Weather Information

15°C (59°F) and 29.92 in Hg at sea level, with a −2°C per 1,000 ft lapse rate.

Connect points of equal pressure; close spacing means a steep pressure gradient and stronger winds.

Surface winds: slowed and turned by friction, crossing isobars at an angle. Winds aloft: flow roughly parallel to the isobars because of the Coriolis effect.

Stable — stratiform clouds, steady precipitation, poor visibility, smooth air. Unstable — cumuliform clouds, showery precipitation, good visibility, turbulence.

High: descending, diverging air, generally fair. Low: rising, converging air, clouds and precip. Ridge: an elongated high. Trough: an elongated low.

Needs moisture, unstable air, and a lifting force. The three stages:

• Cumulus — building, updrafts
• Mature — most hazardous; up- and downdrafts, heavy precip, lightning, hail (rain reaching the ground marks onset)
• Dissipating — downdrafts dominate; anvil points the direction of movement

A sudden change in wind speed or direction at any altitude — abrupt airspeed changes and severe turbulence; associated with thunderstorms, microbursts, fronts, and temperature inversions.

As they converge (small spread), the air nears saturation — expect fog, clouds, or precipitation.

• Radiation — clear, calm nights over land
• Advection — warm moist air moving over a cooler surface
• Upslope — moist air cooled as it climbs terrain
• Precipitation-induced — warm rain falling through cool air
• Steam — cold air moving over warmer water

• Structural — rime (rough, milky, leading-edge), clear (smooth, dense, hard to see, spreads aft), and mixed
• Induction — including carburetor icing
• Instrument — blocked pitot/static sources

Icing cuts lift, adds weight and drag, reduces thrust, and raises stall speed.

• Standard — full briefing when you haven't gathered anything yet
• Abbreviated — supplement or update an earlier briefing
• Outlook — departure 6+ hours out

From 1800wxbrief.com (Leidos Flight Service) or aviationweather.gov.

METAR — routine hourly surface observation (SPECI for significant changes). TAF — Terminal Aerodrome Forecast, valid 24 or 30 hours for about 5 SM around the airport, issued every 6 hours. Change groups: FROM, BECMG, TEMPO, PROB.

The Graphical Forecasts for Aviation tool — replaced the textual Area Forecast for the CONUS.

AGL. A ceiling is the lowest broken or overcast layer.

AIRMET (valid 6 hr, now mainly the G-AIRMET graphic): Sierra (IFR/mountain obscuration), Tango (turbulence/strong surface winds/LLWS), Zulu (icing/freezing levels). SIGMET (valid 4 hr; 6 hr for volcanic ash/tropical cyclones): severe non-convective hazards to all aircraft. Convective SIGMET (valid 2 hr): severe thunderstorm activity.

FB — forecast wind and temperature at standard altitudes, twice daily (no winds within 1,500 ft of station elevation; no temps within 2,500 ft; above 24,000 ft temps assumed negative). PIREPs — actual in-flight reports; the best source for icing, turbulence, and cloud tops.

Surface analysis: shows fronts and pressure systems. Prog charts: forecast significant weather. Both are now consolidated in the Aviation Weather Handbook.

Three, in increasing order of nastiness:

• Single cell — isolated, short-lived
• Multicell — clusters or squall lines
• Supercell — rotating, longest-lived, most violent; may be embedded in other clouds where you can't see it

A mature cell typically runs about 5–10 minutes of peak intensity over a footprint of a mile or two across.

A microburst is a small, intense downdraft from a convective cell — the most severe form of wind shear I can encounter. Downdrafts can reach 6,000 fpm, the outflow is typically under a mile wide, and an individual burst lasts only about 5–15 minutes — short-lived enough that the aircraft ahead of me may report nothing.

It's most dangerous during takeoff and landing (PIREPs code these DURC — during climb — and DURD — during descent): I'm low, slow, and configured with little energy to spare. The classic encounter sequence:

1. Entering the outflow — a performance increase (headwind; airspeed and lift rise)
2. The downdraft hits
3. The headwind becomes a tailwind — airspeed decays rapidly, the nose pitches down, altitude and attitude bleeding away

The response is immediate: full power and go around. Heed low-level wind shear (LLWS) advisories and microburst alerts. (AIM 7-1; Aviation Weather Handbook)

Ceiling and/or visibility set the category — the worse of the two governs. Easiest to memorize in reverse order of severity, LIFR up to VFR:

These are the colored station dots on the GFA/METAR map — green good, blue marginal, red IFR, magenta the worst. MVFR is legal VFR but deserves real respect in a Warrior: 3 miles and a 1,000-ft ceiling leaves very little room over terrain.

6 | 4 | 2 — validity drops in reverse order of severity. The more severe the advisory, the shorter it lives:

SIGMET turbulence triggers include severe turbulence not associated with thunderstorms — think low-level wind shear, mountain waves (most dangerous on the leeward, downwind side), and building-induced mechanical turbulence.

Legally, often yes — an advisory is a forecast, not a prohibition, and the conditions aren't guaranteed to exist. But a PIREP confirming the hazard changes the analysis: flight into known severe icing or conditions exceeding the Warrior's operating limitations is where I'd actually be stopped. Advisories inform the go/no-go; PIREPs plus my aircraft's limitations decide it.

Rule of thumb: "if you read it, true. if you hear it, magnetic." Written products — METAR, TAF, FB — reference true north. Voice sources — tower, ATIS — give magnetic so they line up with runway numbers. Cloud heights in a METAR are AGL (the sensor is sitting on the ground measuring up).

• AO1 vs. AO2 — automated station type: AO2 has a precipitation discriminator (can tell rain from snow); AO1 cannot.
• SLP114 — sea level pressure; prefix a 10 (or 9) and insert the decimal: 1011.4 hPa.
• P6SM (TAF) — visibility Plus 6 statute miles, i.e., more than 6.
• FU — smoke (from the French fumée).
• $ — the station needs maintenance; treat the data with suspicion.

A three-part sequence:

1. Initial RPM drop — hot, less-dense unfiltered air
2. A momentary roughness or dip — the melting ice passing through as water
3. RPM rises and stabilizes — the ice has cleared

Knowing the sequence keeps me from panicking and pushing carb heat back off at exactly the wrong moment. Carb heat works by routing unfiltered air warmed by the exhaust over the intake — hot air is less dense, which is why performance drops while it's on.

A front is the boundary between two air masses — and it's named after the advancing (stronger) air mass.

One more sequencing fact worth having: lows form in response to highs — pressure systems come as a pair, and air flows out of the high (clockwise, descending) and into the low (counterclockwise, rising).