ZIMSEC O LEVEL GEOGRAPHY FORM 4 – WEATHER

Why measure the weather?

In everyday language weather means such qualities as wet or fine, warm or cold. For most people, such descriptive terms are adequate. However, many industries today require more quantitative assessments of the weather, with the use of standardised terms measured by suitably designed instruments. The science of the study of weather is called meteorology; the meteorologist measures temperature, rainfall, pressure, humidity, sunshine and cloudiness, and makes predictions and forecasts about what the weather will do in the future.

2.2. Temperature

The hotness or coldness of a substance is called its temperature and is measured with a thermometer. The ordinary thermometer consists of a hollow glass bulb attached to a narrow stem with a thread-like bore. The bulb is filled with liquid, usually mercury, but also alcohol when very low temperatures need to be measured, which expands when the temperature rises and contracts when the temperature falls. The amount of expansion and contraction is measured by a calibrated scale.

Whilst thermometers are really measuring their own temperature, they are usually needed to measure the temperature of the surrounding air. To ensure that the temperature of the surrounding air is the same as the thermometer, it must be shaded from sunlight and be exposed to adequate ventilation. These conditions are provided by enclosing the thermometer within a wooden box with louvred sides, called a Stevenson screen (see Figure 2.1).

Figure 2.1. The Stevenson Screen

Most temperature scales today are expressed in degrees Celsius (°C), although one will sometime see Fahrenheit (°F) in use. The Celsius scale is fixed by two points, the freezing and boiling point of water, which at normal atmospheric pressure are 0°C and 100°C respectively. The scale is then divided into 100 units. 0°C is equivalent to 32°F and 100°C to 212°F. The Kelvin temperature scale is the absolute temperature scale. Absolute zero, the coldest temperature possible in the universe is 0K or -273°C. Because one Kelvin is equivalent to one degree Celsius, 0°C is the same as 273K. 15°C is the same as 288K.

Special thermometers are used to indicate the maximum and minimum temperatures reached over a period, usually one day. For the amateur, a popular combined maximum and minimum thermometer is the U-shaped thermometer (see Figure 2.2). Thermometers are also used to measure the temperature of the ground at night, which may fall several degrees below that of the air above.

Figure 2.2. The maximum-minimum thermometer

2.3. Pressure

Like all fluids, the air exerts a pressure on everything within and around it, although we are not aware of it. Pressure is a force, or weight, exerted on a surface per unit area, and is measured in Pascals (Pa). The pressure exerted by a kilogram mass on a surface equals 9.8 Pa. The pressure exerted by the whole atmosphere on the Earth’s surface is approximately 100,000 Pa. Usually, atmospheric pressure is quoted in millibars (mb). 1mb is equal to 100 Pa, so standard atmospheric pressure is about 1000mb. In fact, actual values of atmospheric pressure vary from place to place and from hour to hour. At sea level, commonly observed values range between 970 mb and 1040 mb. Because pressure decreases with altitude, pressure observed at various stations must be adjusted to the same level, usually sea level.

Atmospheric pressure is measured by a barometer. A mercury barometer (see Figure 2.3) measures the pressure by noting the length of mercury which is supported by the weight of the atmosphere. One centimetre of mercury is equal to 13.33 mb, so normal atmospheric pressure can support a column of mercury about 75 cm (or 30 inches) high.

An aneroid barometer is a more compact instrument for measuring pressure. It consists of a box of partially exhausted air which expands and contracts as the pressure falls and rises. The box is connected through a system of levers to a pointer which, in conjunction with a dial, indicates the pressure.

Figure 2.3. The mercury barometer

2.4. Humidity

Some water in the form of invisible vapour is intermixed with the air throughout the atmosphere. It is the condensation of this vapour which gives rise to most weather phenomena: clouds, rain, snow, dew and fog. There is a limit to how much water vapour the air can hold and this limit varies with temperature. When the air contains the maximum amount of vapour possible for a particular temperature, the air is said to be saturated. Warm air can hold more vapour than cold air. In general the air is not saturated, containing only a fraction of the possible water vapour.

The amount of vapour in the air can be measured in a number of ways. The humidity of a packet of air is usually denoted by the mass of vapour contained within it, or the pressure that the water vapour exerts. The variation of absolute humidity (vapour content of saturated air) with temperature is shown in Figure 2.4.

Figure 2.4. Variation of absolute humidity with temperature

Relative humidity is measured by comparing the actual mass of vapour in the air to the mass of vapour in saturated air at the same temperature. For example, air at 10°C contains 9.4 g/m³ of water vapour when saturated. If air at this temperature contains only 4.7 g/m³ of water vapour, then the relative humidity is 50%.

When unsaturated air is cooled, relative humidity increases. Eventually it reaches a temperature at which it is saturated. Further cooling leads to condensation of the excess water vapour. The temperature at which condensation sets in is called the dew point.

The dew point, and other measures of humidity can be calculated from readings taken by a hygrometer (see Figure 2.5). A hygrometer has two thermometers, one dry bulb or standard air temperature thermometer, and one wet bulb thermometer. The wet bulb thermometer is an ordinary thermometer which has the bulb covered with a muslin bag, kept moist via an absorbent wick dipped into water. Evaporation of water from the muslin lowers the temperature of the thermometer. The difference between wet and dry bulb temperatures is used to calculate the various measures of humidity.

Figure 2.5. The hygrometer

2.5. Wind

The air is nearly always in motion, and this is felt as wind. Two factors are necessary to specify wind, its speed and direction. The direction of wind is expressed as the point of the compass from where the wind is blowing. Air moving from the north-east to the south-west is called a north-east wind. It may also be expressed in degrees from true north. A north-east wind would be 45^o. A south- west wind would be 235^o. The wind speed can be expressed in miles or kilometres per hour, meters per second, knots or as a force on the Beaufort scale.

The Beaufort Scale

2.6 Precipitation

The amount of rain, sleet, snow or hail which falls in a specified time is expressed as the depth of water it would produce on a large, level impermeable surface. Usually it is expressed in millimetres although inches may sometimes be used. Rainfall is measured daily (24 hours) by means of a rainguage. Today’s rainguages are simple to use with pre-calibrated scales on their sides. When measuring rainfall, certain precautions have to be taken against the effects of obstructions, wind, splashing and evaporation.

2.7 Sunshine

Many weather stations record the amount of bright sunshine during daylight hours. A typical measuring instrument is the Campbell- Stokes sunshine recorder. This consists of a glass ball which focuses the Sun’s rays, burning a hole in an index card. As the Sun moves around the sky, so the burnt hole extends along the index, which is marked in hours to record the amount of direct sunshine.