Vertical Motion and Clouds

Meteorology · Chapter 8 · DGCA CPL and ATPL

Why air rises and sinks, the ten kinds of cloud it makes, and how cloud is measured, reported and flown through.

Remember

Convergence (lows, troughs, cyclones): air piles up and rises → cloud and bad weather, but good visibility. Divergence (highs, ridges): air sinks → fine weather, but poor visibility.
Ten genera by height (tropics): high (CI, CS, CC) 6-18 km, ice; medium (AC, AS) 2-8 km; low (ST, SC, NS) below 2 km; vertical (CU, CB).
ST → drizzle. NS → continuous rain or snow. CB → showers, hail, thunder: the most hazardous. CS gives a halo (ice, no icing); AS gives a corona (water, icing possible).
FEW 1-2/8, SCT 3-4/8, BKN 5-7/8, OVC 8/8. Ceiling: lowest layer of 5/8 or more below 6000 m (20 000 ft). Bases are above ground in METAR and TAF, above sea level in area charts.
Cloud of operational significance: base below 1500 m (5000 ft) or below the highest minimum sector altitude, whichever is greater. No contrails below the mintra level or above the maxtra level.

What makes air go up or down?

Why air goes up

Six cards. Swipe or tap Next.

Vertical motion is slow, but it makes the weather Air moves up and down far more slowly than it blows sideways, yet that slow vertical motion is what builds clouds, rain and turbulence. Local vertical motion (sea breezes, valley winds, thermals) is usually confined to a few kilometres, except in thunderstorms. Large-scale motion from pressure systems, fronts and mountains can cover hundreds of kilometres.
What lifts the air Friction: the rough ground stirs the air into eddies up to about 1 km. Terrain: air rises on the windward slope and sinks on the lee side, sometimes into mountain waves. Convection: heated ground sends up thermals, the bumps that build cumulus and CB. Pressure systems lift or lower whole air masses. Fronts give sloping surfaces: gentle ascent at a warm front, abrupt ascent at a cold front. Vertical wind shear makes eddies, such as the clear-air turbulence near jet streams. Cold front: sharp, sudden ascent → TS and squalls. Warm front: gradual ascent over a wide area → layer cloud.
Air piling in has to go up When more air flows into a region than out of it, it piles up and is forced upward. This convergence goes with lows, troughs, depressions and cyclones. Rising air cools and builds cloud: bad weather, though the visibility is usually good away from the rain.
Air spreading out sinks from above When more air flows out than in, air sinks from above to replace it: subsidence. Divergence goes with highs (anticyclones) and ridges. Sinking air warms and dries, so the sky clears: fine weather, but haze and smoke are trapped and visibility is often poor.
Lifted, cooled, condensed A cloud is a crowd of visible water drops or ice particles. It forms when air is lifted and cools adiabatically until its vapour condenses into drops, or deposits straight into ice. That height is the lifting condensation level, the cloud base. Deposition is vapour turning directly into ice.
Which kind of day? A large high-pressure area sits over central India for a week.

Takeaway: Friction, hills, heating, pressure systems, fronts and wind shear all move air up or down. Rising air makes cloud and weather; sinking air clears the sky.

What makes air go up or down?

What lifts the air

Air moves up and down far more slowly than it blows sideways, but this is what makes the weather. Watch each picture, then tap Next.

Frictional eddies Wind rubbing over rough ground breaks into small eddies, swirls that turn in every direction. In unstable air they can reach about 1 km up. They help make fog and low cloud, and they cause bumps on take-off and landing.
Terrain Wind that meets a mountain is pushed up the windward side and sinks down the lee side. Unstable air makes this stronger; an inversion damps it. Strong winds make mountain waves, which sometimes reach the upper stratosphere, where nacreous clouds can form.
Convection Hot ground warms the air above it, and bubbles of warm air rise as thermals. Gliders use them to climb. Thermals make the ride bumpy. They build cumulus and then cumulonimbus clouds, with strong up- and downdraughts inside.
Pressure systems Lows, cyclones and anticyclones move huge amounts of air up or down over wide areas. In a low, air flows in (convergence) and has to rise. In a high, air sinks and flows out (divergence).
Fronts and wind shear A warm front lifts air gently over a wide area. A cold front lifts it sharply, with convection, thunderstorms and squalls. Where the wind changes a lot with height (vertical wind shear), strong eddies form: clear air turbulence (CAT) near jet streams, made worse over mountains.

Memory hook: Friction, hills, heat, lows, fronts: all lift air.

Takeaway: Air is pushed up or down by friction eddies, hills, convection, pressure systems, fronts and wind shear. Local motions, like breezes and slope winds, stay within a few km. Large-scale ones, like lows, fronts, troughs and mountain waves, cover hundreds of km and reach great heights.