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Heat vs Temperature
Heat and temperature are not the same. Heat is a form of energy that flows from one system or object to another because the two are at different temperatures. Temperature is a measure of the average kinetic energy (motion) of individual molecules in matter. We feel the effect of temperature as the sensible heat transfers from warmer objects to cooler objects.
For instance, when you jump into a cool lake you can sense the heat transfer from your skin to the water as kinetic energy leaves your body and flows to the water; a chill develops. Temperature and heat are related because changes in temperature are caused by the absorption or emission (gain or loss) of heat energy. The term beat energy is frequently used to describe energy that is added to or removed from a system or substance.
Temperature Scales
The temperature at which all atomic and molecular motion in matter completely stops is called 0° absolute temperature (commonly, “absolute zero”). Its value on the different temperature-measuring scales is – 273° Celsius(C), – 459.4° Fahrenheit (F), and 0 Kelvin (K). Figure 5.1 compares these three scales.
Fahrenheit scale
The Fahrenheit scale places the melting point of ice at 32° F and the boiling point of water at 212° F. The scale is named for Daniel G. Fahrenheit, a German physicist (1686-1736). He used these odd values based on the coldest temperature he could achieve in his laboratory (which he called 0° F) and on the approximate temperature of the human body, thought to be about 100° F.
His estimates placed the melting point of ice at 32° F, with 180 subdivisions in his scale to the boiling point of water at 212° F. (Note that there is only one melting point for ice, but there are many freezing points for water ranging from 32° F down to -40° F, depending on its purity and volume and certain conditions in the atmosphere.)
Celsius scale
About a year after the adoption of the Fahrenheit scale, Swedish astronomer Anders Celsius (1701-1744) developed the Celsius scale (formerly called centigrade). He placed the melting point of ice at 0° and boiling temperature of water at sea level at 100°, dividing his scale into 100 degrees using a decimal system.
Kelvin scale
British physicist Lord Kelvin (born William Thomson, 1824-1907) proposed the Kelvin scale in 1848. Science uses this scale because temperature readings start at absolute zero and thus are proportional to the actual kinetic energy in a material. The Kelvin scale’s melting point for ice is 273 K, and its boiling point of water is 373 K. Most countries use the Celsius scale to express temperature.
The United States remains the only major country still using the Fahrenheit scale. The continuing pressure from the scientific community and corporate interests, not to mention the rest of the world, makes adoption of Celsius and SI units inevitable for the United States.
Measuring Temperature
A mercury thermometer or alcohol thermometer is a sealed glass tube that measures outdoor temperatures. (Fahrenheit invented the alcohol and mercury thermometers.) Cold climates demand alcohol thermometers because alcohol freezes at -112°C ( -170°F), whereas mercury freezes at -39°C (-38.2° F).
The principle of these thermometers is simple: When fluids are heated, they expand; upon cooling, they contract. A thermometer stores fluid in a small reservoir at one end and is marked with calibrations to measure the expansion or contraction of the fluid, which reflects the temperature of the thermometer’s environment.
Thermistors measure temperature by sensing the electrical resistance of a semiconducting material, resistance changes at 4% per °C (a thermistor is in the shelter in Figure 5.3). Figure 5.2 shows a mercury minimum-maximum thermometer. It preserves readings of the day’s highest and lowest temperatures until reset (by moving the markers with a magnet).
Another type, the recording thermometer, creates an inked record on a turning drum; usually set for one full rotation every 24 hours or every 7 days. Thermometers for standardized official readings are placed outdoors, in small shelters that are white (for high albedo) and louvered (for ventilation) to avoid overheating of the instruments (Figure 5.3). They are placed at least 1.2-1.8 m (4-6ft) above a surface, usually on turf; in the United States 1.2 m (4ft) is standard.
Official temperature measurements are therefore in the shade to prevent the effect of direct insolation. Temperature readings are taken daily, sometimes hourly, at more than 15,400 weather stations worldwide. One of the goals of the Global Climate Observing System (GCOS) is to establish a reference network of one station per 250,000 km2 (95,800 mi.2). Some stations with recording equipment also report the duration of temperatures, rates of rise or fall, and variation over time throughout the day and night.
The daily mean temperature is an average of daily minimum-maximum readings. The monthly mean temperature is the total of daily mean temperatures for the month divided by the number of days in the month. An annual temperature range expresses the difference between the lowest and highest monthly mean temperatures for a given year. If you install a thermometer for outdoor temperature reading, be sure to avoid direct sunlight on the instrument and place it in an area of good ventilation, and at least 1.2 m off the ground.
