The correct option is 8 km.
Explanation
The troposphere is the lowermost layer of the atmosphere where the majority of the Earth's weather phenomena occur. Its thickness is not uniform across the globe; it varies significantly with latitude and season due to differences in surface heating and gravity.
Detailed Analysis
The height of the troposphere is determined by the intensity of convection currents, which transport heat upwards.
- At the Equator: Due to intense solar heating, strong convection currents and thermal expansion push the air to greater heights. Consequently, the troposphere extends up to approximately 18 km.
- At the Poles: The temperatures are extremely low, causing the air to be cold and dense. This leads to subsidence (sinking of air) and reduced thermal expansion. As a result, the height of the troposphere is minimal, approximately 8 km.
The average height of the troposphere across the globe is considered to be roughly 13 km.
Key Takeaway: The troposphere is thickest at the Equator (~18 km) due to strong convection and thinnest at the Poles (~8 km) due to cold, dense air.