Understanding Geostrophic Wind: Statements and Explanations

This question asks us to evaluate two statements about geostrophic wind and determine if they are individually true and if Statement II explains Statement I. Let's break down each statement.

Statement I: Geostrophic wind blows above a height of 600 metres, parallel to the isobars.

This statement talks about the characteristics of geostrophic wind, specifically its typical height of occurrence and direction relative to isobars (lines of equal atmospheric pressure).

• Height: Geostrophic wind is a theoretical wind that occurs when the pressure gradient force is balanced by the Coriolis force. This balance is achieved when the effect of friction from the Earth's surface becomes negligible. Friction significantly impacts wind speed and direction in the lower atmosphere, typically below what is known as the "planetary boundary layer". The height of this boundary layer varies, but it is often considered to extend up to approximately 1000 metres or higher, depending on atmospheric conditions. Above this layer, often referred to as the "free atmosphere", friction is minimal. While 600 metres is a specific value, it falls within the range where friction starts becoming less dominant, allowing the geostrophic balance to be approximated. So, stating it occurs above 600 metres is generally considered true in the context of describing where friction effects diminish significantly enough for the geostrophic approximation to be valid.
• Direction: Geostrophic wind is indeed characterized by blowing parallel to the isobars. In the Northern Hemisphere, it blows with lower pressure to the left, and in the Southern Hemisphere, it blows with lower pressure to the right. This parallel flow is a direct consequence of the balance between the pressure gradient force and the Coriolis force.

Based on these points, Statement I appears to be true as it correctly describes key characteristics of geostrophic wind.

Statement II: Geostrophic wind is the horizontal wind velocity, in which the Coriolis force balances the horizontal pressure force.

This statement provides the definition of geostrophic wind based on the forces acting upon the air parcel.

• Definition: The geostrophic wind ($\vec{V}_g$) is defined as the hypothetical horizontal wind that would result from an exact balance between the horizontal pressure gradient force ($\vec{F}_p$) and the Coriolis force ($\vec{F}_c$). Mathematically, this balance is represented as $\vec{F}_p + \vec{F}_c = 0$, or $\vec{F}_p = -\vec{F}_c$. The pressure gradient force acts perpendicular to isobars from high pressure to low pressure, and the Coriolis force acts perpendicular to the wind velocity.
• Forces Involved: It correctly identifies the two primary forces involved in the geostrophic balance: the Coriolis force and the horizontal pressure force (pressure gradient force).

This statement accurately defines geostrophic wind based on the force balance. Therefore, Statement II is also true.

Examining the Relationship between Statements I and II

Now we need to consider if Statement II is the correct explanation for Statement I. Statement I describes where geostrophic wind occurs (above 600m) and how it blows (parallel to isobars). Statement II defines geostrophic wind in terms of the force balance (Coriolis force balancing pressure force).

• The reason geostrophic wind blows parallel to the isobars (part of Statement I) is precisely because of the force balance described in Statement II. The pressure gradient force pushes air from high to low pressure, perpendicular to isobars. As the air parcel accelerates, the Coriolis force acts perpendicular to its motion. In the absence of friction (which is why it's considered above the friction layer, hence relating to the height in Statement I), the wind speed and direction adjust until the Coriolis force exactly opposes the pressure gradient force. This balance results in the wind flowing at a 90-degree angle to the pressure gradient force, which means it flows parallel to the isobars.
• The condition of being above the friction layer (like above 600m) mentioned in Statement I is the atmospheric condition necessary for the force balance described in Statement II to be a valid approximation of the actual wind. In the friction layer (below ~600-1000m), friction also plays a significant role, preventing the pure geostrophic balance and causing the wind to blow across the isobars towards lower pressure.

Thus, the force balance described in Statement II (Coriolis force balancing pressure force) is the fundamental reason why the geostrophic wind blows parallel to the isobars, as stated in Statement I, and why this phenomenon is best observed above the friction layer. Therefore, Statement II provides the correct explanation for the characteristics mentioned in Statement I.

Both statements are individually true, and Statement II explains Statement I.

Conclusion

Based on the analysis:

• Statement I is true.
• Statement II is true.
• Statement II is the correct explanation for Statement I.

This aligns with the option stating that both statements are individually true and Statement II is the correct explanation of Statement I.

Revision Table: Geostrophic Wind Concepts

Additional Information on Atmospheric Forces and Wind

Besides the Pressure Gradient Force and Coriolis Force, other forces influence wind, particularly in the lower atmosphere or in situations involving curved flow:

• Friction: Acts to slow down wind, opposing the direction of motion. It is strongest near the surface and decreases with height, becoming negligible in the free atmosphere (above the boundary layer). Friction causes the wind to blow across isobars towards low pressure in the boundary layer.
• Centripetal Force: An apparent force required to keep air moving in a curved path (e.g., around a low-pressure system). When winds are curved, the balance of forces is slightly different from the geostrophic balance, leading to gradient wind. Geostrophic wind is an approximation for straight flow.

Geostrophic wind is an important concept for understanding large-scale atmospheric circulation, especially at higher altitudes where friction is minimal. However, it is an idealization and actual winds are rarely perfectly geostrophic, though they often approximate it above the friction layer.