Aviation weather and the rules, oriented

Instruments are calibrated to a reference. In the standard atmosphere, temperature falls about 2 °C — roughly 3.5 °F — per thousand feet, holding to about 36,000 feet where it reaches around −55 °C. Pressure falls about 1 inch of mercury per 1,000 feet up to 10,000 feet. None of this describes today; it is the yardstick.

The dew point is the temperature at which air can hold no more moisture. Cool air to its dew point and water condenses out — as fog, dew, frost, cloud, rain or snow. So the question behind every forecast is: what is lifting this air, and how far must it rise to reach its dew point? Terrain gives upslope cloud. Radiational cooling on a still night gives valley fog, and a front shoving one air mass under another gives a cloud line.

The misconception worth killing early is that humidity is about "how much water is in the air". What matters operationally is the gap between temperature and dew point.

Concretely: two airports both report 90 percent humidity. One is 25 °C with a dew point of 4 °C; the other is 6 °C with a dew point of 5 °C. The first is nowhere near cloud. The second is one degree of overnight cooling away from fog on the runway — and the humidity figure told you nothing about which was which.

Rule: when you see a weather product, read the temperature–dew point spread before anything else — it tells you how close the air already is to producing something.

Group the hazards by whether you can see them coming. Fog is the previous chapter's mechanism made visible, and its type predicts its behavior. Radiation fog forms on clear nights with little or no wind, typically in low-lying ground and mountain valleys. The surface cools by radiation until the air reaches its dew point; when the sun rises the fog lifts and burns off, and wind speeds its dissipation. Advection and upslope fog are different animals: they need wind to exist at all. Here is the trap: they may not burn off with the morning sun, and they can persist for days and reach greater heights. If you have only ever waited out radiation fog, you will plan a morning departure that never happens.

Icing needs two things at once: visible moisture and a surface at or below freezing. Absent either, you do not accumulate. One widely repeated claim is explicitly rejected by the handbook: "bridging" does not occur with modern deice boots. The old advice to let ice build before cycling them is obsolete — cycle as soon as you see accumulation.

Low-level wind shear is the quiet one: it can affect any flight at any altitude, and frequently goes undetected and unreported.

Concretely: a coastal airport is fogged in at 09:00 with a light onshore wind. If it is radiation fog it will be gone within the hour. If the wind is carrying moist air over cooler ground — advection — the same scene can still be there at 16:00, and at 09:00 the two look identical from the ramp. The wind is the discriminator, not the fog.

Rule: for each hazard, learn its formation condition rather than its appearance — that is what lets you rule it in or out from a forecast instead of from the windscreen.

A thunderstorm passes through three stages. In the cumulus stage lifting begins; if moisture and instability suffice, the cloud builds vertically, and continuous strong updrafts hold moisture up so nothing falls. Roughly fifteen minutes later comes the mature stage, the most violent period of its life, when moisture finally falls through the rising air. Then it dissipates.

That handover is the lesson. Precipitation reaching the ground is the signature of the mature stage, not a sign the storm is spent.

The microburst is the concentrated form of that downdraft. Its numbers are worth memorizing because they explain why you survive one only by avoiding it. A typical one is 1–2 miles across and about 1,000 feet deep, lasts 5–15 minutes, and can produce downdrafts of up to 6,000 feet per minute. Nothing in a light aircraft's performance answers a 6,000-foot-per-minute descent near the ground.

The misconception: treating a storm as a spatial obstacle to be flown around at a comfortable margin. It is better read as a time-varying one.

Concretely: you study a radar image showing a 20-mile gap between two cells and set course for it. The image was already several minutes old when you saw it. A cell that was building when the picture was taken reaches its most violent stage about fifteen minutes later — so the gap you are flying toward is a description of the past, not a plan.

Rule: judge a convective cell by its stage and its age, not by how it looks — the mature stage arrives about fifteen minutes after the cloud starts building.

Decoding is mechanical and comes with repetition; the organizing idea is worth more. Observations report what a station actually measured. The Aviation Routine Weather Report (METAR) is the standard one. A pilot weather report (PIREP) is an observation too, with a distinctive property: it is the only routine product that reports conditions between stations, and the only one that exists because someone chose to file it.

Forecasts state what is expected. The Terminal Aerodrome Forecast (TAF) covers the vicinity of one airport, which is a narrower promise than pilots often read into it.

Advisories are the third job. Inflight weather advisories interrupt you. The best known is the SIGMET — Significant Meteorological Information — issued for weather severe enough to threaten any aircraft.

The trap is treating a forecast as an observation.

Concretely: the forecast promised 3,000 feet and 6 miles all afternoon; the current observation reports 1,200 feet and 3 miles. The forecast is not "wrong" in a way you can argue with — the atmosphere did something other than expected. What that disagreement actually tells you is how much confidence the rest of that forecast deserves, which is the useful reading and the one most pilots skip.

Rule: before using any product, name its job — observed, forecast, or advisory — because that determines both its shelf life and what it is entitled to claim.

An altimeter is a sensitive barometer calibrated to indicate altitude in the standard atmosphere. Set it to 29.92 inches of mercury and it reads pressure altitude: height above the standard datum plane, which may sit above, at or below sea level on any given day. That construction produces two errors.

Pressure. Fly from an area of high pressure into an area of lower pressure without resetting, and the instrument keeps reporting the old, higher reference — so your true height is lower than indicated. The mnemonic pilots use is "high to low, look out below."

Temperature. Cold air is denser than warm air, so when you operate in temperatures colder than standard, true altitude is lower than the altimeter indication. The size of the error scales with the size of the temperature difference. Fly into a cooler air mass while holding a constant indicated altitude and you are quietly descending.

The misconception is that these are exotic corrections for high-altitude or airline flying. They bite hardest at low level in winter, over terrain — exactly where the margin is thinnest.

Concretely: you set the altimeter before departure, fly two hours into colder air and lower pressure without a fresh setting, and hold 4,500 feet indicated the whole way. Both errors push the same direction, so your actual height above the ridge ahead is less than 4,500 feet, and nothing in the cockpit is telling you by how much.

Rule: both errors point the same way — into the ground — so treat "lower pressure or colder than standard" as a single trigger to reset, re-check, or add margin.

Weather accidents rarely begin with a pilot deciding to fly into cloud. They begin with a sequence in which each step is a small extension of the last. Visibility drops a little, the ceiling comes down a little, and turning back never becomes obviously more attractive than continuing. So decide your triggers before departure, in numbers, while nothing is at stake: the ceiling at which you divert, the visibility at which you turn around, the time at which you stop waiting.

A personal minimum set higher than the legal minimum is not timidity — it is the recognition that the legal figure is a floor for the whole pilot population, not a target calibrated to your currency and this aircraft.

Concretely: conditions are legal at departure, forecast to hold, and deteriorate slowly en route. There is no moment when they become obviously unflyable — which is exactly why a pre-set number is the only thing that stops the chain.

The misconception is that experience substitutes for that discipline. It does the opposite — a pilot who has completed several marginal flights has trained themselves that the chain ends well.

Rule: write your diversion trigger down before engine start, as a number; a decision you have to make in flight is one you will make under pressure to continue.

Under §91.155, visual flight rules (VFR) flight requires a stated flight visibility and distance from cloud, varying by airspace and altitude. The table's shape is the lesson.

In Class B, where every aircraft is under air traffic control (ATC) separation, you need 3 statute miles and merely clear of clouds: separation is being provided, so you need only see. In Class C, D and E below 10,000 feet above mean sea level (MSL), it is 3 statute miles with 500 feet below, 1,000 above, 2,000 horizontal. In Class E at or above 10,000 feet MSL it widens to 5 statute miles with 1,000 below, 1,000 above, and 1 statute mile horizontal. Above 10,000 feet aircraft may be flying much faster, so the same warning time requires far more distance.

Class G at 1,200 feet or less above the surface is the outlier. For aircraft other than helicopters, it is 1 statute mile and clear of clouds by day, rising to 3 statute miles with 500/1,000/2,000 at night.

The misconception is reading the table as a difficulty ladder. It is a closing-speed argument.

Concretely: Class B has the loosest cloud requirement in the whole table — clear of clouds — while uncontrolled Class G at night has one of the tightest. Read as a difficulty ladder that is backwards. Read as "who is separating me, and how fast is the traffic", it is the only arrangement that makes sense.

Rule: derive the row from the question "who is protecting me here, and how fast could the other aircraft be?" — then the table is recalled rather than memorized.

Privileges come from the certificate. Under §61.113, as a private pilot you may not act as pilot in command of an aircraft carrying passengers or property for compensation or hire, nor act as pilot in command for compensation or hire at all. The exception people misread is cost-sharing: you may not pay less than the pro rata share of the operating expenses of a flight with passengers. Those expenses are limited to fuel, oil, airport expenditures, or rental fees. Read it carefully — it is a floor on your own contribution, not permission to be paid.

Currency is separate, and it is a rolling clock rather than a status. To carry persons you need three takeoffs and three landings within the preceding 90 days in the same category and class (§61.57). Night carriage of passengers needs its own three, to a full stop, in the period beginning 1 hour after sunset and ending 1 hour before sunrise.

The flight review is different again: under §61.56 you may not act as pilot in command unless, since the beginning of the 24th calendar month before the current month, you have completed a review of at least 1 hour of flight training and 1 hour of ground training, endorsed in your logbook.

The misconception is treating these as one "am I legal" question. They fail independently.

Concretely: a pilot who flies twice a week is comfortably inside the 90-day takeoff and landing requirement and may still be unable to act as pilot in command at all, because the 24-calendar-month flight review lapsed last month. Currency and the review are different clocks, and being excellent at one says nothing about the other.

Rule: for any scenario, ask the three questions separately — certificate, currency, compensation — because a scenario is usually testing exactly one of them.

Start at §91.3. As pilot in command you are directly responsible for, and the final authority as to, the operation of the aircraft. In an in-flight emergency requiring immediate action you may deviate from any rule of this part to the extent required, with a written report owed to the Administrator only upon request. That is the frame; the numbered rules are the floor under it.

The floor, in the shapes most often tested. Preflight (§91.103): become familiar with all available information, explicitly including runway lengths and takeoff and landing distance data. Minimum altitudes (§91.119): 1,000 feet above the highest obstacle within a 2,000-foot radius over congested areas; 500 feet above the surface elsewhere. Over open water or sparsely populated areas, stay no closer than 500 feet to any person, vessel, vehicle or structure. Speed (§91.117): 250 knots below 10,000 feet MSL, and 200 knots both beneath a Class B shelf and within 4 nautical miles of the primary Class C or D airport at or below 2,500 feet above the surface. Fuel (§91.151): enough to reach the first point of intended landing plus 30 minutes by day, 45 at night for airplanes. Oxygen (§91.211): required crew above 12,500 feet MSL after 30 minutes, throughout above 14,000, and provided to every occupant above 15,000. Cruising altitudes (§91.159) apply only above 3,000 feet above the surface: magnetic course 0–179° takes odd thousands plus 500, 180–359° even thousands plus 500.

The misconception is that §91.3's emergency authority is a general override. It is scoped to an emergency requiring immediate action, and to the extent required.

Concretely: arriving at dusk with the day reserve rather than the night one is not converted into compliance by deciding en route that the situation feels urgent. §91.151 is a planning rule — it binds before the flight begins — so the emergency provision has nothing to reach back and authorize.

Rule: when a scenario pits a rule against a judgment, check whether an actual emergency exists; without one, the floor stands.

Airspace is usually taught as shapes. It is more useful as a lookup: entering any volume, ask what equipment and permission it requires and what weather minimums apply. Those three vary independently, which is the point. Class B requires a clearance and, under §91.215, transponder equipment across a defined veil, but its weather requirement is the loosest in the table — 3 statute miles, clear of clouds — precisely because ATC is separating traffic. Class G requires no clearance and no transponder, and imposes the tightest cloud-clearance discipline at night. A pilot who reasons "controlled airspace is stricter" gets both backwards.

Temporary flight restrictions (TFRs) are the case where the checklist has a fourth entry: the airspace did not exist when you planned. They are published, they move, and they are not visible from the cockpit — which makes them a preflight information problem rather than a navigation one, and squarely inside §91.103's "all available information".

The misconception worth naming: that a transponder requirement follows the airspace boundary you can see on the chart.

Concretely: a pilot plans to stay clear of a Class B area by remaining outside its outermost drawn ring, and concludes no transponder is needed. The equipment requirement in §91.215 is defined in its own terms rather than by that outline, so avoiding the shape on the chart does not by itself answer the equipment question. Check the rule, not the picture.

Rule: at every boundary, run the same three questions — equipment, permission, minimums — rather than recalling a class as a single fact.

Two questions hide inside "is this aircraft legal?". What must be aboard is documentary, and mechanical to check. Whether it may fly today is not. Under §91.213 an aircraft with inoperative instruments or equipment installed may not take off unless specific conditions are met. The principal path is an approved Minimum Equipment List (MEL) for that aircraft, together with a letter of authorization, which together constitute a supplemental type certificate. Absent that framework, a separate path in the same section governs, and neither is "the pilot decides it looks unimportant".

The misconception is that a placard or a maintenance note makes an item's absence self-authorizing. The regulation runs the other way: an inoperative item is disqualifying unless an approved structure permits the flight.

Concretely: a landing light fails the evening before a daytime local flight. The instinct is to reason about whether it matters. The regulation instead asks which authorized path permits departure with it inoperative — and "it is not needed today" is not one of them.

Reporting closes the loop, and the useful distinction is between mandatory and voluntary. §91.3(c) obliges a written report of an emergency deviation only upon the Administrator's request. The Aviation Safety Reporting System (ASRS) is voluntary and serves a different purpose entirely — it collects safety information from people who are not obliged to say anything.

Accident and incident notification itself lives in the National Transportation Safety Board's own regulations (49 CFR Part 830), not in Part 14 — a different title of the Code of Federal Regulations, which is why it is easy to look for in the wrong place.

Rule: separate what is carried from what is airworthy today — the first is a list, the second is a judgment the regulation constrains.