Background Information: Weather
Weather refers to short-term changes in atmospheric conditions or elements. Here, short-term means that the changes taking place over a matter of seconds, minutes, or hours, or from day to day. It is a term that denotes the state of the atmosphere at a given time and place. Changes in the weather are continuous and sometimes erratic.
Acted on by the combined effects of the earth's motions and energy from the sun, our planet's formless and invisible envelope of air reacts by producing an infinite variety of weather. In the days of Aristotle, air was believed to be one of four fundamental substances that could not be further subdivided into constituent components [the other three substances were fire, earth (soil) and water]. Although this view of environmental physics left much to be desired, nevertheless the clothes we wear and the activities we engage in are strongly influenced by the weather [action of air]. Also the nature of weather is expressed in terms of the same basic elements, which are focus of the meteorologist's measuring routine. The most important are (i) the temperature of the air, (ii) the type and amount of precipitation, (iii) the humidity of the air, (iv) the speed and direction of the wind, (v) the pressure exerted by the air, (vi) the type and amount of cloudiness and (vii) the duration of sunshine.
The main atmospheric conditions (atmospheric elements) are set out in the following, together with the instruments commonly used to measure them in a simple way. It is to be noted that the more sophisticated instruments are not mentioned here and readers are requested to refer to higher level text books for such information.
Measuring Air Temperature
An easy way to measure the air temperature is to use six's thermometer. This contains two liquids, alcohol, and mercury. After the sun rises in the morning, the air temperature generally rises, so causing the alcohol in the left-hand limb to expand. It pushes the mercury down this limb and up the right-hand limb. Sitting on top of the mercury is a small metal index, which is pushed up as the mercury rises. After the highest (maximum) temperature during the day is reached, the alcohol contracts again, and the mercury level falls, leaving behind the index. The maximum temperature is read from the bottom of the index. As the temperature falls, the mercury level rises up the left-hand limb, so pushing up another metal index that has been placed there. The minimum temperature is read from the bottom of this left-hand index. To ascertain the average temperature for the day, the maximum and minimum temperatures are added together and divided by two.
Measuring Rainfall
Rainfall is measured by a rain gauge, which collects the total rainfall in a 24-hour period. An example of simple rain gauge very similar to the direct rainfall gauge used by schools in the Schools of the Pacific Rainfall Climate Experiment (SPARCE) program. Rain falls through the funnel into the measuring cylinder and can be read directly in millimeters. If the rain is very heavy, the measuring cylinder will fill up and water will then collect in the overflow tube. The rain gauge should be read at the same time each day.
If the diameter of the open funnel is different to the measuring cylinder, an appropriate scaling factor will need to be applied. It is common for the measuring cylinder to be of smaller diameter, so allowing greater accuracy (fine resolution) of measurements. The gauge should be mounted at ground level and clear from trees and buildings (i.e., should have an open exposure).
Measuring Humidity
The wet and dry bulb hygrometer consists of two thermometers, called the wet bulb and the dry bulb. Around the wet bulb is a piece of wet cloth, from which the water is evaporating. This evaporation causes the wet bulb to cool, and so the wet bulb temperature is always lower than the dry bulb.
The wet bulb is therefore completely wet at all times. The dry bulb is partially wet from the humidity in the atmosphere. The degree of atmospheric humidity is ascertained from the difference in the temperatures of the two bulbs.
To take a humidity reading, the procedure is as follows: You simply record the temperatures of the wet and the dry bulbs. Then subtract the wet bulb temperature from the dry bulb temperature to get the wet bulb depression. Then, use special humidity tables to determine the relative humidity.
Measuring Wind Direction
Wind direction is measured by a wind vane, which consists of a rotating metal arrow that pivots on a vertical shaft. The arrow head points to the direction from which the wind blows, and the wind is named after this direction. Both graphical and digital means of recording wind speed and wind direction are easily available in modern professional instrumentation.
