Wind Dynamics: Forces and Measurement Techniques

Wind Dynamics: Key Factors

The Pressure Force

The wind that originates and is called gradient wind. Wind speed is directly proportional to the pressure difference between points.

The Coriolis Force

This force is a result of the Earth's rotation. A mass of air originating from the north, through inertia, retains its direction and speed along the meridian. However, an observer in the Northern Hemisphere, moving with Earth's rotation, will find this air mass increasingly deflecting to the right of its original direction. Conversely, in the Southern Hemisphere, the deflection is to the left. This phenomenon is the effect of the Coriolis force.

The Friction Force

Air masses close to the ground constantly experience friction from the Earth's surface. The intensity of this friction varies with the nature of the terrain and its topography; it is greater over continents and lower over oceans. Friction alters wind direction and reduces its speed, consequently diminishing the Coriolis (deflecting) force.

Wind Measurement Techniques

Wind Direction

Wind direction is measured with a weathervane or anemoscope. Wind direction indicates the direction from which the wind originates. For example, a NE wind blows from the northeast. For more precise measurements, such as soundings in the free atmosphere, directions are expressed in sexagesimal degrees from 1 to 360°, measured clockwise from true North.

Wind Speed

Wind speed is typically expressed in m/s, km/h, or knots. Common unit conversions include:

• 1 m/s = 3.6 km/h
• 1 m/s = 2.24 mph
• 1 m/s = 1.94 knots

Instruments for Measuring Wind Speed

The primary instruments for measuring wind speed are anemometers and anemographs. Anemographs record changes in wind speed over time. Two primary types of anemometers are commonly used:

Rotating Anemometers

The most common is the cup anemometer, featuring three or four cups mounted on arms around a vertical axis. Another type is the vane anemometer, which uses a propeller-shaped rotor. The rotation of the propeller, driven by the wind, is transmitted to an indicator device.

Pressure Anemometers

These consist of a weather vane that keeps an orifice tube pointed directly into the wind. The air blowing through this opening creates a pressure differential within the tube. This pressure is then transmitted to indicators for measurement.

The Beaufort Scale

Beyond instruments, the Beaufort Scale provides a visual estimation of wind speed based on observed sea conditions or land effects. Originally developed for maritime use, it was later adapted for land-based observations.