Essential Weather Measurements: Humidity and Wind Dynamics

Relative Humidity: Definition and Measurement

Relative humidity (RH) relates the amount of moisture actually present in the atmosphere to the maximum amount it can hold at a given temperature. If the air is saturated, the relative humidity is 100%.

Measuring Relative Humidity with a Hygrometer

Relative humidity is measured using a wet and dry bulb thermometer, sometimes known as a hygrometer. The measurement process relies on evaporative cooling:

• Water from a bottle soaks the cotton gauze covering the wet bulb, keeping the thermometer wet.
• As water evaporates from the damp cotton, it cools and reduces the temperature of the wet bulb thermometer.
• Consequently, the wet bulb temperature is lower than the dry bulb temperature due to this cooling effect.

Calculating Relative Humidity

The relative humidity is calculated based on the difference between the wet bulb temperature and the dry bulb temperature, typically using a reference table (e.g., 25°C dry bulb minus 22°C wet bulb equals a 3°C difference).

The relationship between the temperature difference and humidity is inverse:

• If there is a small difference between the readings, the humidity is high.
• If there is a large difference between the readings, the humidity is low.

Understanding Wind Speed

Wind speed is typically measured in kilometers per hour (km/h). The Beaufort scale is a very effective reference chart used for visually assessing wind speed based on observed effects.

Instruments for Measuring Wind Speed

Instruments specifically designed to measure wind speed are called anemometers. The most common type features metal cups that are rotated by the wind. This mechanical movement is transmitted electrically to a meter, which displays the speed of the wind.

Official Measurement Standards

Officially, wind speed should be measured at 10 meters above the ground or water surface. If using a hand-held instrument, it should be held at arm’s length to minimize turbulence caused by the observer's body.

Estimating Wind Speed with a Wind Sock

A wind sock can help estimate wind speed, as it shows the direction, strength, and speed of the wind:

• If the sock is extended straight out, the wind speed is approximately 20 knots (56 km/h).
• If the sock is limp, the conditions are calm.

Determining Wind Direction

Wind direction tells you the direction from which the wind originates.

Methods for Assessing Direction

Several methods can be used to determine wind direction:

1. The Finger Method: Wet your finger and hold it up. The side of the finger that dries first indicates the direction from which the wind is blowing.
2. Weather Vane: A weather vane on a nearby church or building provides a reliable indication of wind direction.
3. Wind Sock: Similar to airfields, a wind sock can be used to quickly visualize the direction of airflow.