Weather forecasting starts with careful measuring. Every day, thousands of small weather stations record the same set of numbers using the same set of instruments, in the same standard way. Get the instrument or the units wrong and the reading is worthless — so a huge part of measuring the weather well is knowing exactly what measures what, in which unit, and how the equipment is set up to be fair.
Why this matters — Paper questions on the weather station (0460) reward precise recall: naming the correct instrument, giving the correct unit, and explaining the design of the Stevenson screen. These are exact facts, so they are reliable marks if you learn them properly.
The instruments and their units
| Element | Instrument | Unit |
|---|---|---|
| Temperature | Maximum–minimum thermometer | °C (degrees Celsius) |
| Rainfall | Rain gauge | mm (millimetres) |
| Air pressure | Barometer | mb (millibars) |
| Wind speed | Anemometer | km/h or knots |
| Wind direction | Wind vane | compass direction |
| Humidity | Hygrometer (wet-and-dry bulb) | % (percentage) |
| Sunshine | Sunshine recorder | hours |
| Cloud cover | Observed by eye | oktas (eighths of the sky) |
Reading the instruments
- Maximum–minimum thermometer uses two scales and two small metal markers to record the highest and lowest temperature since it was last reset. The temperature range = maximum − minimum.
- Rain gauge is a funnel above a measuring cylinder, sunk into the ground with its rim about 30 cm above the surface to stop splash; rainfall is read in mm.
- Barometer measures air pressure; low pressure often brings unsettled, wet weather and high pressure brings calm, dry weather.
- Anemometer has spinning cups; the faster they turn, the higher the wind speed. A wind vane points into the wind to show the direction the wind is coming from.
- Cloud cover is estimated by eye in oktas (eighths): 0 oktas = clear sky, 8 oktas = completely overcast.
The Stevenson screen
Thermometers and the hygrometer are kept inside a Stevenson screen so that all stations measure in fair, comparable conditions. Its design is deliberate:
- Painted white — white reflects sunlight so the box does not heat up and give a falsely high reading.
- Louvred (slatted) sides — allow air to flow freely around the instruments while blocking direct sunlight and rain.
- Stood about 1.25 m above the ground — a standard height, above the layer of air warmed by the ground, so all stations are comparable.
- Door faces north (in the northern hemisphere) — so that opening it to read the thermometers does not let direct sunlight fall on them.
- Sited over grass, away from buildings — so heat radiating from concrete or walls does not spoil the reading.
Worked example (Explain, 3 marks): "Explain two features of a Stevenson screen that give accurate temperature readings." It is painted white so sunlight is reflected and the box does not overheat, keeping the reading fair (1+1). It has louvred sides so air circulates freely around the thermometer, giving the true air temperature rather than trapped warm air (1).
Common exam mistake: A wind vane shows the direction the wind blows FROM, not towards. A north wind comes from the north. Also, do not confuse the barometer (pressure, mb) with the hygrometer (humidity, %).
Memory hook: "White reflects, slats breathe, legs lift, grass below." Four words, four features of the Stevenson screen.