A weather reading is only as reliable as the instrument, the site and the timing. Good interpretation names a limit and says which way it pushes the numbers.
This lesson belongs to the weather and climate module. It connects the graphs from read a climate graph correctly with the question of where the numbers come from.
Which instruments should you know?
| Instrument | Measures | Unit | One limit to know |
|---|---|---|---|
| Thermometer in a screen | Air temperature | °C | Reads to the nearest scale division only |
| Maximum-minimum thermometer | Highest and lowest temperature | °C | Needs resetting each day |
| Rain gauge | Rainfall | mm | Site, wind and evaporation affect the total |
| Barometer | Air pressure | millibars or hPa | Needs a stable indoor position |
| Anemometer | Wind speed | km/h or m/s | Gusts and sheltered sites change readings |
| Wind vane | Wind direction | compass point | Shows direction of origin only |
If you carry out outdoor fieldwork yourself, follow your school’s safety guidance and supervision rules.
How do you judge a reading, step by step?
- Name what is being measured and the unit.
- Check the site: open, shaded, sheltered, or near buildings?
- Check the timing: how often was the reading taken, and over how long?
- Check the scale: what is the smallest division?
- Say which way the limit biases the result, and whether the conclusion still stands.
Worked example
The data below is invented for teaching.
A school records daily rainfall in mm with a gauge for seven days: 0, 0, 12, 8, 0, 5, 0.
Total: 12 + 8 + 5 = 25 mm. Mean per day: 25 ÷ 7 = 3.57, so 3.6 mm to one decimal place.
Now suppose the gauge sits under a tree. Part of the rain is caught by leaves and does not reach the opening. So the true rainfall is probably higher than 25 mm, and the mean of 3.6 mm per day is likely to be an underestimate.
A second limit: the thermometer shows temperatures to the nearest 0.5 °C. A reading of 23.5 °C could really be anywhere from 23.25 to 23.75 °C. That difference is small compared with a range of 10 °C, so it does not change a conclusion about a wide temperature spread.
The mistake to watch for
Mistaken answer: “The rain gauge is not accurate because instruments are always wrong.”
The student gave a vague reason with no cause and no effect on the data.
The correction is to be specific: “The gauge is under a tree. Leaves intercept some rain, so the total is too low.” A strong answer gives the cause, then the direction of the error.
Check yourself
1. A rain gauge records 3, 0, 7, 0, 5 mm over five days. Find the total and the mean per day.
Show answer
Total = 3 + 0 + 7 + 0 + 5 = 15 mm. Mean = 15 ÷ 5 = 3 mm per day.
2. A thermometer in direct sunlight shows 38 °C while a screened one nearby shows 33 °C. Which is closer to the air temperature, and why?
Show answer
The screened thermometer, because it is shaded from the sun. The one in direct sunlight is heated by sunshine and reads too high.
3. Give one reason why a single day of readings might not represent a month.
Show answer
Weather changes from day to day, so one day may be much wetter, drier, hotter or cooler than the monthly pattern.
Where this leads next
Finally, learn how short records relate to long ones in distinguish a weather event from a climatic pattern. Try the statistics and distribution explorer to work out totals and means. When evaluation answers feel thin, our teachers can work on them in online one-to-one Geography tuition.