Temperature — Page 62, Lesson 52

I want to walk you through the topic of temperature in meteorology, starting from the very beginning. Temperature is one of the most important variables in the atmosphere. The way it varies, both horizontally across the earth and vertically as you go up through the air, has huge significance for understanding weather. Let's start with measurement. There are three scales that can be used to measure temperature, but only two of them are used in meteorology: Celsius and Kelvin. The Fahrenheit scale is mentioned for completeness. On the Fahrenheit scale, the melting point of ice is +32 degrees and the boiling point of water at standard pressure is +212 degrees. On the Celsius scale, also called Centigrade, those same points are 0 and +100 degrees. On the Kelvin or Absolute scale, they are +273 and +373 Kelvin. Notice that Kelvin doesn't use the word 'degrees' — it's just Kelvin. Now, the conversion factors. To convert from Fahrenheit to Celsius, you use: °C equals five-ninths times (°F minus 32). To go the other way, from Celsius to Fahrenheit: °F equals nine-fifths times °C plus 32. And to get Kelvin from Celsius, you simply add 273: K equals °C plus 273. Let's move to the instruments used to measure temperature. The standard means of measurement on the ground is a mercury thermometer placed inside a Stevenson Screen. If the screen isn't easily accessible to the observer, electrical resistance thermometers may be used instead. Inside the screen you'll also find a Thermograph, which works similarly to a barograph but records temperature instead of pressure. The Stevenson Screen itself is a louvred box, and it's positioned 4 feet — that's 1.22 metres — above the ground. This screen is used worldwide, and you can see it in Figure 5.1. For upper air temperature, along with pressure and humidity, we use a device called a Radiosonde. This is shown in Figure 5.3. A Radiosonde transmits continuous readings while being carried aloft beneath a balloon. Its rate of climb is 1200 feet per minute, and its maximum ceiling is between 65,000 and 115,000 feet. Earlier versions of these devices were tracked using radar to determine their position and to calculate wind speed. Modern systems use GPS to provide a three-dimensional position, which is sent along with the data. Aircraft readings are another source of temperature data. They are often the only way to measure atmospheric temperature over oceans and other areas far from meteorological stations. However, they are not as accurate as ground-based measurements because they are affected by compressibility and lag. On some modern aircraft, the electrical thermometer gives a digital readout of temperature, and this can be automatically calibrated and transmitted. Now let's talk about the heating of the troposphere. The main source of heat for the troposphere is the sun. Solar radiation from the sun is of short wavelength, denoted by the Greek letter lambda (λ). This radiation has a wavelength range of 0.15 to 4 microns. A micron, symbol µ, is 10 to the minus 6 metres. This short-wave radiation passes through the troposphere almost without heating it at all. Some of this solar radiation is reflected back to the upper air from cloud tops and from water surfaces on the earth. The rest of this radiation heats the earth's surface. The process by which the surface is heated by solar radiation is called insolation. So the troposphere isn't heated directly by the sun's rays; it's heated from the ground up, as the earth's surface warms and then transfers that heat to the air above it.