1. Air Pressure Fig. 4.1

Instrument: Aneroid Barometer | Unit: Millibars (mb) / hPa

960 980 1000 1020 1040 STORMY VERY DRY
1000 mb
  • Mechanism: Contains a sealed, corrugated metal vacuum chamber (aneroid capsule).
  • How it works: Atmospheric pressure squeezes the chamber in or out radially. Mechanical levers convert this motion to rotate the dial pointer.
  • Weather Forecast: Falling pressure indicates approaching low-pressure storms; rising pressure brings stable, fair conditions.

2. Wind Speed Fig. 4.2

Instrument: Cup Anemometer | Unit: Knots / km/h / mph

15 km/h
  • Mechanism: Three or four open hemispherical cups mounted on horizontal arms.
  • How it works: Wind catches the open cups, turning the shaft. Rotational speed (RPM) matches wind velocity.
  • Placement: Positioned in open spaces elevated above buildings to avoid wind obstruction.

3. Wind Direction Fig. 4.3

Instrument: Wind Vane (Weather Vane) | Unit: Compass Points (N, S, E, W)

N S E W
0° (North)
  • Mechanism: Asymmetrical pointer with a small arrow head and large tail fin mounted on a vertical pivot.
  • How it works: Wind pushes the large tail fin, turning the narrow front pointer directly into the oncoming wind.
  • Interpretation: Points to the source direction from which wind originates.

4. Humidity Fig. 4.4

Instrument: Mason's Hygrometer (Psychrometer) | Unit: Percentage (%)

Dry Wet Water
25°C
60%
Calculated Wet Bulb Temp: 20°C
Depression (Dry - Wet): 5°C
  • Mechanism: Two identical thermometers; one bulb wrapped in a damp cloth wick placed in water.
  • Evaporative Cooling: Water evaporates from the wick, lowering wet-bulb temperature. Drier air causes greater evaporation and higher temperature difference.
  • Relative Humidity: Determined using dry-bulb reading and temperature depression via psychrometric tables.