Unit 3 of 12 · 5 of 5 lessons published
Wind and dynamics
Stability decides whether a parcel of air keeps rising once something lifts it. This unit is about the something: the forces that move air sideways, and the large-scale patterns that push whole regions of it upward for hours at a time.
It starts with the wind itself: the pressure gradient force, the Coriolis effect and friction, and why the wind at the ground blows across the isobars while the wind aloft runs along them. From there it follows the flow upward to the jet stream, the waves in it that forecasters call troughs, ridges and shortwaves, and the spin they carry, measured as vorticity. The last lesson ties these together into the reason air rises across a whole region ahead of a trough, the lift that sets off storms and spreads rain and snow over entire states. The examples are observed maps and soundings from real days.
The unit assumes the first unit's lesson on air pressure, where isobars, constant-pressure charts and geopotential height are introduced, and the second unit's lessons on stability.
- What makes the wind blow
Pressure gradient, Coriolis and friction, and why wind crosses the isobars near the ground.
- The jet stream
Where the jet stream comes from, how it moves, and what jet streaks do.
- Troughs, ridges and shortwaves
The waves in the upper-level flow and the weather under each part of them.
- Vorticity
Spin in the atmosphere, from the jet stream down to the mesocyclone.
- Why air rises: lift on the large scale
Divergence aloft, warm air advection and the quasi-geostrophic picture of ascent.