6.4 Wind, Jet Stream, and Local Wind Systems

ACS PA.I.C.K3b is wind: the factors that create it, plus wind shear and mountain wave. Crosswind components and gust additives are computed in the performance chapter. This section is the weather-theory half: why the wind exists, where it changes abruptly, and which clouds warn you that the atmosphere is not a uniform river.

Pressure gradient, Coriolis, and friction

Air wants to flow from high pressure to low pressure. That push is the pressure-gradient force (PGF). On a surface chart, closely spaced isobars mean a steep gradient and stronger wind; widely spaced isobars mean a weak gradient and lighter wind (Aviation Weather Handbook Chapter 10).

If PGF acted alone, wind would blow straight across the isobars into the low. The Earth’s rotation adds Coriolis force, which in the Northern Hemisphere deflects moving air to the right. PHAK: deflection is greatest at the poles and zero at the equator. Aloft, above the friction layer, PGF and Coriolis come into approximate balance and the wind blows nearly parallel to the isobars — clockwise around highs, counterclockwise around lows.

Surface friction slows the wind. A slower wind feels less Coriolis, so near the ground the flow cuts across the isobars toward lower pressure. PHAK: around 2,000 to 3,000 feet above the surface the wind is faster and more parallel to the isobars; the wind at 2,000 feet AGL is generally 20° to 40° to the right of the surface wind in the Northern Hemisphere. That is why a METAR with a southerly surface wind can sit under southwesterly flow just a couple of thousand feet up — friction, not a new air mass.

Highs are generally outward, downward, and clockwise in the Northern Hemisphere. Lows are inward, upward, and counterclockwise. Rising air at a low or trough favors clouds; sinking air at a high or ridge favors clearing.

Buys Ballot’s law

Buys Ballot’s law is the field version of that Northern Hemisphere circulation. Stand with your back to the wind. Low pressure is on your left; high pressure is on your right. (In the Southern Hemisphere the sense reverses.) If you are flying and the wind is on your tail in the usual mid-latitude westerlies, the stronger low is often toward the pole from you. The law does not replace a surface analysis, but it stops the “which way is the low?” freeze on an oral.

It also explains a common PAR wind item: flying from a high toward a low in the Northern Hemisphere, the wind tends to come from the left and increase as you approach the tighter gradient.

The jet stream — location and implications, not folklore speeds

The Aviation Weather Handbook (Chapter 9) defines jet streams as relatively narrow bands of strong wind in the upper levels. They blow from west to east but meander north and south. They follow the boundaries between warm and cold air. Those boundaries are sharpest in winter, so jets are strongest in winter in both hemispheres.

Handbook geography you may use:

• Polar jet near 50°–60° latitude.
• Subtropical jet near 30° latitude.
• Height typically about FL200 to FL450.
• Speeds can exceed 275 mph (239 knots) — that is the handbook’s published upper illustration, not a cruise-planning number for a 172.

PHAK ties the tropopause to the jet and to clear air turbulence. Jets also follow the Sun: they shift north into Canada in spring and summer and south into the United States in autumn, helping cooler air return. A jet is a core of strongest wind, not a single drawn line; think of a river that is fastest in the middle. For a private pilot the operational points are: strong temperature gradients aloft, possible CAT near the tropopause, and surface weather that organizes under jet streaks and troughs. Do not invent a “jet is always 100 knots at 30,000 feet” factoid. The handbook does not give a single everyday speed.

Sea, land, valley, mountain, and katabatic flow

When the synoptic wind is weak, uneven heating writes local circulations (PHAK Chapter 12; AWH Chapter 10).

• Sea breeze: by day, land heats faster than water. Lower pressure over the warmer land; wind from sea to land; lift and possible cumulus inland; sinking and clearing offshore. Common on calm, sunny summer days. A sea-breeze front can lift a line of cumulus or even showers if the inland air is moist and unstable.
• Land breeze: at night, land cools faster. Wind from land to sea, usually weaker than the daytime sea breeze; lift and possible clouds shift over the water.
• Valley breeze: by day, slopes heat; wind upslope / up-valley; clouds may form over the ridges.
• Mountain breeze: at night, slopes cool; denser air drains downslope into the valley. This nocturnal drainage is the pilot’s usual katabatic wind — cold air flowing downhill because it is denser, not because a charted low is pulling it. Strong katabatic flows in mountainous terrain can be far more than a gentle drainage; treat them as a mountain-weather hazard, not a lake-afternoon curiosity.

PHAK’s pattern reminder: an approach over hot bare ground can balloon; an approach over water or vegetation can sink. That is convective current, the same physics as the sea breeze, at traffic-pattern scale.

Wind shear at fronts, inversions, and the low-level jet

Wind shear is a sudden change in wind speed and/or direction over a short distance. PHAK lists common low-level sources: passing fronts, thunderstorms, temperature inversions, and strong winds aloft (greater than 25 knots) above that inversion.

At a front, the two air masses have different winds. Crossing the front, or sitting in the pattern while it passes, can swap a headwind for a crosswind or tailwind in seconds. A tailwind becoming a headwind increases indicated performance; a headwind becoming a tailwind decreases it. The second case is the low-level trap.

At a nocturnal inversion, surface air decouples from the faster flow above. Just above the inversion a low-level jet — a nighttime wind maximum — often develops. The handbook and PHAK do not hand you a single published private-pilot jet speed to memorize, so do not invent one. What you may say: the shear at the inversion top can produce an abrupt airspeed change on climb or descent, even when the surface METAR looks calm. AIRMET Tango can flag non-convective low-level wind shear below 2,000 feet AGL. If the ATIS is calm and the winds aloft or a PIREP show a strong layer just above the surface at night, treat the first few thousand feet as a shear zone.

Crosswind versus gust factor is conceptual here. The crosswind component is the part of the wind blowing across the runway, not down it. A gust is a brief increase above the sustained speed. AFM/POH technique and the performance chapter add a gust increment to approach speed so the airplane does not stall when the gust dies. Weather theory only requires you to see that a 10-gust-20 wind is not “a 10-knot wind” for either crosswind or energy management.

Mountain wave and standing lenticulars — a wind-system preview

When a stable layer and a strong wind blow nearly perpendicular to a ridge, the air can oscillate downstream as a mountain wave (Aviation Weather Handbook Chapter 16). Visual markers, when moisture is present:

• Standing lenticular clouds (including altocumulus standing lenticular, ACSL) mark wave crests. They can look smooth and harmless. They are stationary relative to the ground while air screams through them.
• Rotor clouds under a crest, on the lee side near or below ridge level, mark breaking, possibly severe turbulence and roll. From a distance a rotor can look like ordinary cumulus; the upwind side is the worst place to “have a look.”

Wave energy can propagate well above the peaks — the handbook notes effects sometimes above 60,000 feet — so altitude is not automatic safety. Trapped lee waves can train lenticulars far downwind and matter even below FL250. The lee side when flying into the wind toward the ridge is PHAK’s classic terrain-collision setup: downdrafts following the slope. Chapter 7 (weather hazards) develops turbulence and rotor severity. This chapter’s job is to see lenticulars as a wind-system flag, not as pretty high clouds.

Scenario: evening arrival at a coastal mountain airport

Jordan arrives at sunset. The ATIS is nearly calm, but the AWOS a few hours earlier had a 12-knot sea breeze straight onto the runway. Land has cooled; a land breeze may already be reversing the flow, so the calm is a shift, not “no wind all night.” Inland valleys will start a mountain breeze draining toward the coast. If a strong inversion sets up, a low-level jet can sit just above the pattern even while the windsock droops. If the inbound course crosses a ridge with standing lenticulars, Jordan treats that as mountain wave until proven otherwise — and does not use the rotor cloud as a landmark to fly toward. Crosswind limits and gust additives wait for the performance numbers. The weather decision is whether the system (reversing local wind, inversion shear, wave) is even a VFR arrival.