Statement I : Pressure gradients determine the velocity of winds. Statement II : When isobars (lines of equal atmospheric pressure) are closely spaced, the wi nd velocity would be gentle.

Statement I is true as the pressure gradient force is the primary driver of wind, where the rate of change of pressure with respect to distance determines wind velocity [4]. A greater pressure difference over a specific distance results in higher wind speeds [1]. Statement II is false because the relationship between isobar spacing and wind velocity is inverse to what is stated. Closely spaced isobars indicate a steep or strong pressure gradient, which leads to high wind velocities rather than gentle ones [2][t1][t5]. Conversely, widely spaced isobars represent a weak or gentle pressure gradient, resulting in lighter or gentle winds [t1][t6]. Therefore, while pressure gradients do determine velocity, closely spaced isobars signify strong winds, making the second statement incorrect.

Sources

1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 23: Pressure Systems and Wind System > Pressure Gradient Force > p. 306
2. [4] https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2000-6A%20Chap%204-6.pdf
3. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 23: Pressure Systems and Wind System > Isobars > p. 304