Geostrophic wind
Topic: Atmospheric Pressure, Winds and General Circulation · NCERT: Class 11, Ch 9 "Atmospheric Circulation and Weather Systems"
Meaning
A geostrophic wind is an upper-air wind that blows parallel to the isobars, which are lines of equal pressure. Above about 2–3 km there is no friction from the ground. Up there, two forces balance each other. The pressure gradient force pushes air from high to low pressure. The Coriolis force, which comes from the Earth's spin, pushes it sideways. The wind therefore flows along straight isobars instead of crossing them. Forecasters use this idea to read upper-air weather maps.
Example
In winter, the subtropical westerly jet stream flows over north India at about 12–14 km. It blows nearly geostrophically, running along the isobars at high speed and steering western disturbances.
Don't confuse with
- Surface wind: near the ground, friction slows the air. So it crosses the isobars at an angle, towards low pressure, instead of running parallel to them.