Weather Instruments IGCSE Geography - Revision Notes

Stevenson Screen

• Wooden box on four legs at 121 cm height to avoid ground heat.
• Painted white to reflect sun’s rays.
• Made of wood to prevent heat transfer; sides slatted for air movement.
• Double-layered wood with airspace for insulation, placed on grass.
• Contains:
  • Maximum-minimum thermometer (Six’s thermometer)
  • Hygrometer with wet and dry bulb
• Instruments outside include rain gauge, wind vane, and anemometer.
• Weather station layout avoids heat sources, severe winds, and direct sun.

Wind Measurement

Wind Direction

• Measured with a wind vane.
• Direction is the compass point from where the wind blows.

Wind Speed

• Measured with anemometers in m/s or km/hr.
• Anemometers have cups that rotate to measure wind speed.
• Digital anemometers send data to apps/computers.

Precipitation

• Measured using a rain gauge in millimeters (mm).
• Collected water poured into a measuring cylinder daily.
• Eye-level reading of the water’s meniscus is recorded.

Temperature

• Measured in Celsius or Fahrenheit.
• Instruments include thermometers or thermographs.
• Traditional Six’s thermometer measures daily min/max temperatures and calculates averages.
• Diurnal temperature range and mean monthly/annual temperatures can be calculated.

Humidity

• Measured with hygrometers using wet and dry bulb thermometers.
• Relative humidity compared to air’s maximum capacity at a given temperature.

Air Pressure

• Measured in millibars (mb) using a barometer.
• Types:
  • Mercury barometer
  • Aneroid barometer
  • Barograph
• Pressure readings shown by changes in mercury or using levers for aneroid/barograph.

Sunshine

• Measured with a Campbell-Stokes sphere.
• Sunlight scorches a card, indicating sunshine duration.
• Positioning: open area, south-facing in the northern hemisphere.

Cloud Cover

• Measured in oktas (eighths of the sky covered by clouds).
• Different cloud types based on shape and height (e.g., cirrus, stratus, cumulus).

Tropical Storms

• Known as typhoons, hurricanes, or cyclones based on location.
• Rated on Saffir-Simpson scale, categories 1-5 based on wind speeds.
• Category 3 and above is considered major.
• Feature intense low-pressure systems with a central eye.