Q.What are the geotrophic winds?

Geostrophic winds are the winds that blow parallel to the isobars (lines of equal pressure) in the upper atmosphere, well above the friction of the Earth's surface. They form when two forces balance each other exactly: the pressure gradient force, which pushes air from high to low pressure, and the Coriolis force, which deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

Because the Coriolis force depends on wind speed, the wind accelerates until its deflection exactly counteracts the pressure gradient. At that point, the wind no longer crosses the isobars but flows along them, with low pressure on its left in the Northern Hemisphere. This balance is the geostrophic balance.

• It occurs only above the friction layer (roughly 1 km up), where surface drag is negligible. …

Picture a wind that never seems to tire, racing along the upper sky, not spiralling into a low or bursting out of a high, but flowing in a straight, steady line along the contours of pressure. That is the geostrophic wind. It is not a wind you feel on your skin at ground level — it lives higher up, where friction has almost vanished, and where two invisible forces strike a perfect, delicate balance.

The story begins with air pressure. Air always wants to move from where it is crowded (high pressure) to where it is sparse (low pressure). That push is called the pressure gradient force, and it is what starts any wind blowing. But as soon as the air starts moving, the Earth’s rotation steps in. Because the planet spins beneath the moving air, the air appears to be deflected sideways — to the right in the Northern Hemisphere, to the left in the Southern Hemisphere. This apparent deflection is the Coriolis force.

Now here is the elegant part. The Coriolis force does not push the air forward or backward; it pushes it at a right angle to its motion. So as the air begins to move from high to low pressure, the Coriolis force bends it. The faster the wind blows, the stronger the Coriolis force becomes. Eventually, the wind reaches a speed where the Coriolis force exactly matches the pressure gradient force — but in the opposite direction. The two forces cancel each other out, and the wind stops accelerating. It settles into a steady, straight-line motion, blowing parallel to the isobars, with high pressure on its right and low pressure on its left in the Northern Hemisphere.

That balanced, frictionless wind is the geostrophic wind. The word itself comes from Greek — geo meaning Earth, and strophe meaning turning — a nod to the Earth’s rotation that makes it possible.