leisure time or sleeps is of particular importance. Our respiratory organs are designed in such a way that air that is too dry or saturated with water vapor is harmful to them. Therefore there are state standards, which regulate what the air humidity in the room should be.
Optimal humidity zone
In general, there are a dozen ways to control air humidity and bring it back to normal. This will create the most favorable conditions for studying, sleeping, playing sports, increasing performance and improving well-being.
There are many open bodies of water on Earth, from the surface of which water evaporates: oceans and seas occupy about 80% of the Earth's surface. Therefore, there is always water vapor in the air.
It is lighter than air because the molar mass of water (18 * 10-3 kg mol-1) is less than the molar mass of nitrogen and oxygen, of which air is mainly composed. Therefore, water vapor rises. At the same time, it expands, since in the upper layers of the atmosphere the pressure is lower than at the surface of the Earth. This process can be approximately considered adiabatic, because during the time it occurs, the heat exchange of steam with the surrounding air does not have time to occur.
1. Explain why the steam cools.
They do not fall because they soar in rising air currents, just as hang gliders soar (Fig. 45.1). But when the drops in the clouds become too large, they begin to fall: it's raining(Fig. 45.2).
We feel comfortable when the water vapor pressure at room temperature (20 ºC) is about 1.2 kPa.
2. What part (in percentage) is the indicated pressure of the saturated vapor pressure at the same temperature?
Clue. Use the table of saturated water vapor pressure values at various temperatures. It was given in the previous paragraph. We provide a more detailed table here.
You have now found the relative humidity. Let's define it.
Relative air humidity φ is the ratio of the partial pressure p of water vapor to the pressure pн of saturated vapor at the same temperature, expressed as a percentage:
φ = (p/pн) * 100%. (1)
Comfortable conditions for humans correspond to a relative humidity of 50-60%. If the relative humidity is significantly lower, the air seems dry to us, and if it is higher, it appears humid. When relative humidity approaches 100%, the air is perceived as damp. In this case, the puddles do not dry out, because the processes of water evaporation and steam condensation compensate each other.
So, the relative humidity of the air is judged by how close the water vapor in the air is to saturation.
If air with unsaturated water vapor in it is compressed isothermally, both the air pressure and the unsaturated vapor pressure will increase. But the water vapor pressure will only increase until it becomes saturated!
As the volume decreases further, the air pressure will continue to increase, but the water vapor pressure will remain constant - it will remain equal to the saturated vapor pressure at a given temperature. Excess steam will condense, that is, turn into water.
3. The vessel under the piston contains air whose relative humidity is 50%. The initial volume under the piston is 6 liters, the air temperature is 20 ºС. The air begins to be compressed isothermally. Assume that the volume of water formed from steam can be neglected compared to the volume of air and steam.
a) What will be the relative humidity when the volume under the piston becomes 4 liters?
b) At what volume under the piston will the steam become saturated?
c) What is the initial mass of the steam?
d) By how many times will the mass of steam decrease when the volume under the piston becomes equal to 1 liter?
e) What mass of water will condense?
2. How does relative humidity depend on temperature?
Let's consider how the numerator and denominator in formula (1), which determines the relative humidity of the air, change with increasing temperature.
The numerator is the pressure of unsaturated water vapor. It is directly proportional to the absolute temperature (recall that water vapor is well described by the equation of state of an ideal gas).
4. By what percentage does the pressure of unsaturated vapor increase when the temperature increases from 0 ºС to 40 ºС?
Now let's see how the saturated vapor pressure in the denominator changes.
5. How many times does the saturated vapor pressure increase when the temperature increases from 0 ºС to 40 ºС?
The results of these tasks show that as the temperature increases, the saturated vapor pressure increases much faster than the unsaturated vapor pressure. Therefore, the relative air humidity determined by formula (1) quickly decreases with increasing temperature. Accordingly, as the temperature decreases, the relative humidity increases. We'll look at this in more detail below.
The equation of state of an ideal gas and the table above will help you in completing the next task.
6. At 20 ºС, the relative humidity was 100%. The air temperature increased to 40 ºС, but the mass of water vapor remained unchanged.
a) What was the initial pressure of water vapor?
b) What was the final pressure of water vapor?
c) What is the saturated vapor pressure at 40 ºС?
d) What is the relative humidity in the final state?
e) How will this air be perceived by a person: as dry or as wet?
7. On a damp autumn day, the temperature outside is 0 ºС. The room temperature is 20 ºС, relative humidity is 50%.
a) Where is the partial pressure of water vapor greater: in the room or outside?
b) In which direction will water vapor flow if you open the window - into the room or out of the room?
c) What would the relative humidity in the room become if the partial pressure of water vapor in the room became equal to the partial pressure of water vapor outside?
8. Wet objects are usually heavier than dry ones: for example, a wet dress is heavier than a dry one, and damp firewood is heavier than dry ones. This is explained by the fact that the weight of the moisture contained in it is also added to the body’s own weight. But with air the opposite is true: humid air is lighter than dry air! How to explain this?
3. Dew point
As the temperature decreases, the relative humidity of the air increases (although the mass of water vapor in the air does not change).
When relative humidity reaches 100%, water vapor becomes saturated. (Under special conditions, supersaturated steam can be obtained. It is used in cloud chambers to detect traces (tracks) elementary particles on accelerators.) With a further decrease in temperature, condensation of water vapor begins: dew falls. Therefore, the temperature at which a given water vapor becomes saturated is called the dew point for that vapor.
9. Explain why dew (Fig. 45.3) usually falls in the early morning hours.
Let's consider an example of finding the dew point for air of a certain temperature with a given humidity. For this we need the following table.
10. A man with glasses entered the store from the street and discovered that his glasses were fogged up. We will assume that the temperature of the glass and the layer of air adjacent to it is equal to the air temperature outside. The air temperature in the store is 20 ºС, relative humidity 60%.
a) Is the water vapor in the layer of air adjacent to the glasses saturated?
b) What is the partial pressure of water vapor in the store?
c) At what temperature is the pressure of water vapor equal to the pressure of saturated vapor?
d) What could be the air temperature outside?
11. A transparent cylinder under the piston contains air with a relative humidity of 21%. The initial air temperature is 60 ºС.
a) To what temperature must the air be cooled at a constant volume in order for dew to form in the cylinder?
b) How many times must the volume of air be reduced at a constant temperature for dew to form in the cylinder?
c) The air is first compressed isothermally and then cooled at constant volume. Dew began to fall when the air temperature dropped to 20 ºC. How many times was the volume of air reduced compared to the initial volume?
12. Why is extreme heat more difficult to tolerate when humidity is high?
4. Humidity measurement
Air humidity is often measured with a psychrometer (Fig. 45.4). (From the Greek "psychros" - cold. This name is due to the fact that the readings of a wet thermometer are lower than those of a dry thermometer.) It consists of a dry and wet thermometer.
Wet bulb readings are lower than dry bulb readings because the liquid cools as it evaporates. The lower the relative humidity, the more intense the evaporation.
13. Which thermometer is located to the left in Figure 45.4?
So, according to the readings of thermometers, you can determine the relative humidity of the air. To do this, use a psychrometric table, which is often placed on the psychrometer itself.
To determine the relative humidity of the air, you need to:
– take thermometer readings (in this case 33 ºС and 23 ºС);
– find in the table a row corresponding to the dry thermometer readings and a column corresponding to the difference in thermometer readings (Fig. 45.5);
– at the intersection of the row and column, read the relative air humidity value.
14. Using the psychrometric table (Fig. 45.5), determine at what thermometer readings the relative air humidity is 50%.
Additional questions and tasks
15. In a greenhouse with a volume of 100 m3, the relative humidity must be maintained at least 60%. Early in the morning, at a temperature of 15 ºС, dew fell in the greenhouse. The temperature in the greenhouse during the day rose to 30 ºС.
a) What is the partial pressure of water vapor in a greenhouse at 15 ºС?
b) What is the mass of water vapor in the greenhouse at this temperature?
c) What is the minimum allowable partial pressure of water vapor in a greenhouse at 30 ºC?
d) What is the mass of water vapor in the greenhouse?
e) What mass of water must be evaporated in the greenhouse in order to maintain the required relative humidity in it?
16. On a psychrometer, both thermometers show the same temperature. What is the relative humidity? Explain your answer.
Word Moisture
The word Moisture in Dahl's dictionary
and. liquid in general: | phlegm, dampness; water. Vologa, oily liquid, fat, oil. Without moisture and heat, no vegetation, no life.
What does air humidity depend on?
There is now a foggy moisture in the air. Moist, filled with moisture, damp, wet, soggy, watery. Wet summer. Wet meadows, fingers, air. Damp place. Humidity g. dampness, mokrel, phlegm, wet state. To moisten something, to moisten, to make moist, to water or saturate with water. Moisture meter m.
hygrometer, a device showing the degree of air humidity.
The word Moisture in Ozhegov's dictionary
MOISTURE, -i, f. Dampness, water contained in something. Air saturated with moisture.
The word Moisture in Efremova’s dictionary
Accent: moisture
- Liquid, water or vapor contained in something
The word Moisture in the Vasmer Max dictionary
moisture
borrowed
from Tslav., Wed. old glory moisture (Sup.). See vologa.
The word Moisture in the dictionary of D.N. Ushakova
MOISTURE, moisture, plural. no, female (book). Dampness, water, fumes. Plants require a lot of moisture. The air is saturated with moisture.
The word Moisture in the Dictionary of Synonyms
alcohol, water, phlegm, humidity, liquid, dampness, raw material
The word Moisture in the dictionary Synonyms 4
water, phlegm, dampness
The word Moisture in the dictionary Complete accentuated paradigm according to A. A. Zaliznya
moisture,
moisture,
moisture,
moisture,
moisture,
moisture,
moisture,
moisture,
moisture,
moisture,
moisture,
moisture,
moisture
August's psychrometer consists of two mercury thermometers mounted on a stand or located in a common case.
The ball of one thermometer is wrapped in a thin cambric cloth, lowered into a glass of distilled water.
When using the August psychrometer, absolute humidity is calculated using the Rainier formula:
A = f-a(t-t1)H,
where A is absolute humidity; f is the maximum voltage of water vapor at the wet-bulb temperature (see.
table 2); a - psychrometric coefficient, t - dry thermometer temperature; t1 - wet thermometer temperature; H - barometric pressure at the time of determination.
If the air is completely motionless, then a = 0.00128. In the presence of weak air movement (0.4 m/s) a = 0.00110. Maximum and relative humidity are calculated as indicated on p.
What is air humidity? What does it depend on?
| Air temperature (°C) | Air temperature (°C) | Water vapor tension (mmHg) | Air temperature (°C) | Water vapor tension (mmHg) | |
| -20 - 15 -10 -5 -3 -4 0 +1 +2,0 +4,0 +6,0 +8,0 +10,0 +11,0 +12,0 |
0,94 1.44 2.15 3.16 3,67 4,256 4,579 4,926 5,294 6,101 7,103 8.045 9,209 9,844 10,518 |
+13,0 +14,0 +15,0 +16,0 +17,0 +18,0 +19,0 +20,0 +21,0 +22,0 +24,0 +25,0 +27,0 +30,0 +32,0 |
11,231 11,987 12,788 13,634 14,530 15,477 16.477 17,735 18,650 19,827 22,377 23,756 26,739 31,842 35,663 |
+35,0 +37,0 +40,0 +45,0 +55,0 +70,0 +100,0 |
42,175 47,067 55,324 71,88 118,04 233,7 760,0 |
Table 3.
Determination of relative humidity by readings
aspiration psychrometer (percentage) 
Table 4. Determination of relative air humidity according to the readings of dry and wet thermometers in the August psychrometer under normal conditions of calm and uniform motion air in the room at a speed of 0.2 m/s 
There are special tables for determining relative humidity (tables 3, 4).
More accurate readings are provided by the Assmann psychrometer (Fig. 3). It consists of two thermometers enclosed in metal tubes, through which air is evenly drawn in using a fan located at the top of the device.
The mercury reservoir of one of the thermometers is wrapped in a piece of cambric, which is moistened with distilled water using a special pipette before each determination. After the thermometer has been wetted, turn on the fan with the key and hang the device on a tripod.
After 4-5 minutes, record the readings of the dry and wet thermometers. Since moisture evaporates and heat is absorbed from the surface of a mercury ball, a wet thermometer, it will show a lower temperature. Absolute humidity is calculated using the Sprung formula: 
where A is absolute humidity; f is the maximum voltage of water vapor at the wet-bulb temperature; 0.5 - constant psychrometric coefficient (correction for air speed); t - dry bulb temperature; t1 - wet thermometer temperature; H - barometric pressure; 755 - average barometric pressure (determined according to table 2).
The maximum humidity (F) is determined using Table 2 based on the dry bulb temperature.
Relative humidity (R) is calculated using the formula:
where R is relative humidity; A - absolute humidity; F- maximum humidity at dry bulb temperature.
To determine fluctuations in relative humidity over time, a hygrograph device is used.
The device is designed similarly to a thermograph, but the receiving part of the hygrograph is a fat-free tuft of hair.
Rice. 3. Assmann aspiration psychrometer:
1 - metal tubes;
2 - mercury thermometers;
3 - holes for the outlet of sucked air;
4 - clip for hanging the psychrometer;
5 - pipette for wetting the wet thermometer.
The weather forecast for tomorrow
Compared to yesterday, it has become a little colder in Moscow; the ambient temperature has dropped from 17 °C yesterday to 16 °C today.
The weather forecast for tomorrow does not promise significant changes in temperature; it will remain at the same level from 11 to 22 degrees Celsius.
Relative humidity has increased to 75 percent and continues to rise. Atmospheric pressure over the past 24 hours has decreased slightly by 2 mm mercury, and became even lower.
Actual weather today
According to 2018-07-04 15:00 It's raining in Moscow, the wind is blowing lightly
Weather norms and conditions in Moscow
The weather patterns in Moscow are determined, first of all, by the location of the city.
The capital is located on the East European Plain, and warm and cold air masses move freely over the metropolis. The weather in Moscow is influenced by Atlantic and Mediterranean cyclones, which is why precipitation levels here are higher and winters are warmer than in cities at this latitude.
The weather in Moscow reflects all the phenomena characteristic of a temperate continental climate. The relative instability of weather is expressed, for example, in cold winter, with sudden thaws, cold snap in summer, loss of large quantity precipitation. These and others weather conditions is by no means uncommon. In summer and autumn, fogs are often observed in Moscow, the cause of which lies partly in human activity; thunderstorms that occurred even in winter.
In June 1998, a severe squall killed eight people and injured 157 people. In December 2010, strong freezing rain, caused by the temperature difference at altitude and on the ground, turned the streets into a skating rink, and giant icicles and trees breaking under the weight of the ice fell on people, buildings and cars.
The minimum temperature in Moscow was recorded in 1940, it was −42.2°C, the maximum was +38.2°C recorded in 2010.
The average July temperature in 2010 was 26.1° - close to normal United Arab Emirates and Cairo. And in general, 2010 became a record holder for the number of temperature maximums: 22 daily records were set during the summer.
The weather in the center of Moscow and on the outskirts is not the same.
What and how does relative air humidity depend on?
The temperature in the central regions is higher; in winter the difference can be up to 5-10 degrees. It is interesting that official weather data in Moscow are provided from the weather station at the All-Russian Exhibition Center, located in the northeast of the city, and this is several degrees lower temperature values weather station on Balchug in the center of the metropolis.
Weather in other cities of the Moscow region›
Dry matter and moisture
Water is one of the most abundant substances on earth; it is a necessary condition for life and is included in all food products and materials.
Water, not being a nutrient itself, is vital as a stabilizer of body temperature and a carrier of nutrients ( nutrients) and digestive waste, a reagent and reaction medium in a number of chemical transformations, a stabilizer of the conformation of biopolymers and, finally, as a substance that facilitates the dynamic behavior of macromolecules, including the manifestation of their catalytic (enzymatic) properties.
Water is the most important component of food products.
It is present in a variety of plant and animal products as a cellular and extracellular component, as a dispersing medium and solvent, determining consistency and structure. Water affects appearance, taste and stability of the product during storage. Through physical interaction with proteins, polysaccharides, lipids and salts, water makes a significant contribution to the structure of food.
The total moisture content of a product indicates the amount of moisture in it, but does not characterize its involvement in chemical and biological changes in the product.
In ensuring its stability during storage, the ratio of free and bound moisture plays an important role.
Associated moisture- This is associated water, tightly bound to various components - proteins, lipids and carbohydrates due to chemical and physical bonds.
Free moisture– this is moisture that is not bound by a polymer and is available for biochemical, chemical and microbiological reactions to occur.
Using direct methods, moisture is extracted from the product and its quantity is determined; indirect (by drying, refractometry, by the density and electrical conductivity of the solution) - determine the content of dry substances (dry residue). Indirect methods also include methods based on the interaction of water with certain reagents.
Determination of moisture content drying to constant weight (arbitration method) based on the release of hygroscopic moisture from the object under study at a certain temperature.
Drying is carried out to constant weight or by accelerated methods at elevated temperatures for a given time.
Drying of samples sintered into a dense mass is carried out with calcined sand, the mass of which should be 2-4 times greater than the mass of the sample.
Sand gives the sample porosity, increases the evaporation surface, and prevents the formation of a crust on the surface, which makes it difficult to remove moisture. Drying is carried out in porcelain cups, aluminum or glass bottles for 30 minutes, at a certain temperature, depending on the type of product.
The mass fraction of dry substances (X,%) is calculated using the formula
where m is the mass of the bottle with a glass rod and sand, g;
m1 – mass of a bottle with a glass rod, sand and
weighed before drying, g;
m2 – mass of a bottle with a glass rod, sand and a sample
after drying, g.
Drying in a HF apparatus is carried out using infrared radiation in an apparatus consisting of two massive round or rectangular plates connected to each other (Figure 3.1).
Figure 3.1 – HF apparatus for determining humidity
1 – handle; 2 – top plate; 3 – control unit; 4 - bottom plate; 5 – electric contact thermometer
In working condition, a gap of 2-3 mm is established between the plates.
The temperature of the heating surface is controlled by two mercury thermometers. To maintain a constant temperature, the device is equipped with a contact thermometer connected in series with the relay. The contact thermometer sets the desired temperature. The device is plugged in 20...25 minutes before drying begins to heat up to the set temperature.
A sample of the product is dried in a rotary paper bag measuring 20x14 cm for 3 minutes at a certain temperature, cooled in a desiccator for 2-3 minutes and quickly weighed to the nearest 0.01 g.
Humidity (X, %) is calculated by the formula
where m is the mass of the package, g;
m1 – mass of the bag with the sample before drying, g;
m2 – mass of the package with dried sample, g.
Refractometric method used for production control when determining the content of dry substances in objects rich in sucrose: sweet dishes, drinks, juices, syrups.
The method is based on the relationship between the refractive index of the object under study or an aqueous extract from it and the concentration of sucrose.
Air humidity
The refractive index depends on temperature, so measurements are made after thermostatting the prisms and the test solution.
The mass of dry substances (X, g) for drinks with sugar is calculated using the formula
where a is the mass for dry substances, determined
refractometric method, %;
P – volume of drink, cm3.
for syrups, fruit and berry and milk jelly, etc.
according to the formula
where a is the mass fraction of dry substances in the solution, %;
m1 – mass of dissolved sample, g;
m – weight of the sample, g.
In addition to these common methods for determining dry substances, a number of other methods are used to determine the content of both free and bound moisture
Differential scanning colorimetry.
If the sample is cooled to a temperature below 0°C, the free moisture will freeze, but the bound moisture will not. By heating a frozen sample in a colorimeter, the heat consumed when the ice melts can be measured.
Non-freezing water is defined as the difference between total water and freezing water.
Dielectric measurements. The method is based on the fact that at 0°C the dielectric constants of water and ice are approximately equal. But if some of the moisture is bound, then its dielectric properties should differ greatly from the dielectric properties of bulk water and ice.
Heat capacity measurement.
The heat capacity of water is greater than the heat capacity of ice, because As the temperature in water increases, hydrogen bonds break. This property is used to study the mobility of water molecules.
The value of heat capacity, depending on its content in polymers, provides information about the amount of bound water. If at low concentrations water is specifically bound, then its contribution to the heat capacity is small. In the region of high humidity values, it is mainly determined by free moisture, the contribution of which to the heat capacity is approximately 2 times greater than that of ice.
Nuclear magnetic resonance (NMR). The method consists of studying the mobility of water in a stationary matrix.
In the presence of free and bound moisture, two lines are obtained in the NMR spectrum instead of one for bulk water.
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SEE MORE:
Air humidity. Units. Impact on aviation operations.
Water is a substance that can simultaneously be in different states of aggregation at the same temperature: gaseous (water vapor), liquid (water), solid (ice). These conditions are sometimes called phase state of water.
Under certain conditions, water can change from one (phase) state to another. So water vapor can go into a liquid state (condensation process), or, bypassing the liquid phase, go into a solid state - ice (sublimation process).
In turn, water and ice can go into a gaseous state - water vapor (evaporation process).
Humidity refers to one of the phase states - water vapor contained in the air.
It enters the atmosphere by evaporation from water surfaces, soil, snow, and vegetation.
As a result of evaporation, part of the water turns into a gaseous state, forming a layer of steam above the evaporating surface.
Relative humidity
This steam is transported by air currents in vertical and horizontal directions.
The evaporation process continues until the amount of water vapor above the evaporating surface reaches complete saturation, that is maximum quantity possible in a given volume at constant air pressure and temperature.
The amount of water vapor in the air is characterized by the following units:
Water vapor pressure.
Like any other gas, water vapor has its own elasticity and exerts pressure, which is measured in mmHg or hPa. The amount of water vapor in these units is indicated: actual – e, saturating - E. At weather stations, by measuring elasticity in hPa, observations of the humidity of water vapor are made.
Absolute humidity. Represents the amount of water vapor in grams contained in one cubic meter of air (g/).
Letter A– is indicated by the actual quantity, by the letter A– saturating space. Absolute humidity is close in value to the elasticity of water vapor, expressed in mm Hg, but not in hPa, at a temperature of 16.5 C e And A are equal to each other.
Specific humidity represents the amount of water vapor in grams contained in one kilogram of air (g/kg).
Letter q — is indicated by the actual quantity, letter Q - saturating space. Specific humidity is a convenient value for theoretical calculations, since it does not change with heating, cooling, compression and expansion of air (unless condensation occurs). The specific humidity value is used for all kinds of calculations.
Relative humidity represents the percentage of the amount of water vapor contained in the air to the amount that would saturate a given space at the same temperature.
Relative humidity is indicated by the letter r.
According to definition
r=e/E*100%
The amount of water vapor that saturates a space can vary, depending on how many vapor molecules can escape from the evaporating surface.
The saturation of air with water vapor depends on the air temperature; the higher the temperature, the more quantity water vapor, and the lower the temperature, the less it is.
Dew point– this is the temperature to which the air must be cooled so that the water vapor contained in it reaches complete saturation (at r = 100%).
The difference between air temperature and dew point temperature (T-Td) is called dew point deficiency.
It shows how much the air needs to be cooled so that the water vapor it contains reaches a saturation state.
With a small deficit, air saturation occurs much faster than with a large saturation deficit.
The amount of water vapor also depends on the state of aggregation of the evaporating surface and its curvature.
At the same temperature, the amount of saturating steam is greater over one and less over ice (ice has strong molecules).
At the same temperature, the amount of vapor will be greater over a convex surface (droplet surface) than over a flat evaporating surface.
All these factors play a big role in the formation of fog, clouds and precipitation.
A decrease in temperature leads to the saturation of the water vapor in the air, and then to the condensation of this vapor.
Air humidity has a significant impact on the weather, determining flight conditions. The presence of water vapor leads to the formation of fog, haze, cloudiness, complicating the flight of thunderstorms, and freezing rain.
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 the maximum amount of 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. The life activity of animals and plant organisms is 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 a moisture content of 5% and 10% will contain different amounts of 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 quantity characterizing the content of water vapor in different parts of the 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 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. IN subtropical 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(°С) | -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 regions (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, there is about 28.5 kg of water vapor in the air above each square meter of the earth's surface.
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
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 are talking 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 humidity level atmospheric air is not the same - both in a vertical section and in a horizontal (latitudinal) one. 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. It is a relative value that shows how much moisture the air contains 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 state 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.
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