2.4 Weather: Interpret weather data and identify instruments and their uses.

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Cambridge IGCSE Geography 0460 – Topic 2: The Natural Environment

Objective 2.4 – Weather: Interpret weather data and identify instruments and their uses

1. Key Definitions (AO1)

  • Weather: the short‑term state of the atmosphere at a particular place and time, described by temperature, humidity, pressure, wind speed & direction and precipitation.
  • Climate: the long‑term average of weather (normally a 30‑year period) for a region. Climate tells us what weather is typical; weather tells us what is happening now.

Understanding the difference is the foundation for later topics on climate change and global‑scale weather patterns.

2. Typical Weather Patterns

  • Diurnal cycle: temperature rises after sunrise, peaks in the early afternoon and falls through the night; wind usually weakens after sunset.
  • Seasonal pattern: systematic changes in temperature, pressure systems and prevailing winds that repeat each year (e.g., warm summers, cold winters).
  • Synoptic (day‑to‑day) pattern: movement of high‑ and low‑pressure systems and associated fronts that cause rapid changes in weather.

3. Interpreting Weather Data (AO2)

When presented with a data set, follow these steps:

  1. Identify each variable and its unit (°C, hPa/mb, km h⁻¹, mm, %).
  2. Check the time‑scale – is it hourly, daily, weekly or monthly?
  3. Look for trends, cycles or abrupt changes (e.g., a rapid pressure fall).
  4. Compare the values with the normal range for the location (climatological normals).
  5. Represent the data visually – tables, line graphs, bar charts or combined plots.
  6. Answer the guiding questions:
    • What are the highest and lowest values?
    • When did the extremes occur?
    • Is there a regular cycle (diurnal, seasonal) evident?
    • Do any values indicate an unusual event (e.g., sudden wind‑speed increase, heavy rain)?

3.1 Example – Five‑day Weather Table

Day Max Temp (°C) Min Temp (°C) Mean Sea‑Level Pressure (hPa) Wind Speed (km h⁻¹) Precipitation (mm)
Mon22121015150
Tue24131012202
Wed19111008305
Thu211210053512
Fri23141009180

3.2 Suggested Graphical Representation

Draw a single time‑axis plot showing:

  • Maximum and minimum temperature as two line series.
  • Mean pressure as a dashed line (use a secondary vertical axis if needed).
  • Wind speed as a bar chart.
  • Precipitation as a separate column chart or a blue‑filled area.

3.3 Interpretation of the Sample Data

  • Pressure trend: a steady fall from 1015 hPa (Mon) to 1005 hPa (Thu) signals a low‑pressure system approaching. The lowest pressure coincides with the highest wind speed (35 km h⁻¹) and the greatest rainfall (12 mm) – typical of a frontal passage.
  • Temperature pattern: the warmest day is Tuesday (max 24 °C). Wednesday shows the largest diurnal range (19 – 11 °C = 8 °C), suggesting clear skies and strong nocturnal cooling.
  • Precipitation: Thursday’s 12 mm of rain, together with strong winds and low pressure, indicates the greatest flood risk for the period.

4. Weather Instruments – Identification, Use, Accuracy, Calibration & Limitations (AO1‑AO2)

Instrument Variable Measured Typical Use in Observation Typical Accuracy Calibration / Maintenance Key Limitation
Thermometer (mercury or alcohol) Air temperature (°C or °F) Record daily maximum and minimum; feed climate‑normal calculations. ±0.2 °C (mercury) / ±0.5 °C (alcohol) Calibrated annually against a standard reference thermometer; housed in a Stevenson screen. Radiation error if not properly shielded; lag in windy conditions.
Barometer (mercury or aneroid) Atmospheric pressure (hPa or mb) Detect pressure trends that indicate approaching highs, lows or fronts. ±0.5 hPa (mercury) / ±1 hPa (aneroid) Checked regularly against a calibrated pressure standard; temperature‑compensated housing required. Mechanical drift; aneroid capsules are temperature‑sensitive.
Hygrometer (psychrometer or electronic) Relative humidity (%) Predict fog, dew point and likelihood of precipitation. ±2 % RH (sling psychrometer) / ±1 % RH (electronic) Sling psychrometer: wet‑bulb must be clean; electronic sensors calibrated against a humidity standard. Wet‑bulb evaporation can be affected by wind; electronic sensors may drift with age.
Anemometer (cup or propeller) Wind speed (km h⁻¹ or m s⁻¹) Provide data for storm warnings, wind‑energy assessments and synoptic analysis. ±0.5 m s⁻¹ (≈±1.8 km h⁻¹) Calibrated on a rotating platform with known speeds; must be level and free from obstructions. Response time slows in gusty conditions; cups can become clogged.
Wind vane (weather vane) Wind direction (compass points) Shows the direction from which the wind is blowing; used with pressure to track systems. ±5° Mounted on a high, unobstructed pole; direction checked against a compass reference. Can be affected by nearby structures; may freeze in cold weather.
Rain gauge (standard 0.2 mm resolution) Precipitation amount (mm) Measure daily rainfall; essential for flood forecasting and water‑resource studies. ±0.2 mm Funnel cleaned regularly; gauge checked for leaks and calibrated with a measured water volume. Cannot record very light drizzle (<0.2 mm); wind can cause under‑catch.
Sunshine recorder (Campbell‑Stokes) Duration of sunshine (hours) Record total hours of direct sunlight; contributes to climate normals. ±0.1 h Glass sphere cleaned; paper strip replaced monthly and compared with a calibrated time standard. Cloud‑cover shading can be misread; requires clear sky for accurate burn marks.

4.1 How the Instruments Work – Brief Descriptions

  • Thermometer: liquid (mercury or coloured alcohol) expands with heat and contracts with cooling, moving a calibrated scale inside a sealed glass tube.
  • Barometer: mercury column height reflects pressure; aneroid type uses a sealed metal capsule that expands/ contracts, moving a pointer.
  • Psychrometer (wet‑and‑dry bulb): the temperature difference between the dry‑bulb and a wet‑bulb (wicked with water) is read on a psychrometric chart to obtain relative humidity.
  • Anemometer: rotating cups (or propeller) generate a number of revolutions per unit time that is converted into wind speed.
  • Wind vane: a fin or arrow aligns with the wind; its position is read against a compass rose on the pole.
  • Rain gauge: rain falls into a funnel and is collected in a graduated cylinder; the depth of water equals the amount of rainfall.
  • Campbell‑Stokes sunshine recorder: a glass sphere focuses sunlight onto a calibrated paper strip; the burn marks record the total duration of direct sunshine.

5. Evaluating Data Reliability & Instrument Limitations (AO3)

  • Site conditions – Instruments (thermometer, hygrometer, barometer) must be placed inside a Stevenson screen to avoid direct solar heating, precipitation and ground radiation.
  • Human error – Mis‑reading analogue dials, incorrect timing of observations, transcription mistakes.
  • Instrument drift – Calibration can shift over time; regular checks against known standards are essential.
  • Resolution limits – e.g., a rain gauge that records to 0.2 mm cannot detect very light drizzles, leading to under‑estimation of total rainfall.
  • Response time – Thermometers and hygrometers may lag behind rapid changes, especially in windy conditions.
  • Maintenance issues – Clogged rain‑gauge funnels, dirty wind‑vane fins, cracked thermometer bulbs cause systematic errors.

How to comment on reliability in an exam answer (AO3)

  1. State which instrument(s) were used and give their typical accuracy.
  2. Identify possible sources of error (site, human, drift, resolution).
  3. Explain how each error could affect the interpretation (e.g., an under‑recorded pressure drop might hide the approach of a low‑pressure system).
  4. Suggest at least one improvement (regular calibration, automated digital sensors, better siting, more frequent observations).

6. Linking Weather to Climate Change (Contextual Note)

Accurate weather observation provides the raw data for climate‑normal calculations. Over decades, trends such as increasing mean temperatures, shifts in pressure patterns or changes in precipitation intensity are the basis for detecting climate change. Understanding how to interpret short‑term weather data therefore underpins the ability to recognise long‑term climate trends.

7. Practice Activity – Interpreting a Data Set

Use the five‑day table in section 3.1 to answer the questions below. Show all working where calculations are required.

  1. Flood risk – Identify the day with the greatest risk of flooding and justify your answer using temperature, pressure, wind and precipitation data.
  2. Average maximum temperature – Calculate the mean of the maximum temperatures for the five‑day period.
  3. Pressure‑wind relationship – Describe the observed relationship between falling pressure and increasing wind speed. Relate this to the passage of a low‑pressure system or front.
  4. Data reliability – List two possible sources of error in the data set and discuss how they might influence your answers to (1)–(3).

8. Suggested Diagram

Schematic layout of a typical weather station showing the placement of a thermometer, barometer, hygrometer, anemometer, wind vane and rain gauge within a Stevenson screen.
A schematic of a standard weather‑station layout (instrument positions and Stevenson screen).

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