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Humidity
Humidity is the concentration of water vapor present in the air. Water vapor, the gaseous state of water, is generally invisible to the human eye. Humidity indicates the likelihood for precipitation (meteorology), precipitation, dew, or fog to be present. Humidity depends on the temperature and pressure of the system of interest. The same amount of water vapor results in higher relative humidity in cool air than warm air. A related parameter is the dew point. The amount of water vapor needed to achieve saturation increases as the temperature increases. As the temperature of a parcel of air decreases it will eventually reach the saturation point without adding or losing water mass. The amount of water vapor contained within a parcel of air can vary significantly. For example, a parcel of air near saturation may contain 8 g of water per cubic metre of air at , and 28 g of water per cubic metre of air at Three primary measurements of humidity are widely employed: abso ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hygrometer
A hair tension dial hygrometer with a nonlinear scale. A hygrometer is an instrument that measures humidity: that is, how much water vapor is present. Humidity measurement instruments usually rely on measurements of some other quantities, such as temperature, pressure, mass, and mechanical or electrical changes in a substance as moisture is absorbed. By calibration and calculation, these measured quantities can be used to indicate the humidity. Modern electronic devices use the temperature of condensation (called the dew point), or they sense changes in electrical capacitance or resistance. The maximum amount of water vapor that can be present in a given volume (at saturation) varies greatly with temperature; at low temperatures a lower mass of water per unit volume can remain as vapor than at high temperatures. Thus a change in the temperature changes the relative humidity. A prototype hygrometer was invented by Leonardo da Vinci in 1480. Major improvements occurred during ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Heat Index
The heat index (HI) is an index that combines air temperature and relative humidity, in shade (shadow), shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the Shade (shadow), shade. For example, when the temperature is with 70% relative humidity, the heat index is (see table below). The heat index is meant to describe experienced temperatures in the shade, but it does not take into account heating from direct sunlight, physical activity or cooling from wind. The human body normally cools itself by evaporation of perspiration, sweat. High relative humidity reduces evaporation and cooling, increasing discomfort and potential Hyperthermia, heat stress. Different individuals perceive heat differently due to body shape, metabolism, level of hydration, pregnancy, or other physical conditions. Measurement of perceived temperature has been based on reports of how hot subjects feel under controlled conditions ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dew Point
The dew point is the temperature the air needs to be cooled to (at constant pressure) in order to produce a relative humidity of 100%. This temperature depends on the pressure and water content of the air. When the air at a temperature above the dewpoint is cooled, its moisture capacity is reduced and airborne water vapor will Condensation, condense to form liquid water known as dew. When this occurs through the air's contact with a colder surface, dew will form on that surface. The dew point is affected by the air's humidity. The more moisture the air contains, the higher its dew point. When the temperature is below the freezing point of water, the dew point is called the frost point, as frost is formed via deposition (phase transition), deposition rather than condensation. In liquids, the analog to the dew point is the cloud point. Humidity If all the other factors influencing humidity remain constant, at ground level the relative humidity rises as the temperature falls; this ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Water Vapor
Water vapor, water vapour, or aqueous vapor is the gaseous phase of Properties of water, water. It is one Phase (matter), state of water within the hydrosphere. Water vapor can be produced from the evaporation or boiling of liquid water or from the Sublimation (phase transition), sublimation of ice. Water vapor is transparent, like most constituents of the atmosphere. Under typical atmospheric conditions, water vapor is continuously generated by evaporation and removed by condensation. It is less dense than most of the other constituents of air and triggers convection currents that can lead to clouds and fog. Being a component of Earth's hydrosphere and hydrologic cycle, it is particularly abundant in Earth's atmosphere, where it acts as a greenhouse gas and warming feedback, contributing more to total greenhouse effect than non-condensable gases such as carbon dioxide and methane. Use of water vapor, as steam, has been important for cooking, and as a major component in energy prod ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Apparent Temperature
Apparent temperature, also known as "feels like", is the temperature equivalent perceived by humans, caused by the combined effects of air temperature, relative humidity and wind speed. The measure is most commonly applied to the perceived outdoor temperature. Apparent temperature was invented by Robert G. Steadman who published a paper about it in 1984. It also applies, however, to indoor temperatures, especially saunas, and when houses and workplaces are not sufficiently heated or cooled. * The heat index and humidex measure the effect of humidity on the perception of temperatures above . In humid conditions, the air feels much hotter, because less perspiration evaporates from the skin. * The wind chill factor measures the effect of wind speed on cooling of the human body below . As airflow increases over the skin, more heat will be removed. Standard models and conditions are used. * The wet-bulb globe temperature (WBGT) combines the Effect of radiation on perceived temperature ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
HVAC
Heating, ventilation, and air conditioning (HVAC ) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. "Refrigeration" is sometimes added to the field's abbreviation as HVAC&R or HVACR, or "ventilation" is dropped, as in HACR (as in the designation of HACR-rated circuit breakers). HVAC is an important part of residential structures such as single family homes, apartment buildings, hotels, and senior living facilities; medium to large industrial and office buildings such as skyscrapers and hospitals; vehicles such as cars, trains, airplanes, ships and submarines; and in marine environments, where safe and healthy building conditions are regulated with respect to temperature and humidity, using fres ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Humidex
The humidex (short for humidity index) is an index number used by Canadian meteorologists to describe how hot the weather feels to the average person, by combining the effect of heat and humidity. The term ''humidex'' was coined in 1965. The humidex is a nominally dimensionless quantity (though generally recognized by the public as equivalent to the degree Celsius) based on the dew point. ''Range of humidex: Scale of comfort'': * 20 to 29: Little to no discomfort * 30 to 39: Some discomfort * 40 to 45: Great discomfort; avoid exertion * Above 45: Dangerous; heat stroke quite possible History The current formula for determining the humidex was developed by J. M. Masterton and F. A. Richardson of Canada's Atmospheric Environment Service in 1979. Humidex differs from the heat index used in the United States in being derived from the dew point rather than the relative humidity, though both dew point and relative humidity (when used in conjunction with air temperature) are direc ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Precipitation (meteorology)
In meteorology, precipitation is any product of the condensation of atmospheric water vapor that falls from clouds due to gravitational pull. The main forms of precipitation include drizzle, rain, rain and snow mixed ("sleet" in Commonwealth usage), snow, ice pellets, graupel and hail. Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor (reaching 100% relative humidity), so that the water condenses and "precipitates" or falls. Thus, fog and mist are not precipitation; their water vapor does not condense sufficiently to precipitate, so fog and mist do not fall. (Such a non-precipitating combination is a colloid.) Two processes, possibly acting together, can lead to air becoming saturated with water vapor: cooling the air or adding water vapor to the air. Precipitation forms as smaller droplets coalesce via collision with other rain drops or ice crystals within a cloud. Short, intense periods of rain in scattered locations are called shower (p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Evaporation
Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evaporation, such as when humidity affects rate of evaporation of water. When the molecules of the liquid collide, they transfer energy to each other based on how they collide. When a molecule near the surface absorbs enough energy to overcome the vapor pressure, it will escape and enter the surrounding air as a gas. When evaporation occurs, the energy removed from the vaporized liquid will reduce the temperature of the liquid, resulting in evaporative cooling. On average, only a fraction of the molecules in a liquid have enough heat energy to escape from the liquid. The evaporation will continue until an equilibrium is reached when the evaporation of the liquid is equal to its condensation. In an enclosed environment, a liquid will evaporate unt ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Saturation Vapor Pressure
Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's thermodynamic tendency to evaporate. It relates to the balance of particles escaping from the liquid (or solid) in equilibrium with those in a coexisting vapor phase. A substance with a high vapor pressure at normal temperatures is often referred to as '' volatile''. The pressure exhibited by vapor present above a liquid surface is known as vapor pressure. As the temperature of a liquid increases, the attractive interactions between liquid molecules become less significant in comparison to the entropy of those molecules in the gas phase, increasing the vapor pressure. Thus, liquids with strong intermolecular interactions are likely to have smaller vapor pressures, with the reverse true for weaker interactions. The vapor p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
