Geography (0460) | Weather

Hello IGCSE Geographers! Welcome to the Weather Chapter!

Weather surrounds us every single day. Whether you're planning a trip, a farmer planting crops, or an airline piloting a plane, understanding and measuring the atmosphere is crucial.
This chapter, Topic 2.4: Weather, is all about the instruments and techniques we use to measure the various elements of the atmosphere. It's a very practical section of the syllabus!
Don't worry if all the instrument names seem tricky at first. We will break them down into simple steps, focusing on their function, their key features, and why they are placed where they are.

1. Collecting Weather Data: The Key Instruments and Techniques

Weather data is collected at weather stations, which must follow strict rules to ensure the measurements are accurate and comparable (meaning stations worldwide can compare their readings fairly).

1.1 The Stevenson Screen: The Weather Instrument 'House'

Most temperature and humidity instruments are stored inside a small wooden box called a Stevenson Screen. This screen is designed to provide accurate readings by protecting the instruments from direct sunlight, rain, and wind, while still allowing air to flow freely.

Characteristics and Design of the Stevenson Screen:

• Colour: It is painted white to reflect the sun’s energy (insolation), preventing the inside temperature from becoming artificially high.
• Walls: The sides are made of louvers (slatted wooden panels). These allow air to circulate freely, ensuring the temperature measured inside is the actual air temperature, but they prevent direct sunlight from reaching the thermometers.
• Material: It is usually made of wood, which is a poor conductor of heat, providing insulation.
• Height: It is set on legs so that the bulbs of the thermometers are typically 1.25 metres (or 1.2 metres) above the ground. This height is standardized globally.
• Siting: It must be placed over short grass or a natural surface, away from any tall buildings or trees that might block the wind or reflect heat.

1.2 Instruments Used to Measure Weather Elements

A. Temperature Instruments

We measure temperature in degrees Celsius (or Centigrade).

• Maximum-Minimum Thermometer:
  • Use: Records the highest (maximum) and lowest (minimum) temperatures reached in a 24-hour period.
  • Features: It is a U-shaped glass tube. The maximum side uses mercury and the minimum side uses alcohol. Small metal indexes (markers) are pushed up or down by the expanding or contracting liquids, and they stay put until reset.
  • Skill Check: We use these readings to calculate the Diurnal Temperature Range (the difference between the Max and Min temperature in one day).

B. Humidity Instruments

• Wet-and-Dry Bulb Thermometer (Hygrometer):
  • Use: Measures humidity (the amount of water vapour in the air).
  • Features: It consists of two identical thermometers. The dry bulb measures the actual air temperature. The wet bulb has a cloth wick soaked in water wrapped around its bulb.
  • How it works: If the air is dry (low humidity), water evaporates quickly from the wet wick, cooling the bulb and giving a lower reading. If the air is saturated (high humidity), little evaporation occurs, and the wet bulb reading will be nearly the same as the dry bulb.

C. Wind Instruments

Wind has two key measurable components: speed and direction.

• Anemometer:
  • Use: Measures wind speed (usually in km/h or knots).
  • Features: Typically consists of three or four cups mounted on a vertical spindle. The faster the wind blows, the faster the cups rotate, and the speed is recorded digitally or mechanically.
• Wind Vane:
  • Use: Measures wind direction.
  • Key Rule: The vane points to where the wind is coming from (e.g., if the vane points north, it is a northerly wind).

D. Precipitation Instruments

• Rain Gauge:
  • Use: Measures the amount of rainfall over a period (in millimetres, mm).
  • Features: A cylinder sunk into the ground, with a collecting funnel leading to a receiving jar.
  • Siting: It must be placed in an open area, away from tall objects (like trees or fences) that might block the rain. The top of the funnel should be 30 cm above the ground to prevent splash-in from the ground.
  • Collection: The water collected is carefully poured into a special, narrow measuring cylinder to get an accurate reading (to the nearest 0.1 mm).

E. Sunshine and Pressure Instruments

• Sunshine Recorder (Campbell-Stokes Recorder):
  • Use: Records the duration of bright sunshine (in hours).
  • Features: Uses a glass sphere to focus the sun's rays onto a specially prepared paper card, burning a mark. As the sun moves across the sky, the burn line shows the number of hours the sun was shining brightly.
• Barometer:
  • Use: Measures atmospheric pressure (the weight of the air, in millibars (mb) or hectopascals (hPa)).
  • Significance: Falling pressure often means bad weather (a depression or low-pressure system) is approaching, while rising pressure means calm, settled weather (high pressure).

1.3 Observing Clouds

While instruments measure most elements, cloud cover is usually observed by eye.

• Cloud Type: Identified by shape and height (e.g., Cirrus are high and wispy; Cumulus are puffy/heaped; Stratus are low layers).
• Cloud Amount: Measured in Oktas. An Okta is one eighth of the sky.
  Example: If the sky is half covered, the cloud amount is 4 Oktas. If it is completely covered, it is 8 Oktas. If there are no clouds, it is 0 Oktas.

2. Making Calculations and Interpreting Data

Once data is collected, we need to process it. The syllabus requires you to make simple calculations and interpret graphs and diagrams.

2.1 Simple Calculations (Using Instrument Readings)

Two main types of calculations you will be asked to perform relate to averages and range.

A. Mean (Average) Temperature:

This is calculated by summing up the temperatures recorded (e.g., throughout the day or month) and dividing by the number of readings.
\[ \text{Mean} = \frac{\text{Sum of all readings}}{\text{Number of readings}} \]

B. Diurnal Range:

This is the simplest range calculation, focusing on the temperature variation in one day.
\[ \text{Diurnal Range} = \text{Maximum Temperature} - \text{Minimum Temperature} \] A large diurnal range means hot days and cold nights (common in deserts). A small range means stable temperatures (common near the sea or in equatorial regions).

C. Annual Range:

The difference between the mean temperature of the hottest month and the mean temperature of the coolest month in a year.

2.2 Interpreting Weather and Climate Data

Geographers use specialized diagrams and maps to show weather information. You must be able to read and understand these.

A. Climographs (Climate Graphs)

These are the most common diagrams and combine two types of data:

1. Temperature: Shown by a line graph (usually using the left axis).
2. Precipitation (Rainfall): Shown by bar graphs (columns, usually using the right axis).

What to look for: Look at the shape of the lines and bars to determine the season of highest rainfall and the annual temperature range.

B. Synoptic Charts (Weather Maps)

These maps use special lines and symbols to show weather conditions over a large area at a specific time.

• Isobars: Lines connecting places that have the same atmospheric pressure.
  Rule: If isobars are very close together, it indicates steep pressure gradient and strong winds. If they are far apart, winds are light.
• Isotherms: Lines connecting places that have the same temperature.
• Wind Symbols: Small arrows or shafts show wind direction and speed (using "feathers" or barbs).

C. Wind Rose Graphs

These diagrams show how frequently the wind blows from different directions over a long period (e.g., a month or a year). They look like a compass with bars extending outward, where the length of the bar shows the frequency.

3. Digital Instruments

Modern weather stations increasingly use simple digital instruments.

• Advantages of Digital Instruments:
  • Data can be collected and logged automatically.
  • Readings are quick and highly precise.
  • Measurements can be easily transmitted remotely.
• How they work: Digital sensors (thermistors for temperature, electronic sensors for pressure) convert physical measurements into electronic signals.