You may be surprised to find out there only a few parameters that are regularly measured in the atmosphere. The most common include temperature, humidity, pressure, and the speed and direction of the wind. These few pieces of information taken at any point in the atmosphere basically provide us with a snapshot of the atmosphere at that point. From these snapshots taken simultaneously at predetermined locations in the country and at various heights in the atmosphere, we can infer nearly every other parameter of the atmosphere we need in order to model the motion of the atmosphere. So what's the catch? The catch is integrating all of the many factors that cause these snapshots to change over time, such as the earth-atmospheric heat budgets, discussed in Session 2, and the relationships between the measured parameters themselves. Modeling atmospheric motion is essential for both weather forecasting and predicting the transport and dispersion of pollutants. In the remainder of Part 2 and Part 3, we will look at those parameters we can measure and a few of the important relationships between them that we need to understand in order describe how the motion in the atmosphere is initiated.

Temperature

When we hear the word "temperature" we normally think of hot and cold. But what is hot and cold? When we measure the temperature of something we are actually measuring the average kinetic energy of molecules. Therefore, when we measure the air temperature, we are measuring the average kinetic energy of the molecules that make up the composition of the air. When air molecules bombard a thermometer, the kinetic energy of the air molecules is transferred to the liquid in the thermometer. This transfer of energy causes the liquid to heat up and expand and the thermometer to "rise." In the upper layer of the atmosphere, the thermosphere, the temperature is very hot, at 115 km it can be as hot as 65 C, but you would freeze because there are so few molecules that high up to bombard your body.

In meteorology we usually record temperatures in degrees centigrade or Celsius (C), but for computations we convert degrees Celsius to degrees Kelvin (K). (Use the temperature converter below to convert between degrees Fahrenheit, Celsius, and Kelvin.) Water freezes at 0 C and boils at 100 C. The centigrade temperature scale was, in fact, derived from these properties of water to create a scale with convenient reference points. The Kelvin scale is referred to as the absolute temperature scale. Zero degrees Kelvin (0 K) is absolute zero. If a substance has a temperature of 0 K then this means that all molecular motion has ceased. The molecules have no motion, thus no kinetic energy, and absolutely no temperature. Zero degrees Kelvin converts to centigrade as -273 C. In fact, this easy conversion is another reason we use the Celsius and the Kelvin scales. All you have to do to convert units of Celsius to Kelvin is add 273 degress to the centigrade temperature.

As in the case of the thermometer, increased molecular motion of the air molecules results in the expansion of the air. In other words, as the speed of the molecules increases, the molecules spread further away from each other making the air less dense. As mentioned earlier in this course, the "normal" density of air is 1.2 kg/m³. Less dense or lighter materials "float" on denser materials like a cork floats on the water. This is the reason warm air rises. Convection in the atmosphere is caused by heating of the air nearer the heat source, usually the ground, pavement, or a body of water. The heat is transferred to the air, causing it to expand and rise. Convection is one way in which pollutants are transported away from the surface of the earth and diluted with cleaner air.

You can convert back and forth between degrees Fahrenheit, Celsius, and Kelvin using this Javascript calculator. Note: You need a version of Netscape as recent as 2.02 in order to use this calculator.

Be sure to click "Reset Values" before trying another conversion.

On to Moisture

Back to Why We Measure the Atmosphere

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