Describe the instruments used to measure the elements of weather and how to use them accurately

What this dot point is asking

SEAB wants you to name the instruments that measure each element of weather, to describe how they are used accurately, and to explain the role of the Stevenson screen and careful siting. The central insight is that a measurement is only trustworthy if it is taken in standard conditions; the instruments and their housing are designed to remove errors so readings can be fairly compared between places and over time.

The answer

The instruments

Each element of weather has its own instrument:

• Thermometer: measures air temperature in degrees Celsius. A maximum-minimum thermometer records the highest and lowest readings since it was last reset.
• Rain gauge: a funnel and measuring cylinder that collects rainfall; the depth of water collected gives the rainfall in millimetres.
• Hygrometer: measures humidity. A wet-and-dry-bulb hygrometer compares two thermometers, one with a wet wick, to find relative humidity.
• Barometer: measures air pressure in millibars or hectopascals.
• Anemometer: measures wind speed; spinning cups turn faster in stronger wind.
• Wind vane: shows the direction the wind is blowing from.
• Sunshine recorder: measures the hours of bright sunshine.

Reading instruments accurately

Accuracy depends on careful use:

• Read a thermometer or measuring cylinder at eye level to avoid a parallax error.
• Empty and reset the rain gauge after each reading, and site it away from overhanging trees or walls that would block or funnel rain.
• Take readings at standard times each day so they are comparable.

The Stevenson screen and siting

The Stevenson screen is a white wooden box that houses the thermometers and hygrometer. Its design removes errors:

• White paint reflects direct sunlight, so the thermometer is not heated by the sun and measures the true air temperature in the shade.
• Louvred (slatted) sides let air circulate freely so the reading reflects the surrounding air.
• It stands about $1.2\ \text{m}$ above a grassed surface, a standard height away from the heat radiated by bare ground or buildings.

Siting matters too: the rain gauge and anemometer are placed in the open, away from buildings and trees that would shelter them and distort readings.

Examples in context

Example 1. The Meteorological Service Singapore's stations. Singapore's official weather stations, such as the long-running one at Changi, use standardised instruments in Stevenson screens and open rain gauges so that temperature and rainfall records stretching back decades are consistent. This consistency is what lets meteorologists say reliably that recent years have been among the warmest on record, since the readings were all taken in the same standard way.

Example 2. A school geographical investigation. When students compare the microclimate of a shaded courtyard with an open field, they must use identical, correctly sited instruments at both sites, ideally shaded thermometers at the same height. If one thermometer sat in the sun, the comparison would reflect the siting, not the real microclimate. Standardised measurement is what makes the fieldwork conclusion trustworthy, the same principle that governs national weather stations.

Try this

Q1. Name the instrument used to measure (a) air pressure and (b) wind direction. [2 marks]

• Cue. (a) A barometer measures air pressure; (b) a wind vane shows the direction the wind is blowing from.

Q2. Explain why a thermometer is housed in a white box rather than a black one. [2 marks]

• Cue. White reflects direct sunlight so the thermometer is not heated by the sun and reads the true shaded air temperature; a black box would absorb sunlight and heat up, giving a reading that is too high.

Q3. Explain why a rain gauge should not be placed under a tree. [2 marks]

• Cue. A tree overhangs the gauge and blocks some of the rain from reaching the funnel, so the gauge collects less than the true rainfall, giving a reading that is too low and not comparable with an open site.