Solar Radiation, Heat Balance and Temperature | Class 11 Geography Notes

Forces Affecting the Velocity and Direction of Wind

Wind is air in motion caused by differences in atmospheric pressure. The velocity and direction of wind are influenced by three main forces: the pressure gradient force, the Coriolis force, and frictional force. The pressure gradient force arises from differences in pressure between two points and causes air to move from high to low pressure areas. The strength of this force depends on the pressure gradient, which is stronger where isobars are closer. The Coriolis force results from the Earth's rotation and deflects moving air to the right in the northern hemisphere and to the left in the southern hemisphere. This deflection increases with wind speed and latitude, being zero at the equator and maximum at the poles. Frictional force acts opposite to the wind direction near the Earth's surface, reducing wind speed and modifying its direction. It is strongest at the surface and decreases with altitude, extending up to 1-3 km. These forces combine to determine the actual wind direction and speed. For example, in the upper atmosphere where friction is negligible, the wind blows parallel to isobars (geostrophic wind) due to the balance between pressure gradient and Coriolis forces. Near the surface, friction causes the wind to cross isobars towards low-pressure areas, leading to convergence and divergence patterns essential for weather phenomena.

📊 Diagram: Figure 9.4: Geostropic Wind; Figure 9.5: Convergence and divergence of winds.

🔗 Connection: This section prepares for understanding wind circulation patterns around pressure systems and the general circulation of the atmosphere.