The influence of humid air parameters on the human body. The influence of air humidity on human life
The concept of air humidity is defined as the actual presence of water particles in a certain physical environment, including the atmosphere. In this case, it is necessary to distinguish between absolute and relative humidity: in the first case we are talking about the pure percentage amount of moisture. According to the law of thermodynamics, the maximum content of water molecules in the air is limited. The maximum permissible level determines the relative humidity and depends on a number of factors:
- Atmosphere pressure;
- air temperature;
- presence of small particles (dust);
- level of chemical pollution;
The generally accepted measurement measure is percentage, and the calculation is carried out using a special formula, which will be discussed below.
Absolute humidity is measured in grams per cubic centimeter, which for convenience are also converted into percentages. As altitude increases, the amount of moisture may increase depending on the region, but once a certain ceiling is reached (approximately 6-7 kilometers above sea level), the humidity drops to around zero values. Absolute humidity is considered one of the main macroparameters: planetary climate maps and zones are compiled on its basis.
Humidity level detection
(A psychometer device - it is used to determine humidity by the temperature difference between a dry and wet thermometer)
Humidity by absolute ratio is determined using special instruments that determine the percentage of water molecules in the atmosphere. As a rule, daily fluctuations are negligible - this indicator can be considered static and does not reflect important climatic conditions. In contrast, relative humidity is subject to strong diurnal fluctuations and reflects the precise distribution of condensed moisture, its pressure and equilibrium saturation. This indicator is considered the main one and is calculated at least once a day.
Determination of relative air humidity is carried out using a complex formula that takes into account:
- current dew point;
- temperature;
- saturated steam pressure;
- various mathematical models;
In the practice of synoptic forecasts, a simplified approach is used when humidity is calculated approximately, taking into account the temperature difference and the dew point (the mark when excess moisture falls in the form of precipitation). This approach allows you to determine the required indicators with 90-95% accuracy, which is more than enough for everyday needs.
Dependence on natural factors
The content of water molecules in the air depends on the climatic characteristics of a particular region, weather conditions, atmospheric pressure and some other conditions. Thus, the highest absolute humidity is observed in tropical and coastal zones and reaches 5%. Relative humidity is further affected by fluctuations in a number of factors discussed earlier. During the rainy season with conditions of low atmospheric pressure, relative humidity levels can reach 85-95%. High pressure reduces the saturation of water vapor in the atmosphere, lowering its level accordingly.
An important feature of relative humidity is its dependence on the thermodynamic state. The natural equilibrium humidity is 100%, which, of course, is unattainable due to the extreme instability of the climate. Technogenic factors also influence fluctuations in atmospheric humidity. In megacities, there is increased evaporation of moisture from asphalt surfaces, simultaneously with the release of large amounts of suspended particles and carbon monoxide. This causes a strong decrease in humidity in most cities around the world.
Effect on the human body
The limits of atmospheric humidity that are comfortable for humans range from 40 to 70%. Prolonged stay in conditions of strong deviation from this norm can cause a noticeable deterioration in well-being, up to the development of pathological conditions. It should be noted that a person is especially sensitive to excessively low humidity, experiencing a number of characteristic symptoms:
- irritation of mucous membranes;
- development of chronic rhinitis;
- increased fatigue;
- deterioration of the skin condition;
- decreased immunity;
Among the negative effects of high humidity, one can note the risk of developing fungal and colds.
Humidity- the main environmental parameter, along with temperature and air speed, which affects the evaporation of water from a wet or wetted surface. The effect of humidity on the human body is especially noticeable during breathing: passing through the upper respiratory tract into the bronchi, the air heats up upon contact with the walls of the vessels of the respiratory tract . These walls are connected to the mucous membrane, which under normal conditions is covered with moisture.
Passing through the respiratory tract into the bronchi, the air is heated and humidified, almost reaching a state of saturation. The exhaled air becomes warm and humid, this is indicated by condensation of water vapor when breathing in a cold room or deposition in the form of droplets on cold surfaces.
The mucous membrane of the airways filters the air, freeing it from various impurities, bacteria, and viruses. The inner surface of the bronchi is covered with ciliated epithelium, which traps foreign particles. These particles are eliminated from the body through secretions, which are removed only if their viscosity with respect to water is not too high. If the humidity is low, then the evaporation of water from the mucous membrane will be too intense, which will lead to its drying out. The filtering ability of the ciliated epithelium in the bronchi also decreases and dirt contained in the air easily enters the respiratory tract. The feeling of dryness of the mucous membrane signals the presence of bacteria or viruses that affect the mucous membrane of the nasal cavity, spreading through the bronchi and reaching the lungs.
The amount of evaporated moisture depends only on the humidity of the inhaled air, since the exhaled air is at body temperature and is saturated. It is also obvious that, for the same moisture content, air with a higher temperature will cause more intense drying of the mucous membrane than air with a low temperature.
Let's give an example: when inhaling air with a moisture content of 3 g/kg. dry air In winter conditions, the feeling of dryness will be less than when inhaling air with the same moisture content (relative air humidity 20%) at a temperature of 20-25 C. Since the temperature of the air in the lung cavity is 34 C, its moisture content under saturation conditions will be equal to 34 g/ kg. dry air.. The amount of water evaporated from the mucous membrane for each kg of inhaled air:
G exp = xlu— xamb=34,6-3=31,6 G kg dry air.
With little physical activity, a person inhales approximately 1 m3/h of air or 1.2 kg/h, thus losing approximately 35 g of water every hour.
In cold conditions this is not noticeable, but this cannot be said about being in high temperature conditions.
Dryness of the mucous membrane of the upper respiratory tract limits its filtering ability, promotes the penetration of contaminants into the body and at the same time increases the amount of water evaporating from the bronchi. An increase in mucus viscosity limits or inhibits the mobility of the ciliated epithelium, reducing the infection barrier.
Drying of the respiratory tract leads to intense vasodilation and profuse sweating. To avoid these phenomena, warm air must be sufficiently humidified. Studies have shown that the minimum permissible relative air humidity is approximately 30%, the maximum permissible humidity is approximately 80-90%.
Air humidity is determined by the evaporation of water from the surface of the seas and oceans. Absolute humidity is the density of water vapor per unit volume, and the percentage ratio of the amount of water vapor in a certain volume of air to the amount of vapor that can saturate this volume at a given temperature is called relative humidity . Relative humidity is subject to daily fluctuations. This is primarily due to temperature changes. The higher the air temperature, the greater the amount of water vapor required to completely saturate it. At low temperatures, less water vapor is needed for maximum saturation.
Indicators of relative humidity and saturation deficit are important. These indicators give an idea of the degree of saturation of the air with water vapor and indicate the possibility of heat transfer through evaporation. As the humidity deficit increases, the air's ability to accept water vapor increases. Under these conditions, heat loss through sweating will occur more intensely.
For humans, relative humidity of 30-60% is considered a hygienic norm. This humidity ensures the normal functioning of the body. This helps to moisturize the skin and mucous membranes of the respiratory tract and inhaled air, and to some extent maintain the constant humidity of the internal environment of the body. Air whose relative humidity is below 20% is rated as dry, between 71 and 85% as moderately humid, and above 86% as highly humid. Humidity less than 20% is accompanied by evaporation of moisture from the mucous membranes of the respiratory tract. This leads to a decrease in their filtering ability and a feeling of dryness in the mouth. The limit of human heat balance is an air temperature of 40ºС and humidity of 30% or an air temperature of 30ºС and humidity of 85%.
Depending on the degree of air humidity, the effect of temperature is felt differently. Thus, high air temperature in combination with low humidity is tolerated by a person much more easily than with high humidity. With an increase in air humidity, body temperature rises, pulse and respiration increase, headache and weakness appear, a decrease in motor activity is observed, and the release of heat from the surface of the body by evaporation decreases (hydration and dehydration of tissues). Saturation of air with water vapor in low temperature conditions will contribute to hypothermia of the body.
Condensation, the thickening of water vapor, is its transition to a liquid state and the formation of water droplets. Condensation occurs when air is saturated or supersaturated with water vapor due to its cooling. The products of condensation in the atmosphere are fog and clouds. Fog is a large amount of condensation products (water droplets and ice crystals) in the ground layers of air. As a result of fog, visibility deteriorates, accidents and injuries occur. It contains dust, which makes breathing difficult.
A person’s tolerance to environmental temperature depends on the relative humidity of the air, that is, the percentage ratio of the amount of water vapor contained in a certain volume of air to the amount that completely saturates this volume at a given temperature. When air temperature falls, relative humidity increases, and when air temperature rises, it decreases.
Relative air humidity of 40–60% at a temperature of 18–21 °C is considered optimal for humans. Air whose relative humidity is below 20% is rated as dry, from 71 to 85% as moderately humid, and above 86% as very humid.
Moderate air humidity ensures normal functioning of the body. In humans, it helps moisturize the skin and mucous membranes of the respiratory tract. Maintaining a constant humidity of the internal environment of the body depends to a certain extent on the humidity of the inhaled air. Combined with temperature factors, air humidity creates conditions for thermal comfort or disrupts it, promoting hypothermia or overheating of the body, as well as hydration or dehydration of tissues.
A simultaneous increase in air temperature and humidity sharply worsens a person’s well-being and shortens the possible length of his stay in these conditions. At the same time, there is an increase in body temperature, increased heart rate, and respiration. Headache, weakness appear, and motor activity decreases. Poor heat tolerance in combination with high relative humidity is due to the fact that, simultaneously with increased sweating in high environmental humidity, sweat does not evaporate well from the surface of the skin. Heat transfer is difficult. The body becomes increasingly overheated and heat stroke may occur.
High humidity is also an unfavorable factor at low air temperatures. In this case, there is a sharp increase in heat transfer, which is dangerous to health. Even a temperature of 0 °C can lead to frostbite of the face and limbs, especially in the presence of wind.
Low air humidity (less than 20%) is accompanied by significant evaporation of moisture from the mucous membranes of the respiratory tract. This leads to a decrease in their filtering ability and to unpleasant sensations in the throat and dry mouth.
The boundaries within which a person’s thermal balance at rest is maintained with considerable stress are considered to be an air temperature of 40 °C and a humidity of 30% or an air temperature of 30 °C and a humidity of 85%.
Patients with hypertension and atherosclerosis are especially sensitive to high humidity. There is an increase in the number of exacerbations of diseases of the cardiovascular system with increasing air humidity.
The body's response to hypoxic exposure
Hypoxia – a condition that occurs as a result of insufficient oxygen supply to tissues.
The body's response to hypoxic effects can be considered using a model of hypoxia during mountain climbing:
Initially, in response to hypoxia, a person’s heart rate, stroke and minute blood volume increase compensatoryly. Additional capillaries in the tissues open, which increases blood flow, as the rate of oxygen diffusion increases;
There is a slight increase in breathing intensity. Shortness of breath occurs only with severe degrees of oxygen starvation. This is explained by the fact that increased breathing in a hypoxic atmosphere is accompanied by hypocapnia, which inhibits the increase in pulmonary ventilation, and only after a certain time (1 - 2 weeks) in hypoxic conditions does a significant increase in pulmonary ventilation occur due to the increased sensitivity of the respiratory center to carbon dioxide;
the number of red blood cells and the concentration of hemoglobin in the blood increases due to increased hematopoiesis;
the oxygen transport properties of hemoglobin change, which contributes to a more complete delivery of oxygen to tissues;
the number of mitochondria in cells increases, the content of respiratory chain enzymes increases, which increases energy metabolism in the cell;
behavior changes occur. For example, physical activity decreases.
The body's response to changes in atmospheric pressure
Atmospheric pressure is the pressure of atmospheric air on objects in it and on the earth's surface. Its distribution over the earth's surface determines the movement of air masses and atmospheric fronts, determines the direction and speed of the wind. Pressure plays an important role in the functioning of the body. The well-being of a person who has lived in a certain area for quite a long time is normal, i.e. The atmospheric pressure characteristic of this region should not cause any particular deterioration in well-being.
Changes in atmospheric pressure can lead to a variety of pathological manifestations. First of all, they relate to the cardiovascular system. Thus, under normal conditions, with an increase in atmospheric pressure, some changes in physiological indicators and sensations are observed: a decrease in pulse and respiratory rate, a decrease in systolic and an increase in diastolic blood pressure, an increase in the vital capacity of the lungs, a dull timbre of the voice, a decrease in skin sensitivity and hearing, a feeling of dry mucous membranes , increased intestinal motility, slight compression of the abdomen due to compression of gases in the intestines. However, all these phenomena are relatively easily tolerated. More unfavorable phenomena are observed during the period of changes in atmospheric pressure - increase (compression) and especially its decrease (decompression) to normal. The slower the change in pressure occurs, the better and without adverse consequences the human body adapts to it.
When atmospheric pressure decreases, opposite changes occur: breathing becomes more frequent and deepening, heart rate increases, a slight drop in blood pressure is observed, and changes in the blood are also observed in the form of an increase in the number of red blood cells. On the other hand, the nerve receptors of the pleura (the mucous membrane lining the pleural cavity), the peritoneum (lining the abdominal cavity), the synovial membrane of the joints, as well as vascular receptors react to fluctuations in atmospheric pressure. The adverse effect of low atmospheric pressure on the body is based on oxygen starvation. It is due to the fact that with a decrease in atmospheric pressure, the partial pressure of oxygen also decreases, therefore, with the normal functioning of the respiratory and circulatory organs, less oxygen enters the body.
The body's response to the action of electromagnetic fields (EMF) and radio frequency radiation
Experimental data from both domestic and foreign researchers indicate high biological activity of EMF in all frequency ranges (Vyalov A.M., 1971; Schwan H.P., 1985, 1988; Semm P., 1980; Milham S., 1985). At relatively high levels of irradiating EMF, modern theory recognizes the thermal mechanism of the effect of EMF on a biological object, in which the electromagnetic energy of the external field is converted into thermal energy and is accompanied by an increase in body temperature or local selective heating of tissues, cell organs, especially those with poor thermoregulation (lens, vitreous body) and others).
At a relatively low level of EMF (for example, for radio frequencies above 300 MHz - this is less than 1 mW/cm2), it is customary to talk about the non-thermal or informational nature of the impact on the body. The mechanisms of action of EMF in this case are still poorly understood.
The effect of radio frequency EMF on the central nervous system at an energy flux density (EFD) of more than 1 m W/cm 2 indicates its high sensitivity to electromagnetic radiation.
Changes in the blood are observed, as a rule, at PES above 10 mW/cm 3; at lower levels of exposure, phase changes in the number of leukocytes, erythrocytes and hemoglobin are observed.
With prolonged exposure to EMFs, physiological adaptation or weakening of immunological reactions occurs.
The severity of the identified disorders is directly dependent on:
wavelength;
radiation intensity and mode;
duration and nature of exposure to the body;
on the area of the irradiated surface and the anatomical structure of the organ and tissue.
Numerous studies in the field of biological effects of EMF will allow us to determine the most sensitive systems of the human body: nervous, immune, endocrine and reproductive. A.M. Vyalov (1971) also considers the hematopoietic system to be critical.
When exposed to low-intensity EMFs from the nervous system, significant deviations occur in the transmission of nerve impulses at the synapse level. Higher nervous activity is depressed and memory deteriorates. The structure of the capillary blood-brain barrier of the brain is disrupted, its permeability increases, which directly depends on the intensity of exposure (Gigoriev Yu.G. et al., 1999). The fetal nervous system exhibits particular sensitivity to electromagnetic influences in the later stages of intrauterine development.
A high-intensity electromagnetic field can contribute to nonspecific immune suppression, as well as the development of an autoimmune reaction, as a result of which the immune system reacts against normal tissue structures inherent in a given organism. This pathological condition is characterized in most cases by a deficiency of lymphocytes formed in the thymus gland (thymus), suppressed by electromagnetic influence.
Research by Russian scientists into the influence of the electromagnetic field on the endocrine system, which began in the 60s of the 20th century, showed that under the influence of the electromagnetic field, stimulation of the pituitary-adrenaline system occurs, accompanied by an increase in the content of adrenaline in the blood and activation of blood coagulation processes. Changes in the composition of peripheral blood (leukopenia, neutropenia, erythrocytopenia) were also observed.
Sexual dysfunction is usually associated with changes in its regulation by the nervous and endocrine systems, as well as with a sharp decrease in the activity of germ cells.
It has been established that the female reproductive system is more sensitive to electromagnetic influences than the male reproductive system. It is believed that electromagnetic fields can cause pathologies in the development of the embryo, affecting different stages of pregnancy. It has been established that the presence of contact of women with electromagnetic radiation can lead to premature birth and slow down the development of the fetus.
In recent years, data have appeared on the inducing effect of electromagnetic radiation on the processes of carcinogenesis (Pauly H., Schwan H.P., 1971, Semm P., 1980).
Prolonged contact with an electromagnetic field in the microwave range can lead to the development of a disease called “radio wave disease.” People who have been in the radiation zone for a long time complain of weakness, irritability, fatigue, weakened memory, and sleep disturbances. Often these symptoms are accompanied by disorders of the autonomic functions of the nervous system. From the cardiovascular system, hypotension, heart pain, and pulse instability are manifested.
The main sources of the electromagnetic field can be identified:
Power lines
Electrical wiring (inside buildings and structures)
Household electrical appliances
Personal computers
TV and radio broadcasting stations
Satellite and cellular communications (devices, repeaters)
Electric transport
Since the mid-90s of the last century, one of the most widespread sources of both industrial and non-industrial exposure to modulated EMFs has been mobile communication devices.
Studies carried out in 13 countries using the case-control method, within the framework of the International INTERPHONE project, found that when using cellular communication devices for more than 10 years, the risk of developing gliomas statistically significantly increases. Based on these data, in May 2011, IARC, when considering the electromagnetic field of the radio frequency range as a risk factor for the development of cancer, classified EMFs created by cellular communication devices as potential carcinogens for the risk of developing gliomas in users with long-term use of mobile phones “more than 10 years ( T.L. Pilat, L.P. Kuzmina, N.I.
Electromagnetic fields generated by personal computers are also seen as a potential risk factor for the health of users. Most of the data concerns computers equipped with video display terminals based on a cathode ray tube as a source of electrostatic and electromagnetic fields in the frequency range up to 400 kHz. According to available data, users have an increased risk of changes in the functional state of the central nervous system, the risk of developing diseases of the cardiovascular system, musculoskeletal system. A high incidence of pathology of the visual organ was noted, the leading role in which is played, first of all, by myopia (24–46%) and functional changes in the visual system in persons with normal visual status.
The body's response to noise
We encounter vibroacoustic factors: noise and vibration every day in transport (cars, trains, subways, etc.), in industrial premises, and in everyday life. It is known that in everyday life more than 30% of the population of large cities live in conditions of vibroacoustic discomfort.
The noise has been called the "gray plague" of the 19th, 20th and 21st centuries. With the increase in labor productivity due to the creation of new machines and mechanisms, increasing their power, and introducing new technological processes, the noise is constantly increasing. From a physiological point of view They call all sorts of unpleasant, unwanted sounds that have a harmful, irritating effect on the human body, interfere with the perception of useful signals, and reduce its performance. From a physical point of view, noise is a random combination of sounds of varying frequencies and intensities. Sound intensity, measured in decibels (dB), is used to assess human exposure to noise.
Depending on the level and nature of noise, its duration, intensity and frequency of sounds, as well as the individual characteristics of a person, the consequences of exposure to noise can be very different.
Intense noise with daily exposure leads to an occupational disease - hearing loss, manifested by gradual hearing loss. Initially, it occurs in the high-frequency region, then hearing loss spreads to lower frequencies, which determine the ability to perceive speech.
In addition to the direct impact on the organs of hearing, noise affects various parts of the brain, disrupting the normal processes of higher nervous activity. This effect occurs even earlier than changes in the hearing organ. Typical complaints are increased fatigue, general weakness, irritability, apathy, memory loss, sweating, etc.
Under the influence of noise, changes occur in the human visual organs (the stability of clear vision and visual acuity decreases, sensitivity to different colors changes, etc.) and the vestibular apparatus; the functions of the gastrointestinal tract are disrupted; intracranial pressure increases, etc.
Noise, especially intermittent and pulsed noise, impairs the accuracy of work operations and makes it difficult to receive and perceive information. As a result of the adverse effects of noise on a working person, labor productivity and accuracy of production operations decrease, the number of defects increases, and preconditions for accidents are created.
Approximate sound pressure levels of common environmental sounds:
10 dB - whisper;
20 dB - noise standard in residential premises;
40 dB - quiet conversation;
50 dB - medium volume conversation;
70 dB - typewriter noise;
80 dB - noise of a running truck engine;
100 dB - loud car signal at a distance of 5-7 m;
110 dB - noise of a running tractor at a distance of 1 m;
120-140 dB - pain threshold;
150 dB - airplane takeoff;
Approximately, the effect of noise depending on its level can be characterized as follows:
Noise level 50-65 dB may cause irritation, but its consequences are only psychological. The impact of low-intensity noise during mental work is especially negative.
In addition, the psychological impact of noise depends on the individual’s attitude towards it. Thus, the noise produced by the person himself does not bother him, while small extraneous noise can cause severe irritation. At noise level
65-90 dB its physiological effects are possible. The pulse and blood pressure increase, the blood vessels narrow, which reduces the body's blood supply, and the person gets tired faster. Functional changes in the state of the nervous system occur (irritability, apathy, memory loss, sweating, etc.). With prolonged exposure to intense noise, significant changes in the ultrastructure of mitochondria (inhibition of oxidative processes) and disruption of the functional structure of synapses are observed. Persistent and irreversible changes develop in the auditory analyzer (hearing impairment).
Exposure to noise levels 90 dB and higher leads to disruption of the hearing organs, and its effect on the circulatory system increases. With such intensity, the activity of the stomach and intestines deteriorates, feelings of nausea, headache and tinnitus appear.
At noise levels above 110 dB sound intoxication occurs;
At sound pressure
145 dB
Damage to the hearing aid may occur, including rupture of the eardrum.
The physiological effect of noise depends on three main parameters:
on the duration of noise exposure;
We have repeatedly heard: “absolute” and “relative air humidity”. What are these indicators? Everything is clear with the absolute value: this is the number of particles contained in one cubic meter of air. But what practical benefit will it bring us to know that five units of moisture per cubic meter are invisibly present in our environment? After all, we cannot even say whether this air is dry, normal or too damp, since its composition changes at different temperatures. After all, the atmospheric environment is like a sponge; the warmer it is, the more water vapor dissolves in it. When it gets very cold (for example, on clear nights), the cold with an invisible hand squeezes the “sponge” and dew falls out. And the heat, coming into contact with a decanter of ice water, leaves “perspiration” on the glass.
So, if “5 units per cubic meter” is an absolute indicator, but relative to the ambient temperature it can be considered dry (in the heat), normal or high (in the cold). It is more convenient to use another indicator for domestic needs, namely “relative air humidity”. At a certain temperature, the atmosphere can hold a certain amount of steam. If it is maximally saturated with vapor, we say that the “dampness” is 100%. This, for example, is a Russian bathhouse, where it is hot, but it is also a thick fog, and being inside a cloud at a considerable height, where it is cold. That is, the absolute amount of water in the form of steam in a bathhouse, fog and cloud is different, but the saturation with water is the same - 100%.
And this relative air humidity plays an important role in changing our well-being. Remember how hard it is to breathe and how sleepy you feel before a thunderstorm. This environment is filled with invisible water: its content increases from normal 50% to 80. But excessive dryness also leads to problems: the body loses a lot of moisture. This is especially evident in winter in our homes.
Look: cold penetrates into the room (let's say it's 10 C outside). Even if the relative humidity outside the window is high, it is low in absolute quantity (because it is cold outside). When heated by a stove or central heating radiators, the percentage in our environment changes from high to low. If the room is + 25 C, frosty masses begin to literally suck moisture from objects and people in the room. Wooden furniture dries out, flowers turn yellow, and people feel a sore mouth and dry skin and hair. It’s not easy for those who wear contact lenses in such a situation: their eyes become red and itchy. Allergy sufferers also have a hard time - excessive dryness aggravates the reaction to dust. That is why it is advised to place saucers of water near the radiators, although this is not a panacea.
To always be aware of the percentage of water vapor contained in the air, you can purchase special humidity meters called hygrometers. After all, in a damp environment, as you know, microbes multiply. Therefore, outbreaks of influenza and acute respiratory infections occur during periods of winter thaws, when the south wind raises the temperature and increases phlegm. In the heat, when it’s steamy and stuffy, the number of heart attacks increases, and it’s not easy for asthmatics. With high humidity, cold and heat are more difficult to bear than with dry conditions. The most optimal thing for our body is 50-60% water saturation of the surrounding atmosphere.
Using two simple thermometers, you can build your own hygrometer. How to measure air humidity at home, without reagents? We place both thermometers in the shade, but wrap the lower part of one of them with a piece of felt soaked in water. The evaporation of moisture cools the thermometer. If the relative humidity is high, the felt dries slowly and both thermometers - wet and dry - show the same temperature. And if it is low, the cloth dries quickly, and the meter covered with perspiration gives lower readings.