Updated October 2026
Wind shear is a sudden change in wind speed or direction over a short distance. Its worst form, the microburst, can push air down at up to 6,000 feet per minute and cost an airplane 30 to 90 knots of headwind in seconds. It is one of seven weather hazards a student pilot learns to recognize, along with turbulence, icing, inversions, the marine layer, Santa Ana winds and thunderstorms.
Southern California gives you a gentle version of most of them and a sharp version of two. The marine layer and the Santa Anas are the hazards a Van Nuys student actually meets. Thunderstorms and structural ice are rarer here, which is exactly why they need to be understood before a long cross-country takes you somewhere they are not rare at all.
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Turbulence: Mechanical, Thermal, Mountain Wave And How To Fly It
Thermal turbulence comes from the ground heating unevenly. The FAA's Pilot's Handbook of Aeronautical Knowledge (chapter 12) blames convective currents for the bumpy air you feel at lower altitudes, and a summer afternoon in the San Fernando Valley produces plenty of them. The NOAA normals put the mean daily high at Van Nuys above 90°F from July to September, which is why we tell students to fly early in summer.
Mechanical turbulence is wind tumbling over something: hangars, trees, ridges. Wind over the Santa Monica Mountains and through the passes north of the Valley produces it reliably. On the lee side of a ridge, the handbook warns, the air "follows the contour of the terrain and is increasingly turbulent," and it tends to push an airplane toward the mountain. Cross ridges at an angle, so that a turn away is short, and with height to spare.
Mountain wave is the large-scale version. A strong wind across a range sets up standing waves downwind, sometimes marked by smooth, lens-shaped lenticular clouds, which the handbook describes as forming over mountains in strong winds. Near the ridge tops beneath the wave, rotor turbulence can be severe. Lenticulars over the San Gabriels mean the lee side is not the place to be that day.
How to fly it: slow to the turbulence or maneuvering speed in your POH, and note that maneuvering speed falls as the airplane gets lighter, which our maneuvering speed guide works through for the Warrior. Hold the attitude rather than chasing the altitude. Keep the wings level, tighten the belts and stow anything loose.
Wind Shear And Microbursts
The handbook defines wind shear as "a sudden, drastic change in wind speed and/or direction over a very small area." Near the surface it comes from passing fronts, thunderstorms, temperature inversions and upper winds stronger than 25 knots. Near the ground it bites hardest, because that is where the airplane is slow and has the least room.
A microburst is the extreme case. The FAA's figures: 1 to 2 miles across, a nominal depth of 1,000 feet and a life of about 5 to 15 minutes, with downdrafts of up to 6,000 feet per minute and headwind losses of 30 to 90 knots. Flown into on approach, the sequence is a headwind that makes the airplane float, then the downdraft, then a tailwind that takes the airspeed away exactly when you need it. A trainer that climbs at about 700 feet per minute cannot outclimb that. The only defense is not to be there, so never take off or land under a thunderstorm or a curtain of rain that is not reaching the ground.
The shear you are likelier to meet at Van Nuys is milder: a gusty Santa Ana or a stiff westerly on final, with the wind changing speed as you descend past the hangars. Many instructors teach adding half the gust factor to the approach speed; check what your POH recommends. If the airspeed swings more than you can manage, go around. Our guide to crosswind landings covers the technique.
Icing: Structural, Induction And Carburetor
Structural ice forms when supercooled water droplets freeze on contact with the airframe. The handbook's rule is to be alert any time the temperature approaches 0°C with visible moisture present. Ice adds weight, but the bigger problem is shape. A rough layer on the wing destroys lift and raises the stall speed, and even frost, the handbook says, "can drastically reduce the production of lift." None of the airplanes a student flies here, the Cessna 162, the Warrior or the DA20, is approved for flight in icing conditions. The VFR rule is simple: no cloud, and no visible moisture near freezing.
Carburetor ice forms inside the carburetor, where vaporizing fuel and fast-moving air can drop the temperature by as much as 60 to 70 Fahrenheit degrees (handbook chapter 7). It is most likely below 70°F, or 21°C, with relative humidity above 80 percent. That is a description of a Van Nuys marine-layer morning. At 4:51 AM on 27 October 2026 the field reported 12°C with a dew point of 12°C, which is 54°F and saturated.
The first sign, in an airplane with a fixed-pitch propeller, is a drop in rpm. Apply carburetor heat and the rpm falls a little further, then climbs as the ice melts. Induction icing is the external cousin: ice building on the air intake itself, which can choke any engine, injected or not.
| Airplane | Engine | Carburetor Ice? |
|---|---|---|
| Cessna 162 Skycatcher (all six) | Continental O-200-D, carbureted | Yes. Carburetor heat is a checklist item. |
| Piper Warrior (N3034Q, N6312C) | Lycoming O-320-D3G, carbureted | Yes |
| Diamond DA20-C1 (N306EF, N308EF) | Continental IO-240-B, fuel-injected | No carburetor; intake icing still possible |
| Piper Arrow (N316EF) | Lycoming IO-360, fuel-injected | No carburetor; intake icing still possible |
Temperature Inversions And The LA Smog Layer
Normally the air cools as you climb, about 2°C per 1,000 feet in the standard atmosphere. In an inversion it warms instead. The handbook describes inversions as shallow layers of smooth, stable air near the ground that act as a lid, "keeping weather and pollutants trapped below." When the humidity is high, visibility inside the layer is poor.
The Los Angeles basin is the textbook case. Cool marine air sits under warmer air for much of the year. The marine layer, the afternoon haze and the brown band you see from the Santa Monica Mountains on a still day are all held down by the same lid. Climb through the top and the visibility often jumps from legal but murky to clear across the basin.
Two hazards follow. Haze looks far worse into a low sun, so flying west late in the day can feel much worse than the METAR says. And the inversion boundary itself is a common place for wind shear, one of the causes the handbook lists, so expect a bump or an airspeed change as you pass through it.
The Marine Layer And June Gloom: Why Mornings Are IFR At Van Nuys
The marine layer is a sheet of low stratus that forms over the cool Pacific, pushes inland overnight under the inversion, and burns off from its inland edge as the land heats. It is the most common reason a morning lesson at Van Nuys moves. Our flyable days at Van Nuys page tracks it with the NOAA normals. The gap between the Van Nuys and Santa Monica mean daily highs widens from 2.8°F in February to 16.6°F in August, as the ocean holds the coast down. May and June are its worst months, the season locals call May Gray and June Gloom.
Van Nuys, fifteen miles inland behind the Santa Monica Mountains, usually clears earlier than the coast. Not always, and not only in June. Here is a late-October morning that looked like nothing much from the ground:
| Local Time | What KVNY Reported | Category |
|---|---|---|
| 3:51 AM | Clear, 10 miles, 13°C, dew point 12°C | VFR |
| 4:25 AM | A quarter mile in fog, sky obscured, vertical visibility 100 feet | LIFR |
| 5:51 AM | Clear, 10 miles | VFR |
| 7:38 AM | Broken at 800 feet | IFR |
| 7:51 AM | Scattered 900 feet, broken 1,400 feet | MVFR |
| 9:51 AM | Clear | VFR |
The practical rule follows the climate. Outside May and June, the first slot of the day is usually the smoothest. In May and June, book late morning or midday and use the early slot for ground school. Our guide to reading a METAR and TAF decodes reports like the ones above.
The real danger is not the cloud. It is the VFR pilot who leaves under a 2,500-foot ceiling inland, heads for the beach, and finds the layer lower there and closing in behind. The handbook notes that advection fog, warm moist air moving over a cooler surface, is common in coastal areas. Turn around early, while the way home is still open.
Santa Ana Winds And Fire Season
Santa Anas are dry offshore winds from the north-east, pouring out of the high desert through the passes and canyons toward the coast. They are mostly an autumn and winter wind, they are forecast days ahead, and they turn calm Valley air into strong gusts with mechanical turbulence over every ridge. The Van Nuys runways point roughly north and south, so a north-easterly gust puts a large part of its strength across them.
Santa Ana days are also fire days. Low humidity and strong wind turn a spark in the Santa Susanas or the San Gabriels into a large fire, and a wildfire TFR under 14 CFR 91.137 can go up within hours. Smoke, coded FU in a METAR, cuts visibility far downwind. Our page on NOTAMs and TFRs covers how to check for them.
For a student, a strong Santa Ana day is usually a ground lesson or a session in the full-motion simulator, which flies in any weather at $120 an hour inside a package. Nobody learns much fighting a 25-knot gust in the pattern on lesson six.
Thunderstorms (Rare Here, Deadly Everywhere)
A thunderstorm needs three things, in the handbook's words: "sufficient water vapor, an unstable lapse rate, and an initial lifting action." The Valley rarely gets all three at once. Storms do build over the mountains and the high desert on summer afternoons, and along winter fronts. They bring every hazard on this page in one package: turbulence, wind shear, hail, icing, lightning and heavy rain. The handbook says hail "competes with turbulence as the greatest thunderstorm hazard to aircraft."
Respect the distances. The handbook advises circumnavigating any storm identified as severe, or giving an extreme radar echo, by at least 20 nautical miles. Shear turbulence has been met 20 miles from a severe storm, gust fronts can run up to 15 miles ahead of the rain, and hail can fall in clear air several miles from the cloud. For a VFR student in a trainer the rule is shorter: a storm near the route or the destination means the flight does not go.
Personal Minimums For Student Pilots
The legal minimums are a floor, not a target. The FAA's Risk Management Handbook gives you a way to think about the rest. Its PAVE checklist looks at the Pilot, the Aircraft, the enVironment and External pressures before each flight, with personal minimums set for each from your own experience. Write yours down with your instructor, keep them in your flight bag, and change them only on the ground.
| Item | Legal Floor | Example Starting Point |
|---|---|---|
| Ceiling | 1,000 ft to fly VFR in the Van Nuys surface area, 91.155(c) | Above 3,000 ft, the VFR flight category |
| Visibility | 3 statute miles by day for a student pilot, 61.89(a)(6) | More than 5 statute miles |
| Wind and crosswind | Whatever your instructor writes in your logbook, 61.89(a)(8) | Well inside the POH demonstrated crosswind, agreed with your instructor |
| Turbulence | No regulatory number | No forecast moderate turbulence, no strong Santa Ana |
| Density altitude | Takeoff and landing data reviewed, 91.103(b) | Recomputed from the POH on any day above 90°F |
| TFRs and NOTAMs | All available information, 91.103 | Checked again within an hour of engine start |
The right-hand column is an example to start a conversation with your instructor, not a school policy. One more line belongs on your sheet: the reasons you might go when you should not. Passengers waiting, a rental car due back, a checkride date. External pressure does not show on any weather chart, which is why it needs a line of its own.
Related Reading
- How to read a METAR and TAF: where these hazards first show up in code.
- VFR, MVFR, IFR and LIFR flight categories: the four buckets behind every go or no-go.
- Flyable days at Van Nuys: the marine layer and the Santa Anas, month by month.
- Mountain flying checkout: wave, rotor and density altitude at Big Bear.
- Maneuvering speed explained: the number to slow to in rough air.
- What happens if the weather is bad: how a weathered-out lesson is handled.
Weather Hazards FAQ
What Is Wind Shear In Aviation?
A sudden, drastic change in wind speed or direction over a very small area, as the FAA's Pilot's Handbook of Aeronautical Knowledge defines it. Its most dangerous form is the microburst, 1 to 2 miles across, with downdrafts of up to 6,000 feet per minute and headwind losses of 30 to 90 knots.
What Causes Turbulence When Flying?
Four things. Uneven heating of the ground gives thermal bumps on hot afternoons. Wind flowing over buildings and terrain gives mechanical turbulence. Strong wind across mountains sets up wave and rotor. Wind shear, including clear air turbulence near the jet stream, covers the rest.
What Temperature Does Carburetor Ice Form?
Well above freezing. The FAA says carburetor ice is most likely below 70°F, or 21°C, with relative humidity above 80 percent, because the temperature inside the carburetor can fall by as much as 60 to 70 Fahrenheit degrees. A humid 54°F morning at Van Nuys is prime carburetor-ice weather.
What Is A Temperature Inversion In Aviation?
A layer in which the air gets warmer with height instead of cooler. It is usually smooth and stable, but it traps haze, smoke and moisture beneath it, cutting visibility, and its boundary is a common place for wind shear. The marine layer over Los Angeles sits under one.
What Do Lenticular Clouds Mean For Pilots?
Strong wind blowing across a mountain range. These lens-shaped clouds mark a mountain wave, and the air below the wave near the ridges can hold severe rotor turbulence. Over the San Gabriels they are a signal to keep well away from the downwind side.
When Is The Marine Layer Worst At Van Nuys?
May and June, when the coast-to-valley temperature contrast is building and low stratus can last into the afternoon. It still appears in other months: on 27 October 2026 Van Nuys went from a quarter mile in fog at 4:25 AM to clear at 9:51 AM.
Can You Fly During Santa Ana Winds?
Sometimes, but rarely usefully as a student. Santa Anas bring gusts, crosswind on the north-south Van Nuys runways and mechanical turbulence over the ridges, and they often come with wildfire TFRs. They are forecast days in advance, so most students move the lesson or fly the simulator instead.
Learn Weather With An Instructor Beside You
The best place to meet a gusty final or a ceiling that will not lift is with an instructor in the right seat. A discovery flight is $229 for 60 minutes from Van Nuys, flown from the left seat, and the go or no-go call is made with you before you walk out. To plan full training, the private pilot license starts at $10,400. You can also book a free consultation or call (818) 330-1318, 9 AM to 9 PM, seven days.
Book A Discovery Flight — $229 Book A Free Consultation Call (818) 330-1318
