Wind & how it forms

Wind is air moving from high pressure toward low. Three things shape it:

• Pressure-gradient force starts the air moving, high to low.
• Coriolis deflects that moving air to the right (Northern Hemisphere).
• Friction drags on the bottom of the wind near the ground.

Aloft (above the friction layer) the pressure gradient and Coriolis balance, and the wind blows parallel to the isobars (the geostrophic wind).

Near the surface friction backs the wind (turns it toward the low, about 30° over land: 10° over water, up to 45° over rugged terrain) and slows it.

Veer and back:

• Veer = direction turns clockwise (e.g. 240° → 300°).
• Back = direction turns counterclockwise (e.g. 300° → 240°).

Climbing, friction fades, so the wind veers and strengthens. Descending, it backs and eases.

Daytime heating mixes faster air down from aloft, so the surface wind is stronger and gustier by day, lighter at night.

Breezes at the coast:

• Sea breeze: onshore by day (land warms faster than the sea).
• Land breeze: offshore at night (land cools faster than the sea).

Trap: veer is clockwise, back is counterclockwise. And a sea breeze comes from the sea (blows onshore), a land breeze comes from the land (blows offshore).

Source note: the weather theory here is grounded in the FAA Aviation Weather Handbook (FAA-H-8083-28), a free public-domain text; the physics is universal and matches the Canadian material (equivalent to TP 9352). Canada-specific products/rules stay cited to the TC AIM.

Practice