temperature-humidity index: see humidity.

temperature, body: see body temperature; fever.

temperature inversion, condition in which the temperature of the atmosphere increases with altitude in contrast to the normal decrease with altitude. When temperature inversion occurs, cold air underlies warmer air at higher altitudes. Temperature inversion may occur during the passage of a cold front or result from the invasion of sea air by a cooler onshore breeze. Overnight radiative cooling of surface air often results in a nocturnal temperature inversion that is dissipated after sunrise by the warming of air near the ground. A more long-lived temperature inversion accompanies the dynamics of the large high-pressure systems depicted on weather maps. Descending currents of air near the center of the high-pressure system produce a warming (by adiabatic compression), causing air at middle altitudes to become warmer than the surface air. Rising currents of cool air lose their buoyancy and are thereby inhibited from rising further when they reach the warmer, less dense air in the upper layers of a temperature inversion. During a temperature inversion, air pollution released into the atmosphere's lowest layer is trapped there and can be removed only by strong horizontal winds. Because high-pressure systems often combine temperature inversion conditions and low wind speeds, their long residency over an industrial area usually results in episodes of severe smog.

temperature, measure of the relative warmth or coolness of an object. Temperature is measured by means of a thermometer or other instrument having a scale calibrated in units called degrees. The size of a degree depends on the particular temperature scale being used. A temperature scale is determined by choosing two reference temperatures and dividing the temperature difference between these two points into a certain number of degrees. The two reference temperatures used for most common scales are the melting point of ice and the boiling point of water. On the Celsius temperature scale, or centigrade scale, the melting point is taken as 0°C; and the boiling point as 100°C;, and the difference between them is divided into 100 degrees. On the Fahrenheit temperature scale, the melting point is taken as 32°F; and the boiling point as 212°F;, with the difference between them equal to 180 degrees. The Réaumur scale, used in some parts of Europe, also sets the melting point at zero, but it has an 80-degree temperature difference between 0°R; and the boiling point at 80°R;. The temperature of a substance does not measure its heat content but rather the average kinetic energy of its molecules resulting from their motions. A one-pound block of iron and a two-pound block of iron at the same temperature do not have the same heat content. Because they are at the same temperature the average kinetic energy of the molecules is the same; however, the two-pound block has more molecules than the one-pound block and thus has greater heat energy. A temperature scale can be defined theoretically for which zero degree corresponds to zero average kinetic energy (see gas laws). Such a point is called absolute zero, and such a scale is known as an absolute temperature scale. The Kelvin temperature scale is an absolute scale having degrees the same size as those of the Celsius temperature scale; the Rankine temperature scale is an absolute scale having degrees the same size as those of the Fahrenheit temperature scale. The relationship between absolute temperature and average molecular kinetic energy is one result of the kinetic-molecular theory of gases. See heat; thermodynamics.

standard temperature and pressure: see STP.

low-temperature physics, science concerned with the production and maintenance of temperatures much below normal, down to almost absolute zero, and with various phenomena that occur only at such temperatures. The temperature scale used in low-temperature physics is the Kelvin temperature scale, or absolute temperature scale, which is based on the behavior of an idealized gas (see gas laws; kinetic-molecular theory of gases). Low-temperature physics is also known as cryogenics, from the Greek meaning "producing cold." Low temperatures are achieved by removing energy from a substance. This may be done in various ways. The simplest way to cool a substance is to bring it into contact with another substance that is already at a low temperature. Ordinary ice, dry ice (solid carbon dioxide), and liquid air may be used successively to cool a substance down to about 80°K; (about -190°C;). The heat is removed by conduction, passing from the substance to be cooled to the colder substance in contact with it. If the colder substance is a liquefied gas (see liquefaction), considerable heat can be removed as the liquid reverts to its gaseous state, since it will absorb its latent heat of vaporization during the transition. Various liquefied gases can be used in this manner to cool a substance to as low as 4.2°K;, the boiling point of liquid helium. If the vapor over the liquid helium is continually pumped away, even lower temperatures, down to less than 1°K;, can be achieved because more helium must evaporate to maintain the proper vapor pressure of the liquid helium. Most processes used to reduce the temperature below this level involve the heat energy that is associated with magnetization (see magnetism). Successive magnetization and demagnetization under the proper combination of conditions can lower the temperature to only about a millionth of a degree above absolute zero. Reaching such low temperatures becomes increasingly difficult, as each temperature drop requires finding some kind of energy within the substance and then devising a means of removing this energy. Moreover, according to the third law of thermodynamics, it is theoretically impossible to reduce a substance to absolute zero by any finite number of processes. Superconductivity and superfluidity have traditionally been thought of as phenomena that occur only at temperatures near absolute zero, but by the late 1980s several materials that exhibit superconductivity at temperatures exceeding 100°K; had been found. Superconductivity is the vanishing of all electrical resistance in certain substances when they reach a transition temperature that varies from one substance to another; this effect can be used to produce powerful superconducting magnets. Superfluidity occurs in liquid helium and leads to the tendency of liquid helium to flow over the sides of any container it is placed in without being stopped by friction or gravity.

low-temperature fusion: see cold fusion.

centigrade temperature scale: see Celsius temperature scale.

body temperature, internal temperature of a living organism. Mammals and birds are termed warm-blooded, or homeothermic, i.e., they are able to maintain a relatively constant inner body temperature, whereas other animals are cold-blooded, or poikilothermic, i.e., their body temperature varies according to the temperature of the environment.

Warm-blooded Animals (Homeotherms)

In humans and other mammals, temperature regulation represents the balance between heat production from metabolic sources and heat loss from evaporation (perspiration) and the processes of radiation, convection, and conduction. In a cold environment, body heat is conserved first by constriction of blood vessels near the body surface and later by waves of muscle contractions, or shivering, which serve to increase metabolism. Shivering can result in a maximum fivefold increase in metabolism. Below about 40°F; (4°C;) a naked person cannot sufficiently increase the metabolic rate to replace heat lost to the environment. Another heat-conserving mechanism, goose bumps, or piloerection, raises the body hairs; although not especially effective in humans, in animals it increases the thickness of the insulating fur or feather layer.

In a warm environment, heat must be dissipated to maintain body temperature. In humans, increased surface blood flow, especially to the limbs, acts to dissipate heat at the surface. At environmental temperatures above 93°F; (34°C;), or at lower temperatures when metabolism has been increased by work, heat must be lost through the evaporation of the water in sweat. People in active work may lose as much as 4 quarts per hour for short periods. However, when the temperature and humidity are both high, evaporation is slowed, and sweating is not effective. Most mammals do not have sweat glands but keep cool by panting (evaporation through the respiratory tract) and by increased salivation and skin and fur licking.

Temperature regulatory mechanisms act through the autonomic nervous system and are largely controlled by the hypothalamus of the brain, which responds to stimuli from nerve receptors in the skin. Continued exposure to heat or cold results in some slow acclimatization, e.g., more active sweating in response to continued heat and an increase in subcutaneous fat deposits in response to continued cold.

Environmental extremes may result in failure to maintain normal body temperature. In both increased body temperature, or hyperthermia, and decreased body temperature, or hypothermia, death may result (see heat exhaustion). Controlled hypothermia is used in some types of surgery to temporarily decrease the metabolic rate. Fever, caused by a resetting of the temperature regulatory mechanism, is a response to fever-causing, or pyrogenic, substances, such as bacterial endotoxins or leucocyte extracts. The upper limit of body temperature compatible with survival is about 107°F; (42°C;), while the lower limit varies.

In humans the inner body temperature alternates in daily activity cycles; it is usually lowest in early morning and is slightly higher at the late afternoon peak. In human females there is also a monthly temperature variation related to the ovulatory cycle. In many mammals and birds the body temperature shows more pronounced cyclic variations than in humans. For example, in hibernators the body temperature may lower to only a few degrees above the environmental temperature during the dormant periods; mammalian hibernators reawake spontaneously and in their active period are homeothermic.

Cold-blooded Animals (Poikilotherms)

Reptiles and other poikilothermic animals bask in warm weather and must hibernate in winter. The body temperature of fishes must remain close to that of the surrounding water, because heat is lost directly into the water during respiration; however, in some fishes, such as the bluefin tuna, a special network of fine veins and arteries called the rete mirabile provides a thermal barrier against loss of metabolic heat. The mechanism of temperature regulation in homeotherms is considered an important evolutionary advance in that physical activity in such animals can be relatively independent of the environment.

absolute temperature scale: see Kelvin temperature scale; temperature.

Réaumur temperature scale: see temperature.

Rankine temperature scale, temperature scale having an absolute zero, below which temperatures do not exist, and using a degree of the same size as that used by the Fahrenheit temperature scale. Absolute zero, or 0°R;, is the temperature at which molecular energy is a minimum, and it corresponds to a temperature of -459.67°F;. Because the Rankine degree is the same size as the Fahrenheit degree, the freezing point of water (32°F;) and the boiling point of water (212°F;) correspond to 491.67°R; and 671.67°R;, respectively. The temperature scale is named after the Scottish engineer and physicist William John Macquorn Rankine, who proposed it in 1859. Another absolute temperature scale, the Kelvin temperature scale, is more commonly used for scientific measurements. See also Celsius temperature scale.

Kelvin temperature scale, a temperature scale having an absolute zero below which temperatures do not exist. Absolute zero, or 0°K;, is the temperature at which molecular energy is a minimum, and it corresponds to a temperature of -273.15° on the Celsius temperature scale. The Kelvin degree is the same size as the Celsius degree; hence the two reference temperatures for Celsius, the freezing point of water (0°C;), and the boiling point of water (100°C;), correspond to 273.15°K; and 373.15°K;, respectively. When writing temperatures in the Kelvin scale, it is the convention to omit the degree symbol and merely use the letter K. The temperature scale is named after the British mathematician and physicist William Thomson Kelvin, who proposed it in 1848. Another absolute temperature scale, the Rankine temperature scale, is used by some engineers. See also Fahrenheit temperature scale.

Fahrenheit temperature scale, temperature scale in which the temperature difference between two reference temperatures, the melting and boiling points of water, is divided into 180 equal intervals called degrees. The freezing point is taken as 32°F; and the boiling point as 212°F;. The scale was established by the German-Dutch physicist Gabriel Daniel Fahrenheit in 1724. William John Macquorn Rankine used it as the basis of his absolute temperature scale, now called the Rankine temperature scale, in 1859. Although the Fahrenheit scale was formerly used widely in English-speaking countries, many of these countries began changing to the more convenient Celsius temperature scale in the late 1960s and early 1970s; a notable exception is the United States, where the Fahrenheit scale is still in common use together with other English units of measurement. Temperatures on the Fahrenheit scale can be converted to equivalent temperatures on the Celsius scale by first subtracting 32° from the Fahrenheit temperature, then multiplying the result by 5/9, according to the formula (F-32)5/9=C.

Celsius temperature scale, temperature scale according to which the temperature difference between the reference temperatures of the freezing and boiling points of water is divided into 100 degrees. The freezing point is taken as 0 degrees Celsius and the boiling point as 100 degrees Celsius. The Celsius scale is widely known as the centigrade scale because it is divided into 100 degrees. It is named for the Swedish astronomer Anders Celsius, who established the scale in 1742. William Thomson Kelvin used it as the basis of his absolute temperature scale, now known as the Kelvin temperature scale, in 1848 (see also absolute zero). Temperatures on the Celsius scale can be converted to equivalent temperatures on the Fahrenheit temperature scale by multiplying the Celsius temperature by 9/5 and adding 32° to the result, according to the formula 9C/5+32=F.

In meteorology, an increase of air temperature with altitude. Such an increase is a reversal of the normal temperature condition of the troposphere, where temperature usually decreases with altitude. Inversions play an important role in determining cloud forms, precipitation, and visibility. An inversion acts as a lid, preventing the upward movement of the air below it. Where a pronounced inversion is present at a low level, convective clouds cannot grow high enough to produce showers and, at the same time, visibility may be greatly reduced by trapped pollutants (see smog). Because the air near the base of the inversion is cool, fog is frequently present there.

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Measure of hotness expressed in terms of any of several arbitrary scales, such as Fahrenheit, Celsius, or Kelvin. Heat flows from a hotter body to a colder one and continues to do so until both are at the same temperature. Temperature is a measure of the average energy of the molecules of a body, whereas heat is a measure of the total amount of thermal energy in a body. For example, whereas the temperature of a cup of boiling water is the same as that of a large pot of boiling water (212°F, or 100°C), the large pot has more heat, or thermal energy, and it takes more energy to boil a pot of water than a cup of water. The most common temperature scales are based on arbitrarily defined fixed points. The Fahrenheit scale sets 32° as the freezing point of water and 212° as the boiling point of water (at standard atmospheric pressure). The Celsius scale defines the triple point of water (at which all three phases, solid, liquid, and gas, coexist in equilibrium) at 0.01° and the boiling point at 100°. The Kelvin scale, used primarily for scientific and engineering purposes, sets the zero point at absolute zero and uses a degree the same size as those of the Celsius scale.

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