Section 8: Weather Theory — Density Altitude, Fog Families, and the Anatomy of a Storm

Weather lives in the troposphere (the first ~20,000 ft; jets cruise above it on purpose). The constants: temperature drops 2 C per 1,000 ft climbed - 'an important number' that seeds every later calculation; pressure drops 1 inHg per 1,000 ft from the sea-level standard of 29.92 inHg (14.7 psi, 1013.2 mb) - so his 5,000-ft hometown sits near 24.92 on a standard day, and 18,000 ft holds half the sea-level pressure. Circulation runs on uneven heating: warm equatorial air rises, cold polar air sinks.

PRESSURE ALTITUDE corrects your elevation for non-standard pressure - think of it as 'a hyphenated term... one word': (29.92 minus altimeter setting) x 1,000 + field elevation; his example lands at 5,130 ft. DENSITY ALTITUDE then corrects for temperature: (local temp minus standard temp at that altitude) x 120 + pressure altitude; worked to 3,360 ft. Neither formula is exam-required - the CONCEPT is.

The trap he hammers: HIGH pressure = DENSE air = LOW density altitude = GOOD performance. Students hear 'high' twice and pick the wrong direction. His table runs the corners: standard sea level = DA 0; hot sea level = +2,400; cold high-pressure sea level = -3,400; a 5,000-ft field swings from 1,600 (cold, high pressure) to 8,400 (hot, low pressure). Effects of high DA: longer takeoff/landing, slower climb, lower ceiling, less payload, less range, worse economy, mushy handling - the 12,000-ft flight where his drone went soft is the felt version.

Wind is air flowing high-to-low pressure, a ball rolling downhill; isobars (millibar contours - standard 1013.2) packed tight mean a steep hill and fast wind. Northern-hemisphere systems: HIGHS rotate clockwise and outward, bringing good weather; LOWS counterclockwise and inward, bringing bad. Convective currents rise off rock, sand, and concrete, sink over water and vegetation; buildings shed corner turbulence, and mountains dump dangerous descending air on their lee sides. The daily cycle: sea breeze inland by day (land heats faster), land breeze seaward at night (water holds heat) - the mechanism behind the offshore-drift exam scenario.

Low-level wind shear is a rapid, localized wind change - 'deadly' at manned scale, thunderstorm-adjacent by preference. The microburst is its concentrated form: 5-15 minutes long, downdrafts to 6,000 fpm, wind swings of 30-90 knots, in the sequence headwind -> downdraft -> tailwind, each stage subtracting the margin the previous one faked. The stealth clause: 'you may not actually even see water' - dry microbursts leave no visual warning.

Dew point is the temperature at which air saturates; temperature-minus-dewpoint closing toward zero forecasts visible moisture - dew, fog, cloud, frost. FROST needs dew point below freezing AND a below-freezing surface - and frost on props means vibration that can break blades and kill motors.

Air reaches saturation four ways - warm air over a cold surface, air-mass collision, nighttime ground cooling, and upslope lifting at the lapse rate. The fog family: RADIATION (valley fog - stable air, calm night, ground radiates heat away; sun burns it off; its below-minus-25 F cousin ICE FOG is 'just the worst'); ADVECTION (coastal - warm moist air slides over cold land, needs wind up to 15 knots); UPSLOPE (wind-forced up terrain, saturating on the climb - may NOT burn off, as his Mount Humphreys hike showed, forming and clearing in 10-15 minute cycles at the crest); STEAM (cold dry air over warm water - evaporation instantly re-saturating, with low-level turbulence and icing risk).

Four families by height: LOW stratus/stratocumulus/nimbostratus ('nimbo' = rain) from stable air; MID alto-clouds flagging turbulence aloft; HIGH cirrus family carrying ice; VERTICAL cumulonimbus - 'greatest turbulence of all clouds' - and towering cumulus with rain, chop, hail. The standalone star: LENTICULAR, the mountain lens cloud marking 50+ knot winds with violent up/downdrafts - explicitly exam-flagged.

Stability is the resist-vertical-motion property: cool/dry = stable, warm/humid = unstable. The paired forecast table: UNSTABLE air builds cumuliform clouds, showery rain, rough air - but good visibility (Florida). STABLE air lays stratiform cloud and fog, continuous drizzle, smooth air - and poor visibility (Seattle). Every column is a testable contrast.

Coverage in eighths: clear 0, few 1-2, scattered 3-4, broken 5-7, overcast 8. 'Ceiling' applies ONLY to broken or overcast layers. Visibility reports in statute miles - 10 max down to 1/4 (below that: 'less than 1/4') in quarter-mile steps. The regulatory floor for drone flight: 3 SM - though his personal line is stricter: at 5 SM he already calls it 'actually fairly limited visibility.'

Thunderstorms need three ingredients - water vapor, an unstable lapse rate, a lifting force - and live three stages: CUMULUS (building, no rain yet), MATURE (rain reaches ground; lightning is the marker), DISSIPATING (updrafts collapse). FRONTS are state-sized air-mass boundaries announced by a temperature change AND a wind shift: warm fronts (red, rounded) lay nimbostratus with cirrus heralds far ahead; cold fronts (blue, 'pokey' triangles - his snowplow image) bulldoze air up into cumulonimbus and drag squall lines; stationary fronts stall; OCCLUDED fronts - cold overtaking warm - combine everything: 'the worst possible case.'

ICING: visible moisture plus a surface at or below 0 C. On a drone that means electronics intrusion and prop ice - weight, vibration, ruined lift. The guidance is absolute: clouds + cold + moisture = stay down.