AIR HUMIDITY
The subject of meteorology: The part of Earth physics-geophysics which deals with the Earth’s atmosphere is called meteorology. Meteorology studies physical phenomena in the atmosphere related to weather conditions, such as: conditions for the phenomena of air movement, its heating and cooling, evaporation and condensation, wind phenomena, conditions for the creation of clouds and conditions for the creation of atmospheric precipitation . Knowing time as a meteorological category is of great importance for many branches of the economy. Therefore, in every country there are hydrometeorological offices whose main task is to study the phenomena in the atmosphere and provide weather forecasts. To describe the state of the atmosphere at a given moment in time, many numbers serve as meteorological parameters, among which are: atmospheric pressure, air temperature, speed, direction and direction of the wind, air humidity. atmospheric waste etc. You are now familiar with some of the physical quantities that even meteorologists measure. Here we will give a brief overview of air humidity, but also of the conditions for the creation of atmospheric waste.
Air humidity. Absolute and relative humidity: Due to the permanent evaporation of water basins, as is the entire surface of the Earth, there is water vapor in the atmosphere. By air humidity we mean the amount of water vapor in the air. Although the evaporation of large water basins, oceans, seas, lakes, rivers and other surfaces is continuous, the water vapor in the Earth’s atmosphere is not always saturated. In some places on Earth, evaporation prevails, but in other places, condensation prevails. Due to the movement of air masses, air humidity varies from place to place. Even in the same place, air humidity changes over time. It is estimated that during a year a large amount of water passes through the Earth’s atmosphere, approximately 4.25 .10¹⁴ tons, but that water returns to the Earth as atmospheric waste. Air humidity is characterized by these physical quantities: absolute humidity, maximum humidity, relative humidity and frost point.
Absolute humidity (ρ) is determined by the density of water vapor present in the air. It is the mass of water vapor in one cubic meter of air. It is expressed in kilograms per cubic meter (kg/m³), but sometimes also in grams per cubic meter (g/m³). The density of any gas or vapor is proportional to its pressure. Therefore, absolute humidity can also be measured through the partial pressure of water vapor (ρ). Then the absolute humidity is expressed in units of pascal (Pa) or of the unit outside the system allowed bar (bar). This expression of absolute humidity is often used in meteorology.
Maximum humidity (ρ) is a physical quantity that by its nature is equal to absolute humidity, except that it is the behavior of saturation conditions. Therefore, the maximum humidity is the density of water vapor in saturation conditions. It is expressed with the same unit as the absolute humidity. When the maximum humidity is expressed through the partial pressure of water vapor, in reality it is the maximum partial pressure, or water vapor pressure (ρ). Table 1 shows the density of water vapor during saturation and the partial pressure of saturation water vapor (ρ) for the temperature range from 0⁰ C to 30⁰ C. The density of water vapor (ρ) and the partial pressure ( ρ) are related to Klaiper’s equations:
Here μ is the molar mass of water. Т is the absolute temperature, R is the universal gas constant.
From absolute humidity no clear representation of the true degree of humidity in the air can be obtained. Precisely, absolute humidity does not tell us how much of the water vapor currently present in the air is up to saturation. For this reason, humidity is usually expressed in terms of relative humidity.
Relative air humidity (r) is the ratio between the absolute air humidity during the given temperature and the maximum humidity during the same temperature. That is, relative humidity is measured by the ratio between the density of water vapor at saturation during that temperature (ρз). Humidity can also be presented as a ratio between the partial pressure of water vapor present at the moment during the given temperature (ρp) and the maximum partial pressure of water vapor (ρz)
Relative humidity is usually expressed as a percentage:
Relative humidity depends on the density of the air during saturation which, in turn, depends on the temperature of the air. When calculating the relative humidity for ρз and p3 are taken from the table.
I know that when unsaturated water vapor at given pressure cools, it can become saturated. Something like that happens in the atmosphere as well. Therefore, a physical quantity — the frost point — can be introduced as a measure of humidity.
The dew point
The temperature (T) at which the water vapor present in the air will be saturated is called the frost point. (In the example given above 8°C is the frost point). If the frost point is known with the help of the table, the absolute humidity of the air can be easily determined, and then also the relative humidity. Clean air can be cooled even to the lowest temperatures of the frost point, while the water vapor found in it does not condense, we call such vapor unsaturated. This phenomenon is reached if no “condensation centers” can be found in the air. These are various dusts, smoke or similar. Particularly good condensation centers are electrified particles in the air, such as ions. Air humidity is of great importance to general life on Earth. Plants, animals and humans evaporate more or less depending on how humid the air is. In this way, they regulate their temperature. For humans, air humidity of 60% to 70% is not acceptable. In the spaces, this humidity is provided with air conditioners.
Moisture measurement
Air humidity is measured with the help of a hygrometer (the name comes from the Greek word hygro-humidity) and a psychrometer (the name comes from the Greek word psychro-cold). The absolute hygrometer is used to measure the absolute humidity. It uses the property of some substances, which we call hygroscopes, to absorb water vapor. Such a substance is CaCl2. The figure shows the principle scheme of this type of hygrometer. It consists of three Ute tubes in which CaCl2 is placed. These pipes are connected to a large tank filled to the top with water
When water is released from this container, air enters the container through the pipes. At the moment when all the water will flow, air with a volume equal to the internal volume of container A has passed through the pipes U. Among other things, the water vapor has been absorbed in the CaCl2 hygroscope in the pipes U. If the mass of the pipes U was previously known, and now again, the mass of the absorbed water vapor is obtained from the change of the masses, which is equal to the volume of the vessel А. Thus, there are all the elements to determine the absolute humidity.
The string hygrometer is an instrument that directly determines the relative humidity. In this instrument, the property of the hair of the animal (human) to lengthen, proportionally with the humidity of the air, has been used. Practically, it is not worked with one thread, but with a bunch of threads. One end of the rope is clamped at point M, while the other is first twisted around the cylindrical shaft О (on the crane line of that shaft) and then loaded with a small weight P. To the shaft О is connected the arrow S with the help of which can directly read the relative humidity of the AB scale, previously scaled to relative humidity, in percent.
Daniel’s hygrometer is given below. It is a closed vessel, with two parts A and B, inside of which there is ether. The middle of the container A is covered with a silver surface so that the frost drops can be seen more easily. Wet the gauze from part B of the hygrometer with ether. At the moment when the ice is caught in container A, the internal temperature of the thermometer T1 is read. It is the frost point.
The temperature of the thermometer located on the hygrometer pole (Т2) is also read. Once these temperatures are determined, the relative humidity of the air can also be determined with this instrument. If the partial pressures of the saturated water vapor of these two temperatures, pz1 and pz2, are required from the table; from the ratio of those partial pressures, the relative air humidity r can be determined:
Psychrometer is made of two thermometers. One is dry, and the other is called water because its tank is wrapped with a hygrometer, a thin cloth immersed in water in a small container under the thermometer. Through the wet cloth, the thermometer is always in contact with the evaporating water. Therefore, it regularly shows low temperature from the dry bulb. The higher the relative humidity, the smaller the difference between the indicators of these two thermometers, since the rate of evaporation is the greater the lower the humidity of the air. During 100% humidity the water will generally not evaporate, and both thermometers will show the same temperature. Through the temperatures read by the thermometers, with the help of special psychrometric tables, the relative humidity of the air is found.
