Weather Information

The K2 list adds two you should be able to describe on sight (CA.I.C.K2):

• Ceiling and Visibility Analysis (CVA) — a graphical analysis product paired with the Surface Analysis Chart in K2b
• Convective Outlook (AC) — the convective forecast product in K2f, used for planning days ahead of a flight rather than hours

The rest are familiar: METAR/SPECI and PIREPs, TAF, GFA, winds and temperatures aloft (FB), and the inflight advisories (AIRMET, SIGMET, Convective SIGMET). The examiner must assess at least three K2 sub-elements if K2 is selected, and at least three K3 sub-elements if K3 is selected.

National Weather Service and Flight Service are the ACS-named sources (CA.I.C.K1). Practically that is 1800wxbrief.com (Leidos) and aviationweather.gov, plus an FAA-approved vendor if your operator uses one.

The distinction that matters commercially: a product is official when it comes from an approved source and meets the safety and regulatory requirements for aviation weather. Cockpit FIS-B products explicitly do not — PHAK states they are for information only and "should not be used as primary weather products" (PHAK ch. 13).

Flight Information Service–Broadcast is a ground broadcast service delivered through the ADS-B Services network on the 978 MHz UAT data link (PHAK ch. 13).

Products: METARs, SPECIs, TAFs and amendments, NEXRAD regional and CONUS precipitation maps, NOTAMs (D and FDC), AIRMETs, SIGMETs and Convective SIGMETs, status of SUA, TFRs, winds and temperatures aloft, PIREPs, and TIS-B service status.

The number to memorize: some of the rendered data can be 20 to 30 minutes old and not current; consult the individual equipment manual for specific delay times (PHAK ch. 13).

Three named limitations (PHAK ch. 13):

• Base reflectivity does not give enough information to determine cloud layers or to distinguish hail from rain — a pilot may mistake rain for hail, or the reverse.
• Base reflectivity is sampled at the minimum antenna elevation angle, so an individual site cannot depict high-altitude storms directly over the station — leaving a null-coverage cone if no adjacent site covers it.
• Minimum resolution is 1.24 miles. Zoom in to roughly ten miles and you are looking at individual square return boxes, not real storm structure.

Abnormalities to expect: ground clutter, strobes and spurious radar data, sun strobes when the antenna points at the sun, shadows from buildings or mountains, and returns from military chaff.

As a strategic tool only — for deciding which side of a line to be on, or whether to go at all. Never as a tactical penetration aid.

The reasoning stacks: the image may be 20 to 30 minutes old, resolution is 1.24 miles, it cannot separate hail from rain, and a cell that was building when the mosaic was composed may now be a mature cell where the display still shows green. Combine that with a cell's short life cycle and the gap between picture and reality is larger than the clearance you were planning to take.

6 | 4 | 2 — validity drops as severity rises.

An advisory is a forecast, not a prohibition, so under Part 91 you are usually legal. The commercial answer does not stop there.

What changes the analysis: a PIREP confirming the hazard, and the airplane's operating limitations — a normally aspirated single with no known-ice certification has no legal or practical business in forecast moderate icing. And there is a third factor the private pilot did not have: a customer who wants to go. State it explicitly, because the examiner is testing whether external pressure enters your reasoning.

Decide the trigger on the ground, not in the air. The ones worth naming:

• Conditions at destination trending toward or below your personal minimum, not just the legal minimum
• Ceiling or visibility deteriorating faster than the TAF's trend groups predicted
• Convection developing along the route or over the destination — cells move and build faster than the plan
• Any icing or turbulence report exceeding the airplane's certification or your experience
• Fuel state such that continuing consumes the reserve you would need to divert later

The failure mode is waiting until diversion becomes the only option instead of the best option. Pick the last point on the route at which you still have full choices, and treat reaching it as the decision.

Write them down, before the phone rings, and make them specific to the aircraft and the operation — a number, not a feeling: minimum ceiling and visibility for departure, en route, and destination; maximum crosswind and total wind; minimum fuel on landing; a no-go on forecast icing or convection along the route.

The commercial delta is that the minimums must survive contact with money. A personal minimum that moves when a customer is waiting was never a minimum. State the rule that makes it hold: the decision to go is made against the written number, and only I can change the number — and never on the day of the flight.

1. Big picture first — Surface Analysis, prog charts, and the Convective Outlook: what is the system doing over the next 12 to 24 hours?
2. Route — GFA for cloud, visibility, icing, and turbulence over each segment; FB for wind and freezing level.
3. Endpoints — METAR trend and TAF for departure, destination, and at least one alternate, with the change groups (FM, BECMG, TEMPO, PROB) read carefully.
4. Advisories — AIRMETs, SIGMETs, Convective SIGMETs along the route.
5. Truth — PIREPs, the only real-time report of icing, turbulence, and tops.
6. Aircraft and pilot — certification limits, equipment, my currency and recency in these conditions.

Then a stated decision with a reason: go, no-go, or go with a defined turn-back point.

• Know the latency of your specific box. "Up to 20 or 30 minutes" is the PHAK ceiling; your equipment manual has the real number, and the timestamp on the display is the one to read.
• Set the range deliberately. Below about ten miles the NEXRAD square boxes are resolution artifacts, not weather.
• Decide what you look at when. Strategic weather is a cruise task. In the terminal area the display gets one glance, and only for the picture you already expect.
• Turn layers off. Traffic, terrain, weather, and airspace all at once on one MFD is how a single pilot ends up heads-down in the phase of flight that least tolerates it.
• Cross-check against the window and against ATC. Controllers see current radar; your display does not.

Each reporting station with a METAR or TAF available is shown as a flag from the center of the airport symbol, color-coded to the flight category currently reported there, with a legend on the display. Setting the range out (up to 2,000 miles on some systems) lets you pan the route and see the pattern (PHAK ch. 13).

That pattern view is the real value: a line of blue and red flags shows you where a front actually is far faster than reading twelve METARs, and it makes the divert field obvious. Just remember the flags are as old as the underlying observations plus the link latency.

Ceiling and/or visibility set the category — the worse of the two governs.

MVFR is the category commercial pilots get hurt in — legal VFR, entirely lawful to accept, and not enough room over terrain at night or with a passenger asking when you will arrive.

Formation requires moisture, unstable air, and a lifting force. Stages: cumulus (updrafts), mature (most hazardous — up- and downdrafts, heavy precipitation, lightning, hail; rain reaching the ground marks the onset), dissipating (downdrafts dominate; the anvil points the direction of movement).

Microburst is the implication that kills: an intense, small-scale downdraft, most dangerous on takeoff and approach where I am low, slow, and configured. The encounter sequence is a performance increase on entering the outflow (headwind), then the downdraft, then the headwind becoming a tailwind — airspeed decaying, nose dropping. Response: full power, go around, immediately.

Decision implication: convective activity is a reroute or a delay, not a deviation problem. My datalink picture is up to 20 to 30 minutes old, so a cell is where I last saw it plus its movement plus its growth.

Structural icing needs visible moisture and a surface at or below freezing. Rime is rough and milky and stays near the leading edge; clear is smooth, dense, hard to see, and spreads aft; mixed is both. The effects compound: lift decreases, weight and drag increase, thrust decreases, and stall speed rises — with no reliable warning because the AOA at which the contaminated wing stalls is lower than the one you trained to.

Decision implication: the freezing level from the FB and the GFA sets the altitude band I may not fly in visible moisture. In a non-deiced airplane the answer is a hard no, and the plan needs an escape — either terrain-clear air below the freezing level or a route that stays clear of cloud entirely. An AIRMET Zulu is a planning input; a PIREP of icing is a decision.

Mountain wave forms with strong wind roughly perpendicular to a ridge in stable air; the hazard is on the leeward, downwind side, where downdrafts can exceed a light airplane's climb capability. Rotor and lenticular clouds mark it when there is enough moisture, and nothing marks it when there isn't.

Low-level wind shear is a sudden change in wind speed or direction at any altitude, producing abrupt airspeed changes and severe turbulence — associated with thunderstorms, microbursts, fronts, and temperature inversions. AIRMET Tango covers turbulence, strong surface winds, and LLWS.

Decision implication: crossing a ridge with strong perpendicular winds means crossing at an angle with an escape turn planned, at an altitude with real margin, and accepting that the airplane may simply be unable to outclimb the descending air. On the approach, LLWS means a higher target speed, more energy, and a go-around briefed as the expected outcome rather than the exception.

Stability is the single best predictor of what the sky will look like. The mechanism is heat exchange at the surface (PHAK ch. 12):

• An air mass moving over a warmer surface is heated from below, convective currents form, and the mass becomes unstable — good surface visibility, cumulus clouds, showers, and turbulence.
• An air mass moving over a colder surface forms no convective currents and becomes stable — poor surface visibility, because smoke, dust, and particles cannot rise out and are trapped near the surface. Expect low stratus and fog.

Standard lapse rate is about 2 °C per 1,000 ft. A temperature inversion is the reversal — temperature increasing with height — and it matters twice over: it traps moisture and particulates, contributing to clouds, fog, haze, or smoke and diminished visibility in the inversion layer, and it is a classic low-level wind shear source. Surface-based inversions form on clear, cool nights as the ground cools the air within a few hundred feet of the surface; frontal inversions form when warm air spreads over cooler air.

Decision implication: unstable means bumps and buildups but you will see them; stable means smooth air and you may not see anything at all.

Dew point is the temperature at which the air can hold no more moisture. Cool the air to its dew point and it is saturated, and moisture condenses out as fog, dew, frost, clouds, rain, or snow. The temperature/dew point spread is your convergence tool: unsaturated air lifted cools at about 5.4 °F per 1,000 ft while the dew point drops about 1 °F per 1,000 ft, so they converge at roughly 4.4 °F per 1,000 ft. Divide the surface spread by 4.4 to estimate the cloud base in thousands of feet (PHAK ch. 12).

Cloud formation needs water vapor, condensation nuclei (dust, salt, smoke), and a cooling mechanism. Classification is by base height: low (surface to about 6,500 ft AGL — stratus, stratocumulus, nimbostratus; primarily water droplets but can hold supercooled droplets that produce hazardous icing), middle, high, and clouds with vertical development.

Decision implication: a narrow spread with a stable mass is a ceiling-and-visibility problem; a narrow spread with an unstable mass is a convection problem. Same number, opposite flight.

A front is the boundary between two air masses of different characteristics, and an approaching front of any type always means imminent weather change. The four types: warm, cold, stationary, occluded. No two fronts are the same, but the generalizations hold (PHAK ch. 12).

A warm front advances and replaces colder air, moving slowly at 10 to 25 mph. The warm air slides up over the cool air, cooling and condensing as it goes. Ahead of it expect cirriform then stratiform clouds and fog along the frontal boundary — and in the summer months, cumulonimbus, which is the trap: a warm front is not automatically a benign front.

Turbulence tracks the same causes — convective currents in unstable air, mechanical turbulence from wind over terrain and obstructions, wind shear at frontal surfaces and inversions, and wake turbulence. AIRMET Tango is the product.

Decision implication: the front's timing is the flight-planning variable. Frontal passage two hours earlier or later than forecast is the difference between a legal VFR arrival and a diversion, so brief the front's position and speed, not just the destination TAF.

Fog is a cloud on the surface, formed when air near the ground cools to its dew point. The types, by formation (PHAK ch. 12):

• Radiation fog — clear nights, little or no wind, ground cools by terrestrial radiation. Forms in low-lying areas and mountain valleys. Burns off as the sun raises the temperature, and any increase in wind speeds dissipation. Under 20 ft thick it is ground fog.
• Advection fog — warm, moist air moves over a cold surface. Requires wind (up to 15 kt).
• Upslope fog — moist, stable air forced up sloping terrain. Also requires wind.
• Steam fog (sea smoke) — cold, dry air over warm water. Low-level turbulence and icing are commonly associated.
• Ice fog — water vapor forming directly into ice crystals, usually at −25 °F or colder.

The catch worth stating: advection and upslope fog, unlike radiation fog, may not burn off with the morning sun and can persist for days, and they extend to greater heights. A "it'll lift by ten" plan only works for radiation fog.

Frost is deposited when the collecting surface is below freezing. Dew poses no threat; frost is a definite flight safety hazard — it disrupts airflow over the wing, drastically reducing lift and increasing drag. The airplane must be thoroughly cleaned and free of frost prior to flight. Polishing it smooth is not a thing.

Obstructions to visibility — smoke, haze, dust, volcanic ash — concentrate under stable air and inversions, which is why the smooth morning is often the low-visibility morning.