Discuss the factors affecting the speed and direction of wind.
Wind is air set in motion due to differences in atmospheric pressure. It blows from high pressure to low pressure. The horizontal winds near the earth's surface respond to the combined effect of three forces – the pressure gradient force, the frictional force, and the Coriolis force.
The pressure gradient force is produced by differences in atmospheric pressure. The rate of change of pressure with respect to distance is called the pressure gradient. It is strong where the isobars are close to each other and weak where the isobars are apart. This force determines the initial movement of wind from high to low pressure areas.
The frictional force affects the speed of the wind. It is greatest at the surface and its influence generally extends up to an elevation of 1-3 km. Over the sea surface, the friction is minimal. This force slows down the wind, especially near the ground.
The Coriolis force is exerted by the rotation of the earth about its axis. It affects the direction of the wind. It deflects the wind to the right direction in the northern hemisphere. In the upper atmosphere, 2-3 km above the surface, winds are free from frictional effect and are controlled mainly by the pressure gradient and the Coriolis force. When these two forces are balanced, the resultant wind blows parallel to the isobar, known as the geostrophic wind.
Explanation
This answer covers all three main forces mentioned in the NCERT textbook that affect wind speed and direction: (1) Pressure Gradient Force - responsible for initiating wind movement from high to low pressure, (2) Frictional Force - affects wind speed, strongest at surface, and (3) Coriolis Force - affects wind direction due to Earth's rotation. The answer also mentions the concept of geostrophic wind in the upper atmosphere where friction is absent. The textbook explicitly states that horizontal winds near the earth surface respond to the combined effect of these three forces.