Temperature is a measure of how much internal energy an object or gas has. For example, a gas with fast-moving molecules feels "hot" because when that gas touches something that is cooler, some of the energy of the hot gas is transferred to the cooler object and the cooler object responds by warming up. You sense the transfer of energy as heat. When you touch something that has a lower temperature than your hand, you sense it as being cold because energy leaves your hand and is transferred to the colder object. If there are two objects with different temperatures, energy always flows from the warmer object to the colder object.
In the atmosphere, temperature is related to volume, pressure, and density. Temperature is inversely related to density but directly related to pressure and volume. This means, for example, that when temperature increases, density decreases, and volume and pressure of the gas also increases. So air that is warm and dry will tend to rise when surrounded by cooler air because warm air is less dense than the cooler air around it.
There are three different scales frequently used to measure temperature. Fahrenheit, the most commonly used scale in America, was developed in the early 1700s and is the oldest of the three scales we still use. In this scale, 32Â°F is where water freezes and 212Â°F is where water boils, with a range of 180 degrees between the two. The lowest temperature G. Daniel Fahrenheit was able to reach using a combination of salt, ice, and water was set as zero degrees F in his scale. The second oldest scale is the Kelvin scale, developed by Lord Kelvin in the mid 1800s. This scale begins at absolute zero and has no negative numbers. Absolute zero is when all molecular motion stops, which is not known to exist anywhere in the universe. Even space has a background temperature of 3 K. The Celsius scale was developed after the Kelvin scale in the 1800s. One degree C is the same size as one degree K except that zero is at a different value. In the Celsius scale a change of one degree is equivalent to one Kelvin and 1.8 degrees Fahrenheit. For example, if itâ€™s 80Â°F outside one day, then it is 27Â°C and 300 K.
Measurement of Temperature
|Figure B: Electronic Temperature Sensor|
|Image from the Illinois State Climatologist Office|
The temperature of an object or gas can be measured either using direct contact or with remote sensing. Typically, in direct contact a thermometer containing an expandable liquid like mercury or alcohol is placed into the substance to be measured and allowed to reach equilibrium. The thermometer is calibrated so that it can accurately measure the temperature of the substance. More modern thermometers use electronic sensors to measure temperatures using the thermal properties of the sensor to determine how hot an object or gas is. Satellites use cameras to measure the amount of light radiation an object like a cloud gives off from a distance and uses that information to calculate the temperature. In a satellite picture, the brightest clouds are often the highest and coldest clouds.
Figure B shows a temperature sensor that is contained within this unit and shaded from the sun. Vents on the outside allow for the wind to flow over the temperature sensor inside.