Temperature fundamentally describes the hotness or coldness of a substance. It measures the average kinetic energy of the atoms or molecules within that substance. When a substance warms, its particles move more rapidly. Conversely, as a substance cools, these particles slow down.
How Temperature Is Measured
Measuring temperature involves a thermometer, a device that relies on how materials respond to changes in thermal energy. Many traditional liquid-in-glass thermometers contain a liquid like alcohol or mercury. As the liquid heats, it expands and rises; as it cools, it contracts and descends. This change is read against a calibrated scale.
Three temperature scales are used globally. The Fahrenheit scale, prevalent in the United States, sets water’s freezing point at 32 degrees Fahrenheit (°F) and its boiling point at 212°F. The Celsius scale, adopted in most other countries and for scientific applications, defines water’s freezing point as 0 degrees Celsius (°C) and its boiling point as 100°C.
The Kelvin scale is the base unit of thermodynamic temperature in the International System of Units (SI) and is used in scientific contexts. On this absolute scale, 0 Kelvin (0 K) represents absolute zero, the theoretical point where molecular motion is at its minimum. Water freezes at 273.15 K and boils at 373.15 K, meaning a change of one kelvin equals a change of one degree Celsius.
Temperature and The Human Body
The human body maintains a regulated internal temperature for proper biological function. Normal body temperature ranges between 36.5 and 37.5 degrees Celsius (97.7 to 99.5 degrees Fahrenheit), though individual variations exist. This stability is managed by the body’s thermoregulation system, centered in the hypothalamus.
When body temperature rises, two conditions can occur: fever or hyperthermia. A fever is a controlled increase in the body’s temperature set point, often triggered by the immune system in response to infections. Oral temperatures above 38°C (100.4°F) indicate a fever. This elevated set point helps create a less hospitable environment for pathogens.
Hyperthermia, in contrast, involves an unregulated elevation of body temperature where the body’s heat-dissipating mechanisms are overwhelmed. This can result from excessive heat exposure, such as during heat stroke, where temperatures can climb above 40°C (104°F). Unlike fever, the body’s internal thermostat remains unchanged in hyperthermia, making it a serious condition.
Conversely, hypothermia occurs when the body’s core temperature drops below 35.0°C (95.0°F). Prolonged cold exposure can deplete the body’s energy, leading to this condition. Initial signs of mild hypothermia include uncontrollable shivering and mental confusion. As temperature falls, shivering may cease, confusion can worsen, and physiological processes like breathing and heart rate slow down, potentially leading to organ failure.
Physical Effects of Temperature
Temperature influences the physical world, particularly by governing the states of matter. Most substances exist as a solid, liquid, or gas, and transitions between these states are driven by temperature changes. Water, for example, clearly demonstrates these transformations.
At temperatures below 0°C (32°F), water exists as solid ice, where molecules are closely bound. As ice absorbs thermal energy and its temperature rises to or above 0°C, it melts into liquid water, allowing molecules to move more freely. Further heating to 100°C (212°F) causes liquid water to boil and transform into steam, a gaseous state where molecules are widely dispersed and move rapidly.
Temperature also plays a role in large-scale phenomena, including weather patterns. Uneven heating of the Earth’s surface by the sun creates temperature differences in the atmosphere. Warmer air expands and rises, while cooler, denser air sinks. This differential heating and movement of air masses generate winds, which are air circulating horizontally to replace rising warm air.
Temperature is key to the water cycle. Evaporation, where liquid water turns into vapor and rises into the atmosphere, is accelerated by higher temperatures. As this water vapor ascends to cooler altitudes, it condenses into liquid droplets or ice crystals, forming clouds. This condensation releases heat, influencing atmospheric circulation and leading to precipitation, completing the cycle that distributes water across the planet.
Exploring Temperature Extremes
Temperature extends across a vast scale, from the coldest possible point to extremely hot environments. At one extreme lies Absolute Zero, defined as 0 Kelvin (K), equivalent to approximately -273.15 degrees Celsius or -459.67 degrees Fahrenheit. This theoretical temperature represents the point where particles of matter possess their lowest possible energy.
Temperatures in the universe can reach high levels. The surface of our Sun, for example, is around 5,500 degrees Celsius (9,900 degrees Fahrenheit). Hotter environments exist within the cores of massive stars, particularly during supernovas, where temperatures can reach approximately 100 billion Kelvin.
Extreme temperatures have been observed around supermassive black holes, with estimated core temperatures reaching around 10 trillion Kelvin. On Earth, scientists have recreated conditions seen shortly after the Big Bang in experiments like those at the Large Hadron Collider. These experiments have produced quark-gluon plasma at temperatures exceeding 5 trillion Kelvin, providing insights into the universe’s earliest moments.